[Method of immunocytochemical demonstration of cholinergic neurons in the central nervous system of laboratory animals].

A protocol of immunocytochemical demonstration of choline acetyltransferase (ChAT), a key enzyme of acetylcholine synthesis, in paraffin sections of the brain of some laboratory animals, is presented. The method is simple, gives fairly reproducible results and allows for demonstration of ChAT in neurons, nerve fibers, and terminals in preparations of at least three species of laboratory animals including rat, rabbit, and cat. Different kinds of fixation (10% formalin, 4% paraformaldehyde, or zinc-ethanol-formaldehyde) were found suitable for immunocytochemical visualization of ChAT, however, optimal results were obtained with the application of zinc-ethanol-formaldehyde

Preparation and characterization of two isozymes of choline acetyltransferase from squid head ganglia.

Two isozymes of choline acetyltransferase (Acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6) have been isolated and purified from squid head ganglia. Each isozyme contains multiple isoelectric forms with isoelectric points ranging from pH 5.0 to 6.2. The isozymes differ in their affinities for cellulose phosphate on column chromatography, as well as in their heat stabilities and in their capacities to be activated by salt. Both isozymes are stabilized by sucrose and by sulfhydryl-protecting reagents such as mercaptoethanol and dithiothreitol.

Establishment of an efficient enzyme-linked immunosorbent assay for the determination of human choline acetyltransferase.

A specific and sensitive two-side enzyme-linked immunosorbent assay (sandwich-ELISA) was established for the reliable quantification of human brain and placental choline acetyltransferase (ChAT). In contrast to the radiometric assay developed by Fonnum, which is widely used for the measurement of enzyme activity, the sandwich-ELISA particularly recognized inactivated forms of the antigen. In the assay, affinity-purified polyclonal synthetic peptide antibodies adsorbed to the polystyrene surface of the microtiter plate were employed as capture reagent. Based on standard peroxidase protocols, immobilized ChAT was detected using monoclonal antibodies raised against human placental ChAT. By use of this ELISA, ChAT was determined at various purification stages of the enzyme, in body fluids, during recovery experiments and in sera of patients with severe brain damage.

Production and functional expression of an epitope-tagged human choline acetyltransferase.

cDNA containing the entire coding region of the human choline acetyltransferase gene (hChAT) was fused to the influenza virus hemagglutinin (HA) epitope preceded by a Kozak sequence. The recombinant HA-hChAT was then inserted into an expression vector under the transcriptional control of the cytomegalovirus (CMV) promoter. After transient transfection into COS-1 cells, expression was assayed by Northern and Western blot analysis and immunofluorescence. The chimeric HA-hChAT protein was compared to native hChAT for its ability to synthesize acetylcholine. It behaves identically to unmodified hChAT showing that the HA epitope does not affect ChAT activity. This approach enables one to distinguish the expression of the HA-hChAT from endogenous ChAT. Genetically engineered cells that express a high level of HA-hChAT could be used as a promising experimental tool for gene transfer and neurografting techniques as well as to produce and study transgenic mice.

An endogenous inhibitory factor for choline acetyltransferase.

Chromatography of partially purified choline acetyltransferase (CAT) over carboxymethyl cellulose may result in the loss of up to 95% of the enzyme activity. This loss of activity can be prevented by running the chromatographs at low protein concentration with a large gradient volume suggesting that interactions between CAT and other endogenous proteins are involved in the mechanism of inactivation. Further experiments showed that CM-cellulose chromatography separates an endogenous inhibitory factor(s) and an endogenous activating factor(s) which protects the enzyme from the action of the former. The inhibitory factor elutes with CAT and produces almost complete inactivation unless the protein concentration is maintained below 0.05 mg/ml. Mixing experiments demonstrated that the activating factor is capable of blocking the effect of the inhibitory factor. The low degree of temperature dependence of the inhibitory factor essentially rules out the possibility that the inhibitor is a proteolytic enzyme. The I50 was estimated to be 10(-7) M or less suggesting a possible physiological role of these factors in the regulation of CAT activity.

Recovery of choline acetyltransferase activity without sprouting of the residual acetylcholine innervation in adult rat cerebral cortex after lesion of the nucleus basalis.

In view of the divergent literature concerning the long-term effects of ibotenic acid lesions of the nucleus basalis of Meynert (NBM) on the choline acetyltransferase (ChAT) activity in adult rat cerebral cortex, we have critically reassessed the issue of an eventual recovery of this enzymatic activity by sprouting of the residual acetylcholine (ACh) innervation. At short (1 week) and long survival time (3 months) after unilateral ibotenic acid lesion, ChAT activity was biochemically measured in the ipsi and contralateral fronto-parietal cortex of several rats in which the extent of ACh neuronal loss in NBM was also estimated by counts of ChAT-immunostained cell bodies on the lesioned vs. non-lesioned side. In other lesioned rats, particular attention was paid to the distribution of the residual cortical ACh (ChAT-immunostained) innervation, and that of immunostained vasoactive intestinal polypeptide (VIP) axon terminals known to belong in part to intrinsic cortical ACh neurons which co-localize this peptide. One week after NBM lesion, profound decreases of ipsilateral cortical ChAT activity were tightly correlated with the extent of ACh cell body loss in the nucleus. A significant recovery of cortical ChAT activity could be documented after 3 months, despite persistence of NBM cell body losses as severe as after 1 week. At both survival times, the number of ChAT-immunostained axons was markedly reduced throughout the ipsilateral fronto-parietal cortex, demonstrating that most ACh fibers of extrinsic origin had been permanently removed. This result also indicated that the long-term recovery of ChAT activity had occurred without sprouting of the residual ACh innervation. The laminar distribution and number of VIP-immunostained terminals remained the same on the lesioned and intact side and comparable to normal, ruling out an extensive sprouting of intrinsic ACh/VIP or VIP alone fibers. The return to a near normal cortical ChAT activity in severely ACh-denervated cortex suggested that the intrinsic ACh innervation was primarily responsible for this recovery.

Membrane-bound choline acetyltransferase of the torpedo has characteristics of an integral membrane protein and can be solubilized by proteolysis.

Due to Triton X-114 fractionation of synaptosomes isolated from the electric organ of the fish Torpedo, the existence of a hydrophilic and an amphiphilic form of the enzyme choline-O-acetyltransferase (ChAT) was revealed. Amphiphilic ChAT which represents about 10% of total enzyme activity in synaptosomes, reached 40% of ChAT activity measured in preparations of synaptosomal plasma membranes (SPM) which were washed with solutions of increasing ionic strength. ChAT activity bound to washed SPM could be partially solubilized using proteinase K but not phospholipase C. No ChAT solubilization occurred by treating intact synaptosomes with proteinase K. Water/Triton X-114 partition coefficients of hydrophilic and amphiphilic ChAT were found to be 6.5 and 0.17, respectively. Sedimentation coefficients determined by centrifugation in linear density gradients of sucrose containing Triton X-100, were 4.2S and 4.4S for amphiphilic and hydrophilic ChAT, respectively. On the other hand, removal of Triton X-114 from the detergent phase containing amphiphilic ChAT activity led to enzyme aggregation. Finally, amphiphilic ChAT was slightly more acidic (pH 6.6) than was hydrophilic enzyme (6.8-7.0). We conclude that in Torpedo synaptosomes two forms of ChAT activity, a soluble and a membrane-bound form, are indeed present which differ in their hydrophobicity. The soluble form is hydrophilic. The membrane-bound form is amphiphilic and it aggregates upon removal of detergent. These are two characteristics of integral membrane proteins. Membrane-bound ChAT is most probably intracellularly oriented and not bound to membrane through a 'receptor' protein.

A comparative study of choline acetyltransferase from normal and Alzheimer brain.

A comparative study was made of the enzyme choline acetyltransferase (ChAT) from normal and Alzheimer (senile dementia of the Alzheimer type) brain. The number of molecular weight and charge forms of the enzyme were determined in the caudate region of both brains. Efficient purification of active ChAT was achieved using immuno-affinity purification. It was shown that the purified enzyme was identical in both cases, exhibiting a single charge (apparent pI approximately 8.2) and a single molecular weight (mol. wt. = 68,000). The idea of a selective loss of one particular isoform to explain the reduced levels of ChAT observed in Alzheimer's disease can be ruled out.

Membrane-bound choline acetyltransferase is a cell surface marker of the differentiated NS-20Y cholinergic cell line.

When differentiated cells of the cholinergic NS-20Y type were incubated with an antiserum to choline acetyltransferase (ChAT), in the presence of complement, immunolysis occurred as demonstrated by release of 20% of total lactate dehydrogenase and 70% of total ChAT. No significant immunolytic effects were observed when either undifferentiated (dividing or non-dividing) or partially differentiated cells were incubated under identical conditions. When the Triton X-114 phase separation technique was employed using membranes from differentiated cells, a small but significant proportion of choline acetyltransferase was recovered in the detergent-rich phase. These results suggest that a membrane-bound form of ChAT is a surface marker of the NS-20Y cell line.

Kinetic and inactivation studies of recombinant Drosophila choline acetyltransferase.

A cDNA for Drosophila choline acetyltransferase (ChAT) was expressed in E. coli and the recombinant enzyme partially purified. Kinetic analysis yielded the following constants for the recombinant enzyme; KmAcCoA = 29 microM, KmCoA = 25 microM, Kmcholine = 330 microM, and Kmacetylcholine = 2 mM. The recombinant Drosophila enzyme, like the enzyme from other species, exhibited an increase in activity as a function of increased salt concentration. Chemical modification studies using dithio-bis-nitro-2-carboxylate, butanedione, and diethylpyrocarbonate showed that the recombinant enzyme contains active site cysteine, arginine, and histidine residues. These studies demonstrate that the recombinant Drosophila ChAT possesses the same catalytic properties as the enzyme from a variety of other sources.

Ultrastructural localization of choline acetyltransferase in the CNS of rat.

The ultrastructural localization of choline acetyltransferase in various regions of rat's CNS is studied. It is established that the reaction product is located in some structures of the neuronal perikarya--on the outer surface of the rough endoplasmic reticulum sacs, along the vesicular profiles and in the mitochondria. Electron dense material is found in the axon terminal as well as, where enzyme activity is detected in the cytoplasm, on the membrane of spherical and flat synaptic vesicles and in the mitochondria. These findings, the control results and literature data are discussed.

Choline Acetyltransferase from the electric organ of Electrophorus electricus (L.)--physicochemical characterization and immunochemical identification.

It is well known that the regulation of choline acetyltransferase (ChAT) activity, under physiological conditions, is important for the development and neuronal activities of cholinergic systems. The purification of ChAT has been obtained from many sources such as electric organs of fishes, Drosophila melonogaster, and mammals. We have prepared choline acetyltransferase from a pool of supernatants obtained by differential centrifugation of electric organ homogenates from Electrophorus electricus (L.) in Tris-phosphate buffer, 0.05 M, pH 7.6. The first step of the enzyme purification was performed by ammonium sulfate precipitation at 40% and 80%. The precipitate at 80% was solubilized with sodium-phosphate buffer 0.05 M, pH 7.6, dialyzed, chromatographed on DEAE-52 column and the active fraction submitted to FPLC system columns (Mono-Q: ion exchange- Superose-12: gel filtration). ChAT activity from the eluates was estimated by Fonnun's method [Fonnun, 1975], with Acetyl-Coenzyme A tritium labelled ([3H]AcCoA) as substrate, and the synthesis of 3HACh formed was measured. The peak from gel filtration showed a relative molecular mass of 80 offkDa with highest activity in the order of 77,42 nmoles ACh/min/mg protein. This fraction was analyzed by SDS-PAGE and a band of 42 kDa was detected with Coomassie blue stain, indicating that the enzyme is formed by two subunits. Employing an antibody, the presence of ChAT was confirmed with the Western blotting technique. Isoelectrofocusing analysis demonstrated two isoforms with pI of 6,49 and 6,56, respectively.

Structural localization of the elements of the acetylcholine system in the rat cerebellum.

The elements of the acetylcholine system (choline acetyltransferase, acetylcholinesterase, acetylcholine receptors) were analysed in the cerebellum of the rat by histochemical and biochemical means. All the elements were found. Besides the granular layer, the molecular layer exerts CAT activity. The 3H-QNB binding site is highest in the molecular layer and lowest in the intracerebellar nuclei, while the 125I-alpha-bungarotoxin distribution in these structures is the opposite. The results support the view that cholinergic transmission may be present in the rat cerebellum.

[New in situ hybridization technique with non-radio-labeled probe--detection of choline-acetyltransferase gene expression in rat spinal cord].

We have established a new in situ hybridization method utilizing non-radiolabeled probes. Using this technique, we have attempted to detect the choline-acetyltransferase (ChAT) gene expression in rat spinal cord. It was revealed that the ChAT gene was expressed mainly in the cytoplasm of motor neurons and para-central cells. On the other hand, ChAT protein has already been reported to exhibit a diffused distribution in the cholinergic fibers. Comparing the localization of the ChAT gene with that of the ChAT protein, the ChAT gene was shown to exist only in the cytoplasm surrounding the nuclei. However, the ChAT gene was not expressed in axon terminals where ChAT protein synthesized acetylcholine. This result indicates that the ChAT gene is translated into protein around the nuclei and is thereafter transported toward the action site. We now think that there are two different patterns of neurotransmitter gene distribution. After mRNA is translated into protein, this protein is carried to the action site. On the other hand, mRNA itself is delivered to the action site and translated into protein. After the translation, this protein form exerts its own function. The ChAT gene is suspected as belonging to the first category of gene distribution. In Alzheimer disease, not only the acetylcholine system but also its biosynthetic enzyme, ChAT, system are supposedly destroyed by an unknown factor. If we can clarify the regulatory mechanism of the ChAT gene, this will lead us to the molecular pathogenesis of Alzheimer disease. Additionally, this new in situ hybridization technique should shed some light on the complex brain networks.

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.