A unique gene organization for two cholinergic markers, choline acetyltransferase and a putative vesicular transporter of acetylcholine

More than one way to toy with ChAT and VAChT

Expression of choline acetyltransferase (ChAT) and of the vesicular acetylcholine transporter (VAChT) is required for the acquisition and the maintenance of the cholinergic phenotype. The ChAT and VAChT genes have been demonstrated to share a common gene locus and this suggests a coordinate regulation of their expression. In the present work, we examined the effects of several differentiating treatments on the modulation of ChAT and VAChT expression at the mRNA and protein levels in growing and differentiating NG108-15 cells. In cells grown in the presence of serum, all the agents tested-retinoic acid, dexamethasone and dibutyrylcyclicAMP (dbcAMP)-induced an increase of ChAT and VAChT mRNA levels but with different efficacy. Treatment with dbcAMP plus dexamethasone resulted in the largest increase of VAChT mRNA amount while retinoic acid mostly enhanced ChAT mRNA level. However, while ChAT activity was increased by all agents, no change in the VAChT protein level was detected. In cells differentiated by serum deprivation, only retinoic acid was effective in inducing an increase of VAChT and ChAT mRNA and ChAT activity, while we observed a downregulation by dbcAMP and dexamethasone. Treatment with the antimitotic agent cytosine arabinoside led to an increase of ChAT activity which was further largely enhanced by the addition of dbcAMP plus dexamethasone, but to only a slight change in VAChT activity. We further showed that complex glycosylation processes which might play a role in targeting and/or stability of the membrane protein VAChT are deficient in these cells. Indeed, in transient transfection assays with the reporter soluble enzyme luciferase placed under regulatory and promoter regions of the VAChT gene, we observed a modulation of luciferase expression by differentiating agents. These data illustrate the complexity of the processes which participate to the expression of the ChAT and VAChT genes, both at the transcriptional and posttranslational levels.

Independent patterns of transcription for the products of the rat cholinergic gene locus

The cholinergic phenotype requires the expression of the vesicular acetylcholine transporter and choline acetyltransferase proteins. Both genes are encoded at one chromosomal location called the cholinergic gene locus. We have identified by in situ hybridization histochemistry distinct patterns of transcription from the cholinergic gene locus in the subdivisions of the rat cholinergic nervous system. The vesicular acetylcholine transporter and choline acetyltransferase are co-expressed in cholinergic neurons at all developmental stages in all major types of cholinergic neurons. The relative levels of vesicular acetylcholine transporter and choline acetyltransferase transcripts, however, change substantially during development in the CNS. They also differ dramatically in distinct subdivisions of the mature cholinergic nervous system, with vesicular acetylcholine transporter mRNA expressed at high levels relative to choline acetyltransferase mRNA in the peripheral nervous system, but at equivalent levels in the CNS. Expression of the R-exon, the presumptive first non-coding exon common to both the vesicular acetylcholine transporter and choline acetyltransferase, was not detectable at any developmental stage in any of the cholinergic neuronal subtypes in the rat nervous system. Thus, in contrast to less complex metazoan organisms, production of the vesicular acetylcholine transporter and choline acetyltransferase via a common differentially spliced transcript does not seem to occur to a significant extent in the rat. We suggest that separate transcriptional start sites within the cholinergic gene locus control vesicular acetylcholine transporter and choline acetyltransferase transcription, while additional elements are responsible for the specific transcriptional control of the entire locus in cholinergic versus non-cholinergic neurons. Independent transcription of the vesicular acetylcholine transporter and choline acetyltransferase genes provides a mechanism for regulating the relative expression of these two proteins to fine-tune acetylcholine quantal size in different types of cholinergic neurons, both centrally and peripherally.

Identification of candidate genes for controlling development of the basilar pons by differential display PCR

The basilar pons, a major hindbrain nucleus involved in sensory-motor integration, has become a model system for studying long-distance neuronal migration, axon-target recognition by collateral branching, and the formation of patterned axonal projections. To identify genes potentially involved in these developmental events, we have performed a differential display PCR screen comparing RNA isolated from the developing basilar pons with RNA obtained from developing cerebellum and olfactory bulb, as well as the mature basilar pons. Using 400 different combinations of primers, we screened more than 11,000 labeled DNA fragments and identified 201 that exhibited higher expression in the basilar pons than in the control tissues. From these, 138 distinct gene fragments were cloned. The differential expression of a large subset of these fragments was confirmed using RNase protection assays. In situ hybridization analysis revealed that the expression of many of these genes is limited to the basilar pons and only a few other brain regions, suggesting that they may play specific roles in pontine development.

Development of cholinergic terminals around rat spinal motor neurons and their potential relationship to developmental cell death

Neuron death seems to be regulated mainly by postsynaptic target cells. In chicks, nicotinic antagonists such as alpha-bungarotoxin (alphaBT) can prevent normal cell death of somatic motor neurons (SMNs). For this effect, however, alphaBT could be acting at peripheral neuromuscular junctions and/or central cholinergic synapses. To investigate this issue, we first studied the development of cholinergic terminals in the rat spinal cord by using vesicular acetylcholine transporter immunocytochemistry. Labeled terminals were seen in the ventral horn as early as embryonic day 15 (E15), the beginning of the cell death period. Thus, central cholinergic synapses form at the correct time and place to be able to influence SMN death. We next added alphaBT to organotypic, spinal slice cultures made at E15. After 5 days in vitro, the number of SMNs in treated cultures was substantially greater than in control cultures, indicating that alphaBT can reduce SMN cell death in rats as it does in chicks. Moreover, peripheral target removal led to extensive loss of SMNs, and such a loss occurred even in the presence of alphaBT, indicating the necessity of peripheral target for the alphaBT effect. Finally, to determine whether central cholinergic terminals also may be involved in SMN death, we delayed the alphaBT treatment until after central cholinergic terminals had disappeared from the slice cultures. The increased number of surviving SMNs, even in the absence of central terminals, argued that alphaBT acts at peripheral, not central, cholinergic synapses to rescue SMNs from developmental cell death.

Truncated forms of the dual function human ASCT2 neutral amino acid transporter/retroviral receptor are translationally initiated at multiple alternative CUG and GUG codons

The sodium-dependent neutral amino acid transporter type 2 (ASCT2) was recently identified as a cell surface receptor for endogenously inherited retroviruses of cats, baboons, and humans as well as for horizontally transmitted type-D simian retroviruses. By functional cloning, we obtained 10 full-length 2.9-kilobase pair (kbp) cDNAs and two smaller identical 2.1-kbp cDNAs that conferred susceptibility to these viruses. Compared with the 2.9-kbp cDNA, the 2.1-kbp cDNA contains exonic deletions in its 3' noncoding region and a 627-bp 5' truncation that eliminates sequences encoding the amino-terminal portion of the full-length ASCT2 protein. Although expression of the truncated mRNA caused enhanced amino acid transport and viral receptor activities, the AUG codon nearest to its 5' end is flanked by nucleotides that are incompatible with translational initiation and the next in-frame AUG codon is far downstream toward the end of the protein coding sequence. Interestingly, the 5' region of the truncated ASCT2 mRNA contains a closely linked series of CUG(Leu) and GUG(Val) codons in optimal consensus contexts for translational initiation. By deletion and site-directed mutagenesis, cell-free translation, and analyses of epitope-tagged ASCT2 proteins synthesized intracellularly, we determined that the truncated mRNA encodes multiple ASCT2 isoforms with distinct amino termini that are translationally initiated by a leaky scanning mechanism at these CUG and GUG codons. Although the full-length ASCT2 mRNA contains a 5'-situated AUG initiation codon, a significant degree of leaky scanning also occurred in its translation. ASCT2 isoforms with relatively short truncations were active in both amino acid transport and viral reception, whereas an isoform with a 79-amino acid truncation that lacked the first transmembrane sequence was active only in viral reception. We conclude that ASCT2 isoforms with truncated amino termini are synthesized in mammalian cells by a leaky scanning mechanism that employs multiple alternative CUG and GUG initiation codons.

Stimuli that induce a cholinergic neuronal phenotype of NG108-15 cells upregulate ChAT and VAChT mRNAs but fail to increase VAChT protein

The vesicular acetylcholine transporter (VAChT) and choline acetyltransferase (ChAT) are encoded by genes organized in a single gene locus, and coregulation of the transcription of the two genes has been repeatedly reported in cholinergic tissues. In the present study, different stimuli were used to induce the differentiation of the hybridoma cells NG108-15 and we examined their effects on the modulation of VAChT and ChAT expression at the mRNA and protein levels. All agents upregulated the VAChT and ChAT mRNA levels, but to a different extent. ChAT activity was increased by retinoic acid, dexamethasone, and dibutyrylcyclic AMP (dbcAMP), and a synergistic effect was observed with a combined dexamethasone and dbcAMP treatment. Nonetheless, no changes in the VAChT protein level could be observed, as judged from ligand binding studies as well as from immunochemical detection. Hemicholinium-3-sensitive choline uptake, hemicholinium-3 binding, and acetylcholine content were increased by differentiating agents, with a rank order of potency comparable to their effects on ChAT activity. Prominent changes were observed in the expression of vesicular protein markers, particularly with the associated treatment dexamethasone and dbcAMP. Thus, it appears that although the different stimuli we have been using are able to stimulate neuronal features and activate the transcription of cholinergic genes, they did not contrive to increase the level of VAChT protein in these cells.

The biological role of non-neuronal acetylcholine in plants and humans

Acetylcholine, one of the most exemplary neurotransmitters, has been detected in bacteria, algae, protozoa, tubellariae and primitive plants, suggesting an extremely early appearance in the evolutionary process and a wide expression in non-neuronal cells. In plants (Urtica dioica), acetylcholine is involved in the regulation of water resorption and photosynthesis. In humans, acetylcholine and/or the synthesizing enzyme, choline acetyltransferase, have been demonstrated in epithelial (airways, alimentary tract, urogenital tract, epidermis), mesothelial (pleura, pericardium), endothelial, muscle and immune cells (granulocytes, lymphocytes, macrophages, mast cells). The widespread expression of non-neuronal acetylcholine is accompanied by the ubiquitous expression of cholinesterase and acetylcholine sensitive receptors (nicotinic, muscarinic). Both receptor populations interact with more or less all cellular signalling pathways. Thus, non-neuronal acetylcholine can be involved in the regulation of basic cell functions like gene expression, proliferation, differentiation, cytoskeletal organization, cell-cell contact (tight and gap junctions, desmosomes), locomotion, migration, ciliary activity, electrical activity, secretion and absorption. Non-neuronal acetylcholine also plays a role in the control of unspecific and specific immune functions. Future experiments should be designed to analyze the cellular effects of acetylcholine in greater detail and to illuminate the involvement of the non-neuronal cholinergic system in the pathogenesis of diseases such as acute and chronic inflammation, local and systemic infection, dementia, atherosclerosis, and finally cancer.

Neurogenetics of vesicular transporters in C. elegans

The nematode Caenorhabditis elegans has a number of advantages for the analysis of synaptic molecules. These include a simple nervous system in which all cells are identified and synaptic connectivity is known and reproducible, a large collection of mutants and powerful methods of genetic analysis, simple methods for the generation and analysis of transgenic animals, and a number of relatively simple quantifiable behaviors. Studies in C. elegans have made major contributions to our understanding of vesicular transmitter transporters. Two of the four classes of vesicular transporters so far identified (VAChT and VGAT) were first described and cloned in C. elegans; in both cases, the genes were first identified and cloned by means of mutations causing a suggestive phenotype (1, 2). The phenotypes of eat-4 mutants and the cell biology of the EAT-4 protein were critical in the identification of this protein as the vesicular glutamate transporter (3, 4). In addition, the unusual gene structure associated with the cholinergic locus was first described in C. elegans (5). The biochemical properties of the nematode transporters are surprisingly similar to their vertebrate counterparts, and they can be assayed under similar conditions using the same types of mammalian cells (6, 7). In addition, mild and severe mutants (including knockouts) are available for each of the four C. elegans vesicular transporters, which has permitted a careful evaluation of the role(s) of vesicular transport in transmitter-specific behaviors. Accordingly, it seems appropriate at this time to present the current status of the field. In this review, we will first discuss the properties of C. elegans vesicular transporters and transporter mutants, and then explore some of the lessons and insights C. elegans research has provided to the field of vesicular transport.

In vivo imaging of the vesicular acetylcholine transporter and the vesicular monoamine transporter

Validation of the vesicular acetylcholine transporter (VAChT) and the neuronal vesicular monoamine transporter (VMAT2) as important molecular targets in the cholinergic and dopamine neurons, respectively, has sparked interest in the development of radiotracers for studying these markers in vitro and in vivo. Currently, a number of selective high-affinity radiotracers are available for studying these targets in vivo with positron emission tomography (PET) or single photon emission computed tomography (SPECT). PET studies of VMAT2 in neuropathology reveal changes in the density of this marker that can be verified independently. Similarly, in vivo studies with VAChT ligands suggest that the latter are potentially useful in detecting cholinergic lesions in vivo; however, additional development is required to fully realize the potential of these radioligands.

Is RE1/NRSE a common cis-regulatory sequence for ChAT and VAChT genes?

Choline acetyltransferase (ChAT), the biosynthetic enzyme of acetylcholine, and the vesicular acetylcholine transporter (VAChT) are both required for cholinergic neurotransmission. These proteins are encoded by two embedded genes, the VAChT gene lying within the first intron of the ChAT gene. In the nervous system, both ChAT and VAChT are synthesized only in cholinergic neurons, and it is therefore likely that the cell type-specific expression of their genes is coordinately regulated. It has been suggested that a 2336-base pair genomic region upstream from the ChAT and VAChT coding sequences drives ChAT gene expression in cholinergic structures. We investigated whether this region also regulates VAChT gene transcription. Transfection assays showed that this region strongly represses the activity of the native VAChT promoters in non-neuronal cells, but has no major effect in neuronal cells whether or not they express the endogenous ChAT and VAChT genes. The silencer activity of this region is mediated solely by a repressor element 1 or neuron-restrictive silencer element (RE1/NRSE). Moreover, several proteins, including RE1-silencing transcription factor or neuron-restrictive silencer factor, are recruited by this regulatory sequence. These data suggest that this upstream region and RE1/NRSE co-regulate the expression of the ChAT and VAChT genes.

Synthesis and biological characterization of stable and radioiodinated (+/-)-trans-2-hydroxy-3-P[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM)

The vesamicol analogue (+/-)-trans-2-Hydroxy-3-[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM), a potent ligand for the vesicular acetylcholine transporter (VAChT), was evaluated as a potential radiotracer for studying VAChT density in vivo. In radioligand binding experiments, 3'-IBVM displays subnanomolar affinity for VAChT and 100-fold selectivity for VAChT over sigma1 and sigma2 receptors. Consistent with this profile, radioiodinated (+/-)-3'-IBVM distributed heterogenously in the rat brain following a bolus IV injection, displaying high concentrations in the striatum and moderate to low concentrations in the cortex and cerebellum, respectively. However, co-injection of the radiotracer with the sigma ligand haloperidol resulted in significant reductions of radiotracer levels in all brain regions examined. Therefore, radioiodinated (+/-)-IBVM appears to bind to both VAChT and sigma receptors in vivo.

Novel expression of equivocal messages containing both regions of choline/ethanolamine kinase and muscle type carnitine palmitoyltransferase I

For characterization of the detailed gene structure of human muscle type carnitine palmitoyltransferase I (M-CPTI), we analyzed the 5'-upstream region of the M-CPTI transcripts. As a result, we found a cDNA clone containing a nucleotide sequence unexpected from the reported M-CPTI gene structure in the upstream region of its 5' end. Comparison of this nucleotide sequence with that of genomic DNA showed that this sequence was derived from the 3'-untranslated region of the gene encoding choline/ethanolamine kinase-beta (CK/EK-beta) located upstream of the M-CPTI gene. Southern blot analysis showed that there was no other region homologous to the CK/EK-beta gene in the whole human genome. Thus, the overlapping transcript was concluded to be produced from the functional genes of CK/EK-beta and M-CPTI. Furthermore, cDNAs containing both exons of these genes were detected by the polymerase chain reaction using the cDNA of human heart M-CPTI obtained by specific reverse transcription from its 3'-untranslated region as a template. From these results, the production and organization of these overlapping transcripts are discussed.

Somatomotor neuron-specific expression of the human cholinergic gene locus in transgenic mice

We examined the expression pattern of the vesicular acetylcholine transporter in the mouse nervous system, using rodent-specific riboprobes and antibodies, prior to comparing it with the distribution of vesicular acetylcholine transporter expressed from a human transgene in the mouse, using riboprobes and antibodies specific for human. Endogenous vesicular acetylcholine transporter expression was high in spinal and brainstem somatomotor neurons, vagal visceromotor neurons, and postganglionic parasympathetic neurons, moderate in basal forebrain and brainstem projection neurons and striatal interneurons, and low in intestinal intrinsic neurons. Vesicular acetylcholine transporter expression in intrinsic cortical neurons was restricted to the entorhinal cortex. The sequence of the mouse cholinergic gene locus to 5.1kb upstream of the start of transcription of the vesicular acetylcholine transporter gene was determined and compared with the corresponding region of the human gene. Cis-regulatory domains implicated previously in human or rat cholinergic gene regulation are highly conserved in mouse, indicating their probable relevance to the regulation of the mammalian cholinergic gene locus in vivo. Mouse lines were established containing a human transgene that included the vesicular acetylcholine transporter gene and sequences spanning 5kb upstream and 1.8kb downstream of the vesicular acetylcholine transporter open reading frame. In this transgene, the intact human vesicular acetylcholine transporter was able to act as its own reporter. This allowed elements within the vesicular acetylcholine transporter open reading frame itself, shown previously to affect transcription in vitro, to be assessed in vivo with antibodies and riboprobes that reliably distinguished between human and mouse vesicular acetylcholine transporters and their messenger RNAs. Expression of the human vesicular acetylcholine transporter was restricted to mouse cholinergic somatomotor neurons in the spinal cord and brainstem, but absent from other central and peripheral cholinergic neurons. The mouse appears to be an appropriate model for the study of the genetic regulation of the cholinergic gene locus, and the physiology and neurochemistry of the mammalian cholinergic nervous system, although differences exist in the distribution of cortical cholinergic neurons between the mouse and other mammals. The somatomotor neuron-specific expression pattern of the transgenic human vesicular acetylcholine transporter suggests a mosaic model for cholinergic gene locus regulation in separate subdivisions of the mammalian cholinergic nervous system.

Role of mediatophore in connection with proteins of the active zone in synaptic transmission

Mediatophore is a protein purified from Torpedo electric organ synaptosomes, which translocates acetylcholine (ACh) upon calcium action after reconstitution in artificial membranes. After expression in transfected cells, it endows these cells with a calcium-dependent release mechanism displaying clear quantal properties. The role of mediatophore in synaptic transmission is discussed in relation to the ultrastructural organization of the active zone and the cytosolic high calcium microdomains that transiently appear after presynaptic membrane depolarization.

Modification of cysteines reveals linkage to acetylcholine and vesamicol binding sites in the vesicular acetylcholine transporter of Torpedo californica

Properties of cysteinyl residues in the vesicular acetylcholine transporter (VAChT) of synaptic vesicles isolated from Torpedo californica were probed. Cysteine-specific reagents of different size and polarity were used and the effects on [3H]vesamicol binding determined. The vesamicol dissociation constant increased 1,000-fold after reaction with p-chloromercuriphenylsulfonate or phenylmercury acetate, but only severalfold after reaction with relatively small methylmercury chloride or methylmethanethiosulfonate (MMTS). Methylmercury chloride, but not MMTS, protected binding from phenylmercury acetate. Thus, two classes of cysteines react to affect vesamicol binding. Class 1 reacts with only organomercurials, and class 2 reacts with both organomercurials and MMTS. Quantitative analysis of the competition between p-chloromercuriphenylsulfonate and VAChT ligands was possible after defining second-order reaction conditions. The results indicate that each cysteinyl class probably contains a single residue. Acetylcholine protects cysteine 1, but apparently does not protect cysteine 2. Vesamicol, which binds to a different site than acetylcholine does, apparently protects both cysteines, suggesting that it induces a conformational change. The relatively large reagent glutathione removes a substituent from cysteine 1, but not cysteine 2, suggesting that cysteine 2 is deeper in the transporter than cysteine 1 is. The complete sequence of T. californica VAChT is given, and possible identities of cysteines 1 and 2 are discussed.

Neurotransmitter release at rapid synapses

The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca(2+) and they are critically involved in determining the local Ca(2+) microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca(2+) channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15-25 large quanta, or 150-200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca(2+)-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.

Dendritic and axonal targeting of the vesicular acetylcholine transporter to membranous cytoplasmic organelles in laterodorsal and pedunculopontine tegmental nuclei

Autoregulation of cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei has been implicated in many functions, most importantly in drug reinforcement and in the pathophysiology of schizophrenia. This autoregulation is attributed to the release of acetylcholine, but neither the storage or release sites are known. To determine these sites, we used electron microscopy for the immunocytochemical localization of antipeptide antiserum raised against the vesicular acetylcholine transporter (VAchT) that is responsible for the uptake of acetylcholine into storage vesicles. The cellular and subcellular distribution of VAchT was remarkably similar in the two regions by by using each of two methods, immunogold and immunoperoxidase. In both PPT and LDT nuclei, VAchT labeling was seen mainly on membranous organelles including the trans-Golgi network in many somata. VAchT-immunoreactive tubulovesicles resembling saccules of smooth endoplasmic reticulum were often seen near the plasma membrane in dendrites. The VAchT-containing dendrites comprised almost 50% of the labeled profiles (1027/2129) in PPT and LDT nuclei. The remaining VAchT-immunoreactive profiles were primarily small unmyelinated axons and axon terminals. In axon terminals, VAchT was densely localized to membranes of small synaptic vesicles. The VAchT-immunoreactive axon terminals formed either symmetric or asymmetric synapses. The postsynaptic targets of these axon terminals included dendrites that were with (36/110) or without (74/110) VAchT immunoreactivity. Our results suggest that dendrites, as well as axon terminals, have the potential for storage and release of acetylcholine in the LDT and PPT nuclei. The released acetylcholine is likely to play a major role in autoregulation of mesopontine cholinergic neurons.

The cholinergic neuronal phenotype in Alzheimer's disease

The synthesis, storage and release of acetylcholine (ACh) requires the expression of several specialized proteins, including choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT). The VAChT gene is located within the first intron of the ChAT gene. This unique genomic organization permits coordinated activation of expression of the two genes by extracellular factors. Much less is known about factors that reduce the expression of the cholinergic phenotype. A cholinergic deficit is one of the primary features of Alzheimer's disease (AD), and AD brains are characterized by amyloid deposits composed primarily of A beta peptides. Although A beta peptides are neurotoxic, part of the cholinergic deficit in AD could be attributed to the suppression of cholinergic markers in the absence of cell death. Indeed, we and others demonstrated that synthetic A beta peptides, at submicromolar concentrations that cause no cytotoxicity, reduce the expression of cholinergic markers in neuronal cells. Another feature of AD is abnormal phospholipid turnover, which might be related to the progressive accumulation of apolipoprotein E (apoE) within amyloid plaques, leading perhaps to the reduction of apoE content in the CSF of AD patients. ApoE is a component of very low density lipoproteins (VLDL). As a first step in investigating a potential neuroprotective function of apoE, we determined the effects of VLDL on ACh content in neuronal cells. We found that VLDL increases ACh levels, and that it can partially offset the anticholinergic actions of A beta peptides.

Brief exposure to neurotrophins produces a calcium-dependent increase in choline acetyltransferase activity in cultured rat septal neurons

We demonstrate that brief (30-min) exposure of cultured embryonic rat septal neurons to neurotrophins (NTs) increases choline acetyltransferase (ChAT) activity by 20-50% for all tested NTs (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4, each at 100 ng/ml). The increase in ChAT activity was first detected 12 h after NT exposure, persisted at least 48 h, and was not mediated by increased neuronal survival or action-potential activity. Under some conditions, the response to brief NT exposure was as great as that produced by continuous exposure. NT stimulation of ChAT activity was inhibited by inhibitors of p75- and Trk kinase-mediated signaling, by removal of extracellular Ca2+ during the period of NT exposure, and by buffering intracellular Ca2+ with BAPTA. Application of nerve growth factor and brain-derived neurotrophic factor transiently increased [Ca2+] within a subpopulation of neurons. NT stimulation of ChAT activity was not affected significantly by cyclic AMP agonists or antagonists. These findings suggest that brief exposure to NTs can have a long-lasting effect on cholinergic transmission, and that this effect requires Ca2+, but not cyclic AMP.

Targeted neutralization of calcineurin, by expression of an inhibitor peptide under the control of a cholinergic specific promoter in PC12 cells, promotes neurite outgrowth in the presence of NGF

We have characterized a region of the mouse vesicular acetylcholine transporter(VAChT)/choline acetyltransferase (ChAT) gene locus that serves as a cholinergic-specific promoter for the expression of both VAChT and ChAT genes, as well as a reporter gene (LacZ) in vivo. We have used this promoter to direct the expression of an inhibitor peptide, derived from the calcineurin (CalN) autoregulatory domain, to directly neutralize the function of CalN to define the role of this Ca2+/Calmodulin regulated phosphatase in neurite outgrowth. Targeted inhibition of CalN promotes neurite outgrowth in PC12 cells in the presence of NGF, as early as 24 h after transfection. Inhibition of CalN-mediated enhancement of neurite outgrowth in PC12 cells reaches a maximum effect within the first 4 to 6 days after transfection, and does not cause adverse effects when highly expressed for up to 12 days. Cyclosporin A, a nontargeted CalN inhibitor, increases the number of neurites in mock transfected cells by 1.5 fold, while in transfected PC12 cells, the expression of the CalN inhibitor peptide increases the neurite number by 1.8 fold. These data demonstrate that CalN is an important regulator of the neurotrophic response in cholinergic cells and may prove valuable in developing treatment strategies to promote recovery from neurological injury.

Reconstitution of mediatophore-supported quantal acetylcholine release

Synaptic transmission of a nerve impulse is an extremely rapid event relying on transfer of brief chemical impulses from one cell to another. This transmission is dependent upon Ca2+ and known to be quantal, which led to the widely accepted vesicular hypothesis of neurotransmitter release. However, at least in the case of rapid synaptic transmission the hypothesis has been found difficult to reconcile with a number of observations. In this article, we shall review data from experiments dealing with reconstitution of quantal and Ca2+-dependent acetylcholine release in: i) proteoliposomes, ii) Xenopus oocytes, and iii) release-deficient cell lines. In these three experimental models, release is dependent on the expression of the mediatophore, a protein isolated from the plasma membrane of cholinergic nerve terminals of the Torpedo electric organ. We shall discuss the role of mediatophore in quantal acetylcholine release, its possible involvement in morphological changes affecting presynaptic membrane during the release, and its interactions with others proteins of the cholinergic nerve terminal.

Identification and characterization of the high-affinity choline transporter

In cholinergic neurons, high-affinity choline uptake in presynaptic terminals is the rate-limiting step in acetylcholine synthesis. Using information provided by the Caenorhabditis elegans Genome Project, we cloned a cDNA encoding the high-affinity choline transporter from C. elegans (cho-1). We subsequently used this clone to isolate the corresponding cDNA from rat (CHT1). CHT1 is not homologous to neurotransmitter transporters, but is homologous to members of the Na+-dependent glucose transporter family. Expression of CHT1 mRNA is restricted to cholinergic neurons. The characteristics of CHT1-mediated choline uptake essentially match those of high-affinity choline uptake in rat brain synaptosomes.

Tyrosine hydroxylase and vesicular acetylcholine transporter are coexpressed in a high proportion of intramural neurons of the human neonatal and child urinary bladder

Autonomic ganglia of the human pelvic plexus contain sympathetic and parasympathetic neurons which innervate the internal reproductive organs and the lower urinary tract while the urinary bladder also receives innervation from small intramural ganglia embedded in the detrusor muscle. Previous studies have used the immunocytochemical demonstration of tyrosine hydroxylase (TH), either alone or in combination with dopamine beta-hydroxylase, to identify noradrenergic neurons in these ganglia. However until recently a reliable marker for cholinergic neurons in the human autonomic nervous system was not available since antibodies to choline acetyltransferase do not react in this tissue. The present immunohistochemical study has used an antibody to human vesicular acetylcholine transporter (VAChT) to identify cholinergic neurons in the pelvic plexus and intramural bladder ganglia in a series of specimens from human male neonates and children. Immunostaining for TH was also carried out on the same sections and the results showed that while the vast majority of pelvic ganglion neurons were either cholinergic or noradrenergic (as seen by the presence of VAChT or TH respectively), approximately 50% of the neurons in the intramural ganglia were labeled with both immunomarkers. The presence of TH in cholinergic neurons may be due to the immaturity of the tissues examined since previous data on intramural bladder ganglia in the adult have shown that a much smaller proportion of the neurons contain TH than was observed in the present study. It is concluded that the presence of TH alone cannot be regarded as a specific marker for noradrenergic neurons in the genitourinary system of the human neonate and child.

Choline acetyltransferase: the structure, distribution and pathologic changes in the central nervous system

Choline acetyltransferase (ChAT), the enzyme responsible for the biosynthesis of acetylcholine, is presently the most specific indicator for monitoring the functional state of cholinergic neurones in the central and peripheral nervous systems. ChAT is a single-strand globular protein. The enzyme is synthesized in the perikaryon of cholinergic neurones and transported to the nerve terminals probably by both slow and rapid axoplasmic flows. ChAT exists in at least two forms in cholinergic nerve terminals: (i) soluble; and (ii) non-ionically membrane-bound forms. Multiple mRNA species of ChAT (R-, N-and M-types) are transcribed from different promoter regions and produced by different splicing in the mouse, rat, and human. All transcripts encode the same ChAT protein in rodents, while in human M-type mRNA has the capability to generate both large and small forms of ChAT proteins and R-and N-types ChAT mRNA generate a small form, which corresponds to the rodent ChAT. The genomic structure of ChAT is unique compared with other enzymes for neurotransmitters. The first intron of the ChAT gene encompasses the open reading frame encoding another protein, vesicular acetylcholine transporter (VAChT), which is responsible for the transportation of acetylcholine from the cytoplasm into the synaptic vesicles. The expressions of ChAT and VAChT appear to be coordinately regulated by multiple regulatory elements in cholinergic neurones. Immunohistochemical and in situ hybridization studies have revealed the localization of cholinergic neurones in the central nervous system: the medial septal nucleus, the nucleus of the diagonal band of Broca, the basal nucleus of Meynert, the caudate nucleus, the putamen, the nucleus accumbens, the pedunculopontine tegmental nucleus, the laterodorsal tegmental nucleus, the medial habenular nucleus, the parabigeminal nucleus, some cranial nerve nuclei, and the anterior horn of the spinal cord. Focally distributed cholinergic neurones project fibers to many areas in the central nervous system and construct a complicated cholinergic network, playing an important role in neuropsychic activities, such as learning, memory, arousal, sleep and movement. Central cholinergic neurones are involved in several neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis, in which disturbance of the central cholinergic system does not appear to be closely related to the etiology, but rather to the development of clinical symptoms. In addition, abnormalities of ChAT in the brain have been recently demonstrated in schizophrenia and sudden infant death syndrome.

Protein kinase A regulates cholinergic gene expression in PC12 cells: REST4 silences the silencing activity of neuron-restrictive silencer factor/REST

The role of protein kinase A in regulating transcription of the cholinergic gene locus, which contains both the vesicular acetylcholine transporter gene and the choline acetyltransferase gene, was investigated in PC12 cells and a protein kinase A-deficient PC12 mutant, A126.1B2, in which transcription of the gene is reduced. The site of action of protein kinase A was localized to a neuron-restrictive silencer element/repressor element 1 (NRSE/RE-1) sequence within the cholinergic gene. Neuron-restrictive silencer factor (NRSF)/RE-1-silencing transcription factor (REST), the transcription factor which binds to NRSE/RE-1, was expressed at similar levels in both PC12 and A126.1B2 cells. Although nuclear extracts containing NRSF/REST from A126.1B2 exhibited binding to NRSE/RE-1, nuclear extracts from PC12 cells did not. The NRSF/REST isoform REST4 was expressed in PC12 cells but not in A126.1B2. REST4 inhibited binding of NRSF/REST to NRSE/RE-1 as determined by gel mobility shift assays. Coimmunoprecipitation was used to demonstrate interaction between NRSF/REST and REST4. Expression of recombinant REST4 in A126.1B2 was sufficient to transcriptionally activate the cholinergic gene locus. Thus, in PC12 cells, protein kinase A promotes the production of REST4, which inhibits repression of the cholinergic gene locus by NRSF/REST.

N-(3-Iodophenyl)trozamicol (IPHT) and related inhibitors of vesicular acetylcholine transport: synthesis and preliminary biological characterization

Four isomeric N-(halophenyl)trozamicol analogues (6a-d) were synthesized and evaluated as potential vesicular acetylcholine transporter (VAChT) ligands. Of the four compounds, N-(3-bromophenyl) trozamicol (6b) and N-(3-iodophenyl)trozamicol (6d) displayed the highest affinity for the VAChT in vitro, whereas the para-substituted compound 6c showed the lowest affinity for this transporter. Tissue distribution studies of N-(3-[125I]iodophenyl)trozamicol ([125I]6d, [125I)IPHT) suggest that the central distribution of the latter is consistent with cholinergic innervation. However, only moderate target-to-background ratios were obtained, suggesting little improvement over the N-(halobenzyl)trozamicols described previously.

Hydroxylated decahydroquinolines as ligands for the vesicular acetylcholine transporter: synthesis and biological evaluation

Analogues of the potent anticholinergic 2-(4-phenylpiperidino)cyclohexanol (vesamicol, 1) in which the cyclohexyl fragment was replaced with an N-acyl or N-alkyl trans-decahydroquinolyl moiety were synthesized and evaluated as potential ligands for the vesicular acetylcholine transporter (VAChT). The binding of compounds, such as 18, 20, and 21, was both stereospecific and of comparable magnitude to that of the closely related vesamicol analogue 2,3-trans-4a, 8a-trans-3-hydroxy-2-(4-phenylpiperidino)-1,2,3,4,5,6,7, 8-decahydronaphthalene (6) which displays subnanomolar affinity for this transporter. However, these compounds also demonstrated high affinities for sigma(1) and sigma(2) receptors and thus failed to show significantly improved selectivity over previously reported vesamicol analogues.

Initiation of translation in prokaryotes and eukaryotes

The mechanisms whereby ribosomes engage a messenger RNA and select the start site for translation differ between prokaryotes and eukaryotes. Initiation sites in polycistronic prokaryotic mRNAs are usually selected via base pairing with ribosomal RNA. That straightforward mechanism is made complicated and interesting by cis- and trans-acting elements employed to regulate translation. Initiation sites in eukaryotic mRNAs are reached via a scanning mechanism which predicts that translation should start at the AUG codon nearest the 5' end of the mRNA. Interest has focused on mechanisms that occasionally allow escape from this first-AUG rule. With natural mRNAs, three escape mechanisms - context-dependent leaky scanning, reinitiation, and possibly direct internal initiation - allow access to AUG codons which, although not first, are still close to the 5' end of the mRNA. This constraint on the initiation step of translation in eukaryotes dictates the location of transcriptional promoters and may have contributed to the evolution of splicing.The binding of Met-tRNA to ribosomes is mediated by a GTP-binding protein in both prokaryotes and eukaryotes, but the more complex structure of the eukaryotic factor (eIF-2) and its association with other proteins underlie some aspects of initiation unique to eukaryotes. Modulation of GTP hydrolysis by eIF-2 is important during the scanning phase of initiation, while modulating the release of GDP from eIF-2 is a key mechanism for regulating translation in eukaryotes. Our understanding of how some other protein factors participate in the initiation phase of translation is in flux. Genetic tests suggest that some proteins conventionally counted as eukaryotic initiation factors may not be required for translation, while other tests have uncovered interesting new candidates. Some popular ideas about the initiation pathway are predicated on static interactions between isolated factors and mRNA. The need for functional testing of these complexes is discussed. Interspersed with these theoretical topics are some practical points concerning the interpretation of cDNA sequences and the use of in vitro translation systems. Some human diseases resulting from defects in the initiation step of translation are also discussed.

Novel, testis-specific mRNA transcripts encoding N-terminally truncated choline acetyltransferase

Previous studies reported the presence of choline acetyltransferase (ChAT) mRNA and protein in the mammalian testis. We have now found that none of the ChAT mRNAs produced in the testis is capable of encoding a full-length ChAT protein. Two ChAT cDNAs were isolated from an adult rat testis cDNA library encoding N-terminally truncated ChAT proteins of 450 and 414 amino acids (aa), respectively, the former containing a novel N-terminal extension of 69 residues. Rapid Amplification of cDNA Ends (RACE) analysis revealed a complex pattern of 5' untranslated mRNA termini generated from the ChAT gene locus in the testis, all representing truncated versions of the ChAT enzyme. Two of these proteins were produced in transfected fibroblasts and found to lack ChAT activity. Neither did they show binding to the ChAT substrates, acetyl CoA and choline, in a competition assay. These results indicate that mammalian testis lacks a bona fide ChAT enzyme but expresses truncated ChAT proteins with a possible unique function to the testis.

A multigene locus containing the Manx and bobcat genes is required for development of chordate features in the ascidian tadpole larva

The Manx gene is required for the development of the tail and other chordate features in the ascidian tadpole larva. To determine the structure of the Manx gene, we isolated and sequenced genomic clones from the tailed ascidian Molgula oculata. The Manx gene contains 9 exons and encodes both major and minor Manx mRNAs, which differ in the length of their 5′ untranslated regions. The coding region of the single-copy bobcat gene, which encodes a DEAD-box RNA helicase, is embedded within the first Manx intron. The organization of the bobcat and Manx transcription units was determined by comparing genomic and cDNA clones. The Manx-bobcat gene locus has an unusual organization in which a non-coding first exon is alternatively spliced at the 5′ end of two different mRNAs. The bobcat and Manx genes are expressed coordinately during oogenesis and embryogenesis, but not during spermatogenesis, in which bobcat mRNA accumulates independently of Manx mRNA. Similar to Manx, zygotic bobcat transcripts accumulate in the embryonic primordia responsible for generating chordate features, including the dorsal neural tube and notochord, are downregulated during embryogenesis in the tailless species Molgula occulta and are upregulated in M. occulta X M. oculata hybrids, which restore these chordate features. Antisense experiments indicate that zygotic bobcat expression is required for development of the same suite of chordate features as Manx. The results show that the Manx-bobcat gene complex has a role in the development of chordate features in ascidian tadpole larvae.

The cholinergic 'pitfall': acetylcholine, a universal cell molecule in biological systems, including humans

1. Acetylcholine (ACh) represents one of the most exemplary neurotransmitters. In addition to its presence in neuronal tissue, there is increasing experimental evidence that ACh is widely expressed in pro- and eukaryotic non-neuronal cells. Thus, ACh has been detected in bacteria, algae, protozoa, tubellariae and primitive plants, suggesting an extremely early appearance of ACh in the evolutionary process. 2. In humans, ACh and/or the synthesizing enzyme, choline acetyltransferase, has been demonstrated in epithelial cells (airways, alimentary tract, urogenital tract, epidermis), mesothelial (pleura, pericardium) and endothelial and muscle cells. In addition, immune cells express the non-neuronal cholinergic system (i.e. the synthesis of ACh can be detected in human leucocytes (granulocytes, lymphocytes and macrophages)), as well as in rat microglia in vitro. 3. The widespread expression of non-neuronal ACh is accompanied by the ubiquitous expression of cholinesterase activity, which prevents ACh from acting as a classical hormone. 4. Non-neuronal ACh mediates its cellular actions in an auto- and paracrine manner via the activation of the widely expressed nicotinic and muscarinic acetylcholine receptors, which can interfere with virtually all cellular signalling pathways (ion channels and key enzymes). 5. Non-neuronal ACh appears to be involved in the regulation of basic cell functions, such as mitosis, cell differentiation, organization of the cytoskeleton, cell-cell contact, secretion and absorption. Non-neuronal ACh also plays a role in the regulation of immune functions. All these qualities together may mediate the so-called 'trophic property' of ACh. 6. Future experiments should be designed to analyse the cellular effects of ACh in greater detail. The involvement of the non-neuronal cholinergic system in the pathogenesis of chronic inflammatory diseases should be investigated to open up new therapeutic strategies.

Proteolytic processing, axonal transport and differential distribution of chromogranins A and B, and secretogranin II (secretoneurin) in rat sciatic nerve and spinal cord

The chromogranin family comprises chromogranin A and B, and secretogranin II. The present study has focused on the axonal transport of chromogranins/secretogranin II and their detailed distribution in peripheral nerves and the spinal cord. With radioimmunoassay (RIA) and column chromatography, we first studied the processing of chromogranin B and secretogranin II during axonal transport. No larger precursors of these peptides were detected in the sciatic nerves, indicating that they are already processed to a high degree early during axonal transport. We also analysed nerve segments above and below a crush, using RIA, in order to compare these accumulation data with those obtained by the cytofluorimetric-scanning (CFS) technique. For the latter technique, the amounts of accumulation distal to the crush (presumably representing recycling and retrogradely transported peptides) were 30-40% of the amounts in the proximal accumulation for chromogranin A and secretoneurin, in contrast to chromogranin B, which showed 15% recycling. With the RIA, the corresponding values for secretoneurin and PE-11 (antibody against chromogranin B) were 42% and 14%, respectively. Therefore, the data obtained by CFS were in excellent agreement with those obtained by RIA. In crushed sciatic nerves, chromogranin A was present in large axons as well as in small- and medium-sized axons. Chromogranin B was mainly restricted to large axons, while secretoneurin was localized to bundles of small axons. This differential distribution was also found in the spinal roots and in the peripheral terminals. Chromogranin A was present in both ventral and dorsal roots, and chromogranin B was detected in ventral roots and in large sensory axons in the dorsal roots. Secretoneurin was dominant in the dorsal root. Double-labelling studies with antibodies against choline acetyltransferase/vesicular acetylcholine transporter, or against tyrosine hydroxylase, confirmed that chromogranin A was distributed in cholinergic, sensory, as well as adrenergic neurons. Chromogranin B was mainly present in cholinergic motor neurons and large sensory neurons, and secretoneurin was restricted to adrenergic and sensory neurons. The present study demonstrates that chromogranins A and B, and secretoneurin are transported with fast axonal transport in the peripheral nerves, with different amounts of recycling, and that they are differentially distributed in different types of neurons in the peripheral nervous system and the spinal cord, suggesting that each of them may play a special role in subsets of neurons.

Mutational analysis of aspartate residues in the transmembrane regions and cytoplasmic loops of rat vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) is responsible for the transport of the neurotransmitter acetylcholine (ACh) into synaptic vesicles using an electrochemical gradient to drive transport. Rat VAChT has a number of aspartate residues within its predicted transmembrane domains (TM) and cytoplasmic loops, which may play important structural or functional roles in acetylcholine transport. In order to identify functional charged residues, site-directed mutagenesis of rVAChT was undertaken. No effect on ACh transport was observed when any of the five aspartate residues in the cytoplasmic loop were converted to asparagine. Similarly, changing Asp-46 (D46N) in TM1 or Asp-255 (D255N) in TM6 had no effect on ACh transport or vesamicol binding. However, replacement of Asp-398 in TM10 with Asn completely eliminated both ACh transport and vesamicol binding. The conservative mutant D398E retained transport activity, but not vesamicol binding, suggesting this residue is critical for transport. Mutation of Asp-193 in TM4 did not affect ACh transport activity; however, vesamicol binding was dramatically reduced. With mutant D425N of TM11 transport activity for ACh was completely blocked, without an effect on vesamicol binding. Activity was not restored in the conservative mutant D425E, suggesting the side chain as well as the negative charge of Asp-425 is important for substrate binding. These mutants, as well as mutant D193N, clearly dissociated ACh binding and transport from vesamicol binding. These data suggest that Asp-398 in TM10 and Asp-425 in TM11 are important for ACh binding and transport, while Asp-193 and Asp-398 in TM4 and TM10, respectively, are involved in vesamicol binding.

Identification and transgenic analysis of a murine promoter that targets cholinergic neuron expression

Choline acetyltransferase (ChAT) is a specific phenotypic marker of cholinergic neurons. Previous reports showed that different upstream regions of the ChAT gene are necessary for cell type-specific expression of reporter genes in cholinergic cell lines. The identity of the mouse ChAT promoter region controlling the establishment, maintenance, and plasticity of the cholinergic phenotype in vivo is not known. We characterized a promoter region of the mouse ChAT gene in transgenic mice, using beta-galactosidase (LacZ) as a reporter gene. A 3,402-bp segment from the 5'-untranslated region of the mouse ChAT gene (from -3,356 to +46, +1 being the translation initiation site) was sufficient to direct the expression of LacZ to selected neurons of the nervous system; however, it did not provide complete cholinergic specificity. A larger fragment (6,417 bp, from -6,371 to +46) of this region contains the requisite regulatory elements that restrict expression of the LacZ reporter gene only in cholinergic neurons of transgenic mice. This 6.4-kb DNA fragment encompasses 633 bp of the 5'-flanking region of the mouse vesicular acetylcholine transporter (VAChT), the entire open reading frame of the VAChT gene, contained within the first intron of the ChAT gene, and sequences upstream of the start coding sequences of the ChAT gene. This promoter will allow targeting of specific gene products to cholinergic neurons to evaluate the mechanisms of diseases characterized by dysfunction of cholinergic neurons and will be valuable in design strategies to correct those disorders.

C. elegans neuroscience: genetics to genome

From their earliest experiments, researchers using Caenorhabditis elegans have been interested in the role of genes in the development and function of the nervous system. As the C. elegans Genome Project completes the genomic sequence, we review the accomplishments of these researchers and the impact that the Genome Project has bad on their research. We also speculate on future directions in this research that are enabled by the efforts of the Genome Project.

Influence of retinoic acid and of cyclic AMP on the expression of choline acetyltransferase and of vesicular acetylcholine transporter in NG108-15 cells

Treatment of the cholinergic cell line NG108-15 with retinoic acid or cAMP results in an increase of choline acetyltransferase activity (ChAT) whereas none of these agents influences the amount of the vesicular acetylcholine transporter (VAChT) as judged from vesamicol binding and immunoblot studies. We suggest that immaturity of posttranslational events controlling the expression of VAChT protein is responsible for the apparent absence of coregulation of ChAT and VAChT protein expression.

Target-dependent development of the vesicular acetylcholine transporter in rodent sweat gland innervation

Descriptive studies have delineated a developmental change in neurotransmitter phenotype from noradrenergic to cholinergic in the sympathetic innervation of sweat glands in rodent footpads. Transplantation and culture experiments provide evidence that interactions with the target tissue induce this change. Recent studies with an antiserum that recognizes the vesicular acetylcholine transporter (VAChT) suggest, however, that the development of cholinergic function in sympathetic neurons, including those that innervate sweat glands, occurs prior to and does not require target contact. To clarify these apparently contradictory findings, we directly compared the appearance of VAChT immunoreactivity in the sympathetic neurons that innervate sweat glands with the time that axons contact this target. We find that VAChT immunoreactivity is not detectable in either the axons or cell bodies of sweat gland neurons until several days after target innervation. Before and during VAChT acquisition, the developing sweat gland innervation contains vesicular stores of catecholamines. An analysis of mutant mice that lack sweat glands was undertaken to determine whether VAChT expression requires target interactions and revealed that VAChT does not appear in the absence of glands. These findings, together with previous studies, confirm the target dependence of cholinergic function in the sympathetic neurons that innervate sweat glands.

Cholinergic neurons and terminal fields revealed by immunohistochemistry for the vesicular acetylcholine transporter. I. Central nervous system

Antibodies directed against the C-terminus of the rat vesicular acetylcholine transporter mark expression of this specifically cholinergic protein in perinuclear regions of the soma and on secretory vesicles concentrated within cholinergic nerve terminals. In the central nervous system, the vesicular acetylcholine transporter terminal fields of the major putative cholinergic pathways in cortex, hippocampus, thalamus, amygdala, olfactory cortex and interpeduncular nucleus were examined and characterized. The existence of an intrinsic cholinergic innervation of cerebral cortex was confirmed by both in situ hybridization histochemistry and immunohistochemistry for the rat vesicular acetylcholine transporter and choline acetyltransferase. Cholinergic interneurons of the olfactory tubercle and Islands of Calleja, and the major intrinsic cholinergic innervation of striatum were fully characterized at the light microscopic level with vesicular acetylcholine transporter immunohistochemistry. Cholinergic staining was much more extensive for the vesicular acetylcholine transporter than for choline acetyltransferase in all these regions, due to visualization of cholinergic nerve terminals not easily seen with immunohistochemistry for choline acetyltransferase in paraffin-embedded sections. Cholinergic innervation of the median eminence of the hypothalamus, previously observed with vesicular acetylcholine transporter immunohistochemistry, was confirmed by the presence of vesicular acetylcholine transporter immunoreactivity in extracts of median eminence by western blotting. Cholinergic projections to cerebellum, pineal gland, and to the substantia nigra were documented by vesicular acetylcholine transporter-positive punctate staining in these structures. Additional novel localizations of putative cholinergic terminals to the subependymal zone surrounding the lateral ventricles, and putative cholinergic cell bodies in the sensory mesencephalic trigeminal nucleus, a primary sensory afferent ganglion located in the brainstem, are documented here. The cholinergic phenotype of neurons of the sensory mesencephalic trigeminal nucleus was confirmed by choline acetyltransferase immunohistochemistry. A feature of cholinergic neurons of the central nervous system revealed clearly with vesicular acetylcholine transporter immunohistochemistry in paraffin-embedded sections is the termination of cholinergic neurons on cholinergic cell bodies. These are most prominent on motor neurons of the spinal cord, less prominent but present in some brainstem motor nuclei, and apparently absent from projection neurons of the telencephalon and brainstem, as well as from the preganglionic vesicular acetylcholine transporter-positive sympathetic and parasympathetic neurons visualized in the intermediolateral and intermediomedial columns of the spinal cord. In addition to the large puncta decorating motor neuronal perikarya and dendrites in the ventral horn, vesicular acetylcholine transporter-positive terminal fields are distributed in lamina X surrounding the central canal, where additional small vesicular acetylcholine transporter-positive cell bodies are located, and in the superficial layers of the dorsal horn. Components of the central cholinergic nervous system whose existence has been controversial have been confirmed, and the existence of new components documented, with immunohistochemistry for the vesicular acetylcholine transporter. Quantitative visualization of terminal fields of known cholinergic systems by staining for vesicular acetylcholine transporter will expand the possibilities for documenting changes in synaptic patency accompanying physiological and pathophysiological changes in these systems.

Distribution of chromogranins A and B and secretogranin II (secretoneurin) in rat pelvic neurons and vas deferens

The family of chromogranins/secretogranin peptides comprises three major subtypes: chromogranin A, chromogranin B and secretogranin II. We have characterized these proteins in rat vas deferens and pelvic ganglia by using two approaches. Firstly, extracts of rat vas deferens were subjected to molecular sieve chromatography followed by radioimmunoassay. The results indicate that, in the peripheral nerves of this organ, chromogranin B and secretogranin II are processed to small peptides, i.e. PE-11 and secretoneuron, respectively. Secondly, we investigated the localization of each of these peptides in the rat pelvic ganglia and vas deferens. Comparisons with the distribution of tyrosine hydroxylase, choline acetyltransferase, vesicular acetylcholine transporter and SV2 were carried out in double labelling studies. All tyrosine hydroxylase-positive neurons contained neuropeptide Y, but many neuropeptide Y-containing neurons were negative for tyrosine hydroxylase. In the pelvic ganglia, chromogranin A was widely localized in the neuropeptide-positive neurons and 65% of chromogranin A-containing neurons were positive for tyrosine hydroxylase, suggesting their adrenergic nature. However, in nerve terminals of the vas deferens, chromogranin A was present at very low, or undetectable, levels. The chromogranin B-derived peptide PE-11, on the other hand, was absent from the large-sized, tyrosine hydroxylase-positive neurons, but present in some small-sized neurons that were choline acetyltransferase/vesicular acetylcholine transporter-positive and tyrosine hydroxylase-negative. In the vas deferens, PE-11 was present with intense immunoreactivity in nerve terminals of the lamina propria beneath the epithelium, but it was very sparse in the muscular layer and co-localized with vesicular acetylcholine transporter-like immunoreactivity, suggesting a cholinergic nature. The secretogranin II-derived peptide secretoneurin was distributed with strong immunoreactivity in the somata of pelvic ganglion neurons, 72% of which also contained tyrosine hydroxylase, as well as in nerve terminals in the muscular layer and the lamina propria of the vas deferens. Most, if not all, secretoneurin-positive terminals in the pelvic ganglia and the vas deferens were positive for choline acetyltransferase/vesicular acetylcholine transporter-like immunoreactivity. Retrograde tracing with FluoroGold demonstrated that the majority of FluoroGold-labelled neurons in the pelvic ganglia were positive for either chromogranin A or secretoneurin. The present study indicates that chromogranins A and B and secretogranin II are proteolytically processed to a high degree in the nerves of the rat vas deferens. Furthermore, they are heterogeneously localized in subsets of neurons of the pelvic ganglia and in different sets of nerve terminals in the vas deferens, suggesting that each of these peptides may play distinct roles in neurons of the autonomic nervous system to the vas deferens.

Fast axonal transport of the vesicular acetylcholine transporter (VAChT) in cholinergic neurons in the rat sciatic nerve

The sciatic nerve, as a part of the peripheral nervous system (PNS), has been used to study axonal transport for decades. It contains motor, sensory as well as autonomic axons. The present study has concentrated on the axonal transport of the synaptic vesicle acetylcholine transporter (VAChT), using the "stop-flow/nerve crush" method. After blocking fast axonal transport by means of a crush, distinct accumulations of various synaptic vesicle proteins, including VAChT, and peptides developed during the first hour after crush-operation and marked increases were observed up to 8 h post-operative. Semiquantitative analysis, using cytofluorimetric scanning (CFS) of immuno-incubated sections, revealed a rapid rate of accumulation proximal to the crush, and that the ratio between distal accumulations (organelles in retrograde transport) and proximal accumulations (organelles in anterograde transport) was about 40%. Most synaptic vesicle proteins were colocalized in the axons proximal to the crush. VAChT-immunoreactive axons were also immunoreactive for choline acetyltransferase (ChAT). Autonomic axons with VAChT also contained VIP-LI. The results demonstrate (1) that VAChT, as well as other synaptic vesicle proteins, is transported with fast axonal transport in motor axons as well as in autonomic post-ganglionic neurons in this nerve, (2) VAChT colocalized in motor axons with SV2 as well as with synaptophysin, indicating storage in the same axonal particle, (3) in the autonomic postganglionic sympathetic cholinergic fibres, VAChT colocalized with VIP, but VIP-LI was present in rather large granular structures while VAChT-LI was present mostly as small granular elements, (4) in motor as well as in autonomic axons ChAT-LI was present in VAChT-positive axons, and (5) the ratio of recycling (retrogradely accumulated) VAChT-IR was about 40%, in contrast to the recycling fraction of synaptophysin that was about 70%.

The cholinergic locus: ChAT and VAChT genes

The gene encoding the vesicular acetylcholine transporter has been localized within the first intron of the gene encoding choline acetyltransferase and is in the same transcriptional orientation. These two genes, whose products are required for the expression of the cholinergic phenotype, could therefore be coregulated. The promoters of both genes have been identified. The mechanisms that account for the regulation of the expression of both genes are now being investigated.

Acetylcholine release. Reconstitution of the elementary quantal mechanism

Choline acetyltransferase and vesicular acetylcholine transporter genes are the products of two adjacent genes defining a cholinergic locus. The release mechanism is expressed independently of this locus in some cell lines. A cholinergic neuron will therefore have to coordinate the expression of release with that of the cholinergic locus. Transfection of a plasmid encoding Torpedo mediatophore in cells that are unable to release this transmitter endows them with a Ca2(+)-dependent and quantal release mechanism. The synchronization of mediatophore activation results from a control of calcium microdomains by the synaptic vesicles. It is therefore dependent on the proteins that dock vesicles close to calcium channels.

Induction of choline acetyltransferase mRNA in human mononuclear leukocytes stimulated by phytohemagglutinin, a T-cell activator

The induction of mRNA for choline acetyltransferase (ChAT), which catalyzes acetylcholine (ACh) synthesis was investigated in human mononuclear leukocytes (MNL) stimulated by phytohemagglutinin (PHA), a T-cell activator, using the reverse transcription-polymerase chain reaction. Stimulation of MNL by PHA induced the expression of ChAT mRNA, and potentiated ACh synthesis. ChAT mRNA induction required more time than the induction of interleukin-2 mRNA. Expression of the gene encoding the vesicular ACh transporter, which mediates ACh transport in cholinergic neurons, was not observed in PHA-stimulated MNL, suggesting that the mechanisms controlling ACh release from T-lymphocytes differ from those in cholinergic neurons. These findings demonstrate that activation of T-lymphocytes up-regulates ACh synthesis in the blood, and suggest that ACh plays an important role as a neuroimmunomodulator besides its role as a neurotransmitter.

Distribution of the vesicular transporter for acetylcholine in the rat central nervous system

In order to develop another selective marker for cholinergic cell bodies and fibres, we have raised a highly specific polyclonal antibody against a peptide derived from the C-terminus of a recently cloned putative vesicular acetylcholine transporter. This antibody recognizes the vesicular acetylcholine transporter protein on western blots of membranes from transfected monkey fibroblast COS cells as well as from various rat brain regions but not from untransfected COS cells or rat liver. In separate mapping studies, the antibody was found to stain cell bodies and fibres in all of the regions of the nervous system known to be cholinergic, including (i) the various nuclei of the basal nuclear complex and their projections to the hippocampus, amygdala, and cerebral cortex, (ii) the caudate-putamen nucleus, accumbens nucleus, olfactory tubercle, and islands of Calleja complex, (iii) the medial habenula, (iv) the mesopontine cholinergic complex and its projections to the thalamus, extrapyramidal motor nuclei, basal forebrain, cingulate cortex, raphe and reticular nuclei, and some cranial nerve nuclei, and (v) the somatic motor and autonomic nuclei of the cranial and spinal nerves. In many of these cholinergic neurons, it is possible to detect immunoreactivity for the vesicular acetylcholine transporter in proximal portions of processes and their branches, as well as in numerous puncta in close association with them. Some of these puncta are large and surround cell bodies and processes of neurons in several regions, including the somatic motor neurons of cranial nerve nuclei in the brainstem and in the ventral horn of the spinal cord. Double immunofluorescence studies indicated that neurons positive for the vesicular acetylcholine transporter also stained for the biosynthetic enzyme of acetylcholine, choline acetyltransferase. We conclude that antibody against the C-terminus of the putative vesicular acetylcholine transporter provides another marker for cholinergic neurons that, unlike in situ hybridization procedures, labels terminals as well as cell bodies. Therefore this antibody has the potential to reveal changes in number and morphology of cholinergic cell bodies and their terminal varicosities that occur in both physiologic and pathologic conditions.

Acetylcholine release and the cholinergic genomic locus

Choline acetyltransferase and vesicular acetylcholine-transporter genes are adjacent and coregulated. They define a cholinergic locus that can be turned on under the control of several factors, including the neurotrophins and the cytokines. Hirschprung's disease, or congenital megacolon, is characterized by agenesis of intramural cholinergic ganglia in the colorectal region. It results from mutations of the RET (GDNF-activated) and the endothelin-receptor genes, causing a disregulation in the cholinergic locus. Using cultured cells, it was shown that the cholinergic locus and the proteins involved in acetylcholine (ACh) release can be expressed separately ACh release could be demonstrated by means of biochemical and electrophysiological assays even in noncholinergic cells following preloading with the transmitter. Some noncholinergic or even nonneuronal cell types were found to be capable of releasing ACh quanta. In contrast, other cells were incompetent for ACh release. Among them, neuroblastoma N18TG-2 cells were rendered release-competent by transfection with the mediatophore gene. Mediatophore is an ACh-translocating protein that has been purified from plasma membranes of Torpedo nerve terminal; it confers a specificity for ACh to the release process. The mediatophores are activated by Ca2+; but with a slower time course, they can be desensitized by Ca2+. A strictly regulated calcium microdomain controls the synchronized release of ACh quanta at the active zone. In addition to ACh and ATP, synaptic vesicles have an ATP-dependent Ca2+ uptake system; they transiently accumulate Ca2+ after a brief period of stimulation. Those vesicles that are docked close to Ca2+ channels are therefore in the best position to control the profile and dynamics of the Ca2+ microdomains. Thus, vesicles and their whole set of associated proteins (SNAREs and others) are essential for the regulation of the release mechanism in which the mediatophore seems to play a key role.