Coordinated up-regulation of choline acetyltransferase and vesicular acetylcholine transporter gene expression by the retinoic acid receptor alpha, cAMP, and leukemia inhibitory factor/ciliary neurotrophic factor signaling pathways in a murine septal cell line

Post-training intrahippocampal infusion of nicotine prevents spatial memory retention deficits induced by the cyclo-oxygenase-2-specific inhibitor celecoxib in rats

Recently, we demonstrated that intrahippocampal infusion of the cyclo-oxygenase (COX)-2-specific inhibitor celecoxib impaired spatial memory retention in the Morris water maze. In the present work, we investigated the effects of nicotine, infused in the rat dorsal hippocampus several minutes after infusion of celecoxib, on memory retention in the Morris water maze. Rats were trained for 3 days; each day included two blocks, and each block contained four trials. Test trials were conducted 48 h after surgery. As expected, bilateral intrahippocampal infusion of celecoxib (19 microg/side; 0.1 m) increased escape latency and travel distance in rats, indicating significant impairment of spatial memory retention. We also examined the effects of bilateral infusion of nicotine (0.5, 1.0 and 2.0 microg/side) on memory retention. Infusion of 1 microg nicotine significantly decreased escape latency and travel distance but not swimming speed, compared with controls, suggesting memory retention enhancement by nicotine at this concentration. In separate experiments, bilateral infusion of nicotine, infused 5 min after 0.1 m (19 microg/side) celecoxib infusion, was associated with escape latency, travel distance and swimming speed profiles very similar to those in control animals. Brain tissue sections from several of these animals were subjected to immunohistochemical staining analysis with anti-COX-2 antibodies. Quantification analysis by optical density measurements showed that the celecoxib infusion reduced the immunoreactivity of COX-2-containing neurons in the CA1 area of the hippocampus compared with controls, although this reduction was not significant. However, infusion of a combination of celecoxib and nicotine significantly increased this immunoreactivity compared with levels in control and celecoxib-infused groups. These results suggest that nicotine prevented or reversed the adverse effects of celecoxib on spatial memory retention and protected or restored the immunostaining pattern of COX-2 neurons in the rat dorsal hippocampus.

Expression of high affinity choline transporter during mouse development in vivo and its upregulation by NGF and BMP-4 in vitro

An important feature of cholinergic neurons is high-affinity choline transport, which allows them to reuse choline for the synthesis of ACh needed to support cholinergic neurotransmission. The choline transporter, designated CHT, was recently cloned. We applied RT/PCR to monitor the expression of CHT in the developing mouse CNS from embryonic day 14 (E14) to postnatal day 30 (P30). We found that CHT was expressed early in development, predominantly in the regions containing cholinergic neurons. In the spinal cord, CHT mRNA was present at close to adult levels at the earliest time point examined (E14) and showed almost no changes after birth. In the striatum and the septum, CHT mRNA increased steadily during embryonic stages and leveled off after birth. Surprisingly, CHT mRNA expression was also detected in other brain regions, notably in the cerebellum, where it peaked on E19, and then rapidly declined during postnatal development. CHT protein was detected by Western blotting as a band of apparent molecular weight of 70 kDa. The accumulation of this protein during development lagged behind mRNA accumulation in all tissues. We also examined the effects of NGF and BMP-4, the potent inducers of choline acetyltransferase and vesicular acetylcholine transporter genes, on CHT expression. Both factors increased CHT mRNA accumulation in primary septal cultures. The effect of NGF was dependent on the PI3K signaling, as it was abolished by the PI3K inhibitor LY294002. This result indicates that some of the signals regulating other cholinergic-specific genes also control CHT expression.

Working memory deficits in retinoid X receptor gamma-deficient mice

Retinoid signaling has been recently shown to be required for mnemonic functions in rodents. To dissect the behavioral and molecular mechanisms involved in this requirement, we have analyzed the spatial and recognition working memory in mice carrying null mutations of retinoid receptors RARbeta and RXRgamma. Double mutants appeared deficient in spatial working memory as tested in spontaneous alternation in the Y-maze and delayed nonmatch to place (DNMTP) test in the T-maze. These mutant mice did acquire, however, spatial place reference or right/left discrimination tasks in the T-maze set-up, indicating that basic sensorimotor functions, spatial orientation, and motivational factors are unlikely to account for deficits in working memory-sensitive tasks. Double-mutant mice were also deficient in novel object recognition at intermediate, but not short delays. RXRgamma appeared to be the functionally predominant receptor in modulation of the working memory, as RXRgamma, but not RARbeta single null mutant mice exhibited deficits similar to those observed in the double mutants. The mechanism of this modulation is potentially related to functions of RXRgamma in frontal and perirhinal cortex, structures in which we detected RXRgamma expression and which are functionally implicated in working memory processes.

Nutrients as trophic factors in neurons and the central nervous system: role of retinoic acid

In multicellular organisms, death, survival, proliferation, and differentiation of a given cell depend on signals produced by neighboring and/or distant cells, resulting in the coordinated development and function of the various tissues. In the nervous system, control of cell survival and differentiation is achieved through the action of a distinct group of polypeptides collectively known as neurotrophic factors. Recent findings support the view that trophic factors also are involved in the response of the nervous system to acute injury. By contrast, nutrients are not traditionally viewed as potential trophic factors; however, there is increasing evidence that at least some influence neuronal differentiation. During development the brain is responsive to variations in nutrient supply, and this increased sensitivity or vulnerability of the brain to nutrient supply may reappear during neuronal repair, a period during which a rapid membrane resynthesis and reestablishment of synthetic pathways occur. To further evaluate the potential of specific nutrients to act as pharmacologic agents in the repair of injured neurons, the effects of retinoic acid, an active metabolite of vitamin A, and its role as a trophic factor are discussed. This literature review is intended to provide background information regarding the effect of retinoic acid on the cholinergic phenotype and the differentiation of these neurons and to explain how it may promote neuronal repair and survival following injury.

Role of retinoid signalling in the adult brain

Vitamin A (all-trans-retinol) is the parent compound of a family of natural and synthetic compounds, the retinoids. Retinoids regulate gene transcription in numerous cells and tissues by binding to nuclear retinoid receptor proteins, which act as transcription factors. Much of the research conducted on retinoid signalling in the nervous system has focussed on developmental effects in the embryonic or early postnatal brain. Here, we review the increasing body of evidence indicating that retinoid signalling plays an important role in the function of the mature brain. Components of the metabolic pathway for retinoids have been identified in adult brain tissues, suggesting that all-trans-retinoic acid (ATRA) can be synthesized in discrete regions of the brain. The distribution of retinoid receptor proteins in the adult nervous system is different from that seen during development; and suggests that retinoid signalling is likely to have a physiological role in adult cortex, amygdala, hypothalamus, hippocampus, striatum and associated brain regions. A number of neuronal specific genes contain recognition sequences for the retinoid receptor proteins and can be directly regulated by retinoids. Disruption of retinoid signalling pathways in rodent models indicates their involvement in regulating synaptic plasticity and associated learning and memory behaviours. Retinoid signalling pathways have also been implicated in the pathophysiology of Alzheimer's disease, schizophrenia and depression. Overall, the data underscore the likely importance of adequate nutritional Vitamin A status for adult brain function and highlight retinoid signalling pathways as potential novel therapeutic targets for neurological diseases.

Posttraining intrahippocampal infusion of a protein kinase AII inhibitor impairs spatial memory retention in rats

The role of protein kinase AII (PKA II) in spatial memory retention in male rats and its regulation of cholinergic gene expression were explored through the effects of intrahippocampal infusion of H-89, a selective PKA II inhibitor. Alterations in escape latency, travel distance, and swimming speed in a Morris water maze were measured. Animals were trained for 3 days; each day included two blocks, and each block contained four trials. Stereotaxic surgery was employed for the infusions after the last trial on the third day of training, and the animals were tested 48 hr after surgery. Bilateral intrahippocampal infusion of H-89 (2.5 or 5 microM) into the CA1 region generated significant alterations in escape latency and traveled distance but not swimming speed. The response was fairly dose dependent, and the maximal effect was obtained with 5 microM H-89. After behavioral testing, several of the infused animals were transcardially perfused and their brains removed. Brain tissue sections from these rats were subjected to immunohistochemical staining analysis with anticholine acetyltransferase (ChAT) antibodies. These analyses indicated that 5 microM H-89 infusions qualitatively reduced the density of ChAT-containing cholinergic nerve terminals in the dorsal hippocampus. The intrahippocampal infusions with 5 microM H-89 also caused an apparent reduction in the number of ChAT-containing neurons in the medial septum. Our results suggest that PKA II is involved in regulation of cholinergic gene expression and plays an important role in spatial memory retention in rats.

Mechanisms and perspectives on differentiation of autonomic neurons

Neurons share many features in common but are distinguished by expression of phenotypic characteristics that define their specific function, location, or connectivity. One aspect of neuronal fate determination that has been extensively studied is that of neurotransmitter choice. The generation of diversity of neuronal subtypes within the developing nervous system involves integration of extrinsic and intrinsic instructive cues resulting in the expression of a core set of regulatory molecules. This review focuses on mechanisms of growth and transcription factor regulation in the generation of peripheral neural crest-derived neurons. Although the specification and differentiation of noradrenergic neurons are the focus, I have tried to integrate these into a larger picture providing a general roadmap for development of autonomic neurons. There is a core of DNA binding proteins required for the development of sympathetic, parasympathetic, and enteric neurons, including Phox2 and MASH1, whose specificity is regulated by the recruitment of additional transcriptional regulators in a subtype-specific manner. For noradrenergic neurons, the basic helix-loop-helix DNA binding protein HAND2 (dHAND) appears to serve this function. The studies reviewed here support the notion that neurotransmitter identity is closely linked to other aspects of neurogenesis and reveal a molecular mechanism to coordinate expression of pan-neuronal genes with cell type-specific genes.

Clinical variability of CMS-EA (congenital myasthenic syndrome with episodic apnea) due to identical CHAT mutations in two infants

Congenital myasthenic syndromes (CMS) result from mutations in various synapse-associated genes. Mutations in the choline acetyltransferase (CHAT) gene cause a presynaptic CMS associated with episodic apnea (CMS-EA). We present two unrelated Croatian children affected by CMS-EA. Beside other clinical findings characteristic for CMS, both patients manifested intermittent apneas since early infancy. Whereas the course of disease is mild in the female patient (patient 2), the male patient (patient 1) experienced recurrent and severe episodes of apnea despite adequate treatment with AChE-inhibitors and shows a global developmental delay with delayed myelination and signs of hypoxic-ischemic injury in brain imaging. Interestingly, sequencing of the CHAT gene revealed identical, compound heterozygous mutations S694C and T354M in both children. These findings are in line with a remarkable clinical heterogeneity observed in patients with CHAT mutations and emphasize the potential role of apneic crises for the development of secondary hypoxic brain damage and psychomotor retardation.

Comparative study of gene expression of cholinergic system-related molecules in the human spinal cord and term placenta

By reverse transcription-polymerase chain reaction, Southern blot analysis, direct sequencing, and immunohistochemistry, we studied the expression of cholinergic neuronal markers (choline acetyltransferase [ChAT], vesicular acetylcholine transporter [VAChT], and a high-affinity choline transporter [CHT1]), and gene regulatory molecules (repressor element-1 silencing transcription factor/neuron-restrictive silencer factor [REST/NRSF] and CoREST) in the human spinal cord and term placenta, both of which are well known to contain cells synthesizing acetylcholine. H-type, M-type, N2-type, and R-type ChAT mRNAs, VAChT mRNA, and CHT1 mRNA were detected in the spinal cord, but only H-type, M-type, and N2-type ChAT mRNAs, in the term placenta. REST/NRSF and CoREST were detected in the spinal cord and the placenta, but the amounts of both mRNAs were greater in the placenta than in the spinal cord. Further microdissection analyses revealed that the placental trophoblastic cells contained more REST/NRSF and CoREST transcripts than the spinal large motor neurons. Large motor neurons in the anterior horn of the spinal cord were immunohistochemically stained for ChAT and VAChT. In the placenta, stromal fibroblasts, endothelial cells, and trophoblastic cells of the chorionic villi were positively stained with anti-ChAT antibody but not with anti-VAChT antibody. These findings suggest that transcriptions of the R-type ChAT and VAChT mRNAs are coordinately suppressed in the human term placenta, which might be regulated in part by a REST/NRSF complex that binds to a consensus sequence of repressor element 1/neuron-restrictive silencer element (RE1/NRSE) in the 5' region upstream from exon R, whereas transcriptions of the H-type, M-type, and N2-type ChAT mRNAs might be independent of control by RE1/NRSE. It is possible that at least two separate regulatory mechanisms of gene expression are present for the human cholinergic gene locus, which might be selected by different combinations of DNA motifs and binding proteins to function in neuronal and non-neuronal cells.

Disruption of the retinoid signalling pathway causes a deposition of amyloid beta in the adult rat brain

We have disrupted the retinoid signalling pathway in adult rats by a dietary deficiency of vitamin A. After 1 year of this dietary deficiency, there was a deposition of amyloid beta in the cerebral blood vessels. There is a downregulation of retinoic acid receptor alpha in the forebrain neurons of the retinoid-deficient rats and a loss of choline acetyl transferase expression, which precedes amyloid beta deposition. In neocortex of pathology samples of patients with Alzheimer's disease, the same retinoic acid receptor alpha deficit in the surviving neurons was observed. We have identified the retinoid-synthesizing enzymes involved in this process, retinaldehyde dehydrogenase-2 and class IV alcohol dehydrogenase, only the former is downregulated in patients with Alzheimer's disease. This suggests that retinoids are important for the maintenance of the adult nervous system and their loss may in part play a role in Alzheimer's disease.

Cloning of chicken choline acetyltransferase and its expression in early embryonic retina

The enzyme choline acetyltransferase [EC 2.3.1.6] (ChAT) synthesizes the neurotransmitter acetylcholine that plays a key morphogenic role in vertebrate retina development. As the embryonic avian retina is particularly useful for morphogenetic studies, we cloned the complete coding region of chicken ChAT cDNA. At the deduced amino acid level, chicken ChAT is approximately 76% identical to mammalian ChAT proteins. We also report here the cloning of the complete 5' end of the complex cholinergic locus. This locus contains both the ChAT gene and the nested intronless gene for the vesicular acetylcholine transporter (VAChT). The genomic organization of the 5' end of the chicken cholinergic locus is similar to that reported in other vertebrate species. A 5.7 kb mRNA corresponding to the ChAT message was detected in both embryonic retina and post-hatch brain. An analysis of the ChAT mRNA in embryonic chick retina shows that the message can be detected by E6 and its level increased during early retinal development. Vertebrate ChAT mRNAs can contain one or more of three non-coding exons, M, N or R and by RT-PCR we demonstrate, at least, a chicken ChAT mRNA containing exon M.

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

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

VAChT-Cre. Fast and VAChT-Cre.Slow: postnatal expression of Cre recombinase in somatomotor neurons with different onset

The cholinergic gene locus (CGL) consists of the genes encoding the choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT). To establish a cholinergic-specific Cre-expressing mouse, we constructed a transgene expression vector (VAChT-Cre) with 11.3 kb human CGL in which a Cre-IRES-EGFP unit was inserted in the VAChT open reading frame. The activity of Cre, whose expression was driven by the VAChT promoter, was examined by crossing a reporter mouse (CAG-CAT-Z) in which expression of LacZ is activated upon Cre-mediated recombination. Transgenic lines with the VAChT-Cre construct displayed the restricted Cre expression in a subset of cholinergic neurons in the somatomotor nuclei and medial habenular nucleus, but absent in visceromotor and other central and peripheral cholinergic neurons. Cre expression was first observed at postnatal day 7 and later detected in approximately 40-60% of somatomotor neurons. Based on the onset of Cre expression, we generated two mouse lines (two alleles; VAChT-Cre. Fast and VAChT-Cre.Slow) in which Cre expression reaches maximal levels fast and slow, respectively. The use of VAChT-Cre mice should allow us to deliver Cre to a subset of postnatal motor neurons, thereby bypassing lethality and facilitating analysis of gene function in adult motor neurons.

Regulation of the cholinergic gene locus by the repressor element-1 silencing transcription factor/neuron restrictive silencer factor (REST/NRSF)

The cholinergic gene locus is comprised of two genes, the choline acetyltransferase gene and the vesicular acetylcholine transporter gene. The vesicular acetylcholine transporter gene is located within the first intron of the choline acetyltransferase gene. This arrangement permits coordinate regulation of the locus. Protein kinase A regulates expression of the cholinergic gene locus in PC12 cells. This regulation was found to be dependent on the presence of a 21-bp DNA sequence known as the repressor element- (RE- 1)/neuron-restrictive silencer element(NRSE). Repressor element-I silencing transcription factor (REST)/ neuron-restrictive silencer factor (NRSF), which binds to the RE-I/NRSE, is a zinc finger containing transcriptional repressor that blocks the expression of many neuronal RE-I/NRSE containing genes in nonneuronal cells. However, REST/NRSF expression has also been observed in neurons as well as the PC 12 cell line used in these studies. REST/NRSF truncated isoforms were expressed in neuronal cells, suggesting they also function in regulating neuronal gene expression. A study of REST4, one of the REST/NRSF isoforms, suggests that it regulates transcription of the cholinergic gene locus by blocking the repressor activity of REST/NRSF. Protein kinase A regulation of the cholinergic gene locus in PC 12 cells can thus be attributed, at least in part, to increased synthesis of REST4, which in turn derepresses the repressor activity of REST/NRSF.

Autonomic microganglion cells: a source of acetylcholine in the rat carotid body

Hypoxic chemosensitivity of peripheral arterial chemoreceptors and the ventilatory response to O2 deprivation increases with postnatal development. Multiple putative neurotransmitters, which are synthesized in the carotid body (CB), are thought to mediate signals generated by hypoxia. Acetylcholine (ACh) is believed to be a major excitatory neurotransmitter participating in hypoxic chemosensitivity. However, it is not known whether ACh originates from type I cells in the CB. In these studies, we tested the hypothesis that choline acetyltransferase (ChAT) and vesicular ACh transporter (VAChT) mRNAs are expressed in the CB and that mRNA levels would increase with postnatal maturation or exposure to hypoxia. Semiquantitative in situ hybridization histochemistry and immunohistochemistry were used to localize cholinergic markers within neurons and cells of the rat CB, the nodose-petrosal-jugular ganglion complex, and the superior cervical ganglion up to postnatal day 28. We show that the pattern of distribution, in tissue sections, is similar for both ACh markers; however, the level of VAChT mRNA is uniformly greater than that of ChAT. VAChT mRNA and immunoreactivity are detected abundantly in the nodose-petrosal-jugular ganglion complex in a number of microganglion cells embedded in nerve fibers innervating the CB for all postnatal groups, whereas ChAT mRNA is detected in only a few of these cells. Contrary to our hypothesis, postnatal maturation caused a reduction in ACh trait expression, whereas hypoxic exposure did not induce the upregulation of VAChT and ChAT mRNA levels in the CB, microganglion, or within the ganglion complex. The present findings indicate that the source of ACh in the CB is likely within autonomic microganglion cells and cholinergic nerve terminals.

Identification of a region from the human cholinergic gene locus that targets expression of the vesicular acetylcholine transporter to a subset of neurons in the medial habenular nucleus in transgenic mice

We use a transgenic mouse model system to elucidate the regulatory regions within the human cholinergic gene locus responsible for vesicular acetylcholine transporter gene expression in vivo. In this report we characterized two transgenes for their ability to confer cholinergic-specific expression of the encoded vesicular acetylcholine transporter. An 11.2 kb transgene (named hV11.2) that spanned from about 5 kb upstream of the start of vesicular acetylcholine transporter translation down to the first choline acetyltransferase coding exon gave expression in the somatomotor neurons and a subpopulation of cholinergic neurons in the medial habenular nucleus. The second transgene (named hV6.7), a 5-prime truncated version of hV11.2 that was devoid of 4.5 kb of gene-regulatory sequences completely lacked vesicular acetylcholine transporter expression in vivo. Our data indicate that vesicular acetylcholine transporter expression in somatomotor neurons and in the medial habenular nucleus is uniquely specified within the cholinergic gene locus, and separable from cholinergic expression elsewhere. The identification of these two subdivisions of the cholinergic nervous system suggests that other cholinergic neurons in the CNS and PNS are similarly regulated by additional discrete domains within the cholinergic gene locus.

Neural stem cells and cholinergic neurons: regulation by immunolesion and treatment with mitogens, retinoic acid, and nerve growth factor

Degenerative diseases represent a severe problem because of the very limited repair capability of the nervous system. To test the potential of using stem cells in the adult central nervous system as "brain-marrow" for repair purposes, several issues need to be clarified. We are exploring the possibility of influencing, in vivo, proliferation, migration, and phenotype lineage of stem cells in the brain of adult animals with selective neural lesions by exogenous administration (alone or in combination) of hormones, cytokines, and neurotrophins. Lesion of the cholinergic system in the basal forebrain was induced in rats by the immunotoxin 192IgG-saporin. Alzet osmotic minipumps for chronic release (over a period of 14 days) of mitogens [epidermal growth factor (EGF) or basic fibroblast growth factor (bFGF)] were implanted in animals with behavioral and biochemical cholinergic defect and connected to an intracerebroventricular catheter. After 14 days of delivery, these pumps were replaced by others delivering nerve growth factor (NGF) for an additional 14 days. At the same time, retinoic acid was added to the rats' food pellets for one month. Whereas the lesion decreased proliferative activity, EGF and bFGF both increased the number of proliferating cells in the subventricular zone in lesioned and nonlesioned animals. These results are indicated by the widespread distribution of BrdUrd-positive nuclei in the forebrain, including in the cholinergic area. Performance in the water maze test was improved in these animals and choline acetyltransferase activity in the hippocampus was increased. These results suggest that pharmacological control of endogenous neural stem cells can provide an additional opportunity for brain repair. These studies also offer useful information for improving integration of transplanted cells into the mature brain.

Exploring the regulation of the expression of ChAT and VAChT genes in NG108-15 cells: implication of PKA and PI3K signaling pathways

Involvement of different protein kinases regulated by cAMP and implication of muscarinic receptors in the regulation of choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) mRNA levels and ChAT activity has been studied in NG108-15 cells. Dibutyryl cAMP enhanced both ChAT and VAChT mRNA levels and stimulated ChAT activity. Muscarinic stimulation or inhibition did not change ChAT activity or the receptor subtype mRNA pattern. MEK1/2 did not affect the regulation of ChAT and VAChT mRNA levels. However, PKA plays a major role in regulating ChAT and VAChT mRNA levels, because H89 decreased both. Strikingly, inhibition of PI3K by LY294002 had two opposite effects: ChAT mRNA level was decreased and VAChT mRNA level was increased. Such a result consolidates the observation that ChAT and VAChT genes, despite their unusual organization in a single "cholinergic locus," can be differentially or synergistically regulated, depending on the activated signaling pathways.

Regulation of cholinergic gene expression by the neuron restrictive silencer factor/repressor element-1 silencing transcription factor

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 locus is reduced. The site of action of protein kinase A was localized to a neuron restrictive silencer element/repressor element-1 (NRSE/RE-1) within the upstream region of the cholinergic gene locus. The neuron restrictive silencer factor/repressor element-1 silencing transcription factor (NRSF/REST), the transcription factor which binds to NRSE/RE-1, was expressed at similar levels in both PC12 and A126.1B2. 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 repressor element-1 silencing transcription factor-4 (REST4) was found to be expressed in PC12 cells, but not in the protein kinase A deficient PC12 cell line. REST4 inhibited the binding of NRSF/REST to NRSE/RE-1 as determined by gel mobility shift assays. Co-immunoprecipitation was used to demonstrate interaction between NRSF/REST and REST4. Expression of recombinant REST4 in the protein kinase A deficient PC12 cell line was sufficient to transcriptionally activate the cholinergic gene locus. Thus in PC12 cells protein kinase A promotes the production of REST4, which in turn de-represses of the cholinergic gene locus by inactivating the transcription repressor NRSF/REST.

Quantification of transcriptional activities of reporter gene constructs in primary cultures of sympathetic neurons

Primary cultures of sympathetic neurons provide an attractive cellular model for investigating the mechanisms of neurotransmitter phenotypic plasticity. However, it has not been possible to transfect these neurons by conventional techniques, and this has been a major impediment to molecular investigations of neuronal gene expression in this system. Here, reporter plasmids were transferred into the nuclei of cultured sympathetic neurons by microinjection. We developed and improved this procedure and were able to measure the transcriptional activities of two coinjected promoters in small groups of neurons, and even from a single neuron. Promoter activities can thus be quantified and normalized relative to that of a constitutively expressed promoter, allowing correction for variability in the injection and assay procedures. High and low promoter activities can be reliably quantified. Importantly, this method can be used not only for reporter plasmids but also for DNA fragments containing only a promoter and reporter gene without any vector sequence that might interfere with promoter. Using this approach, we measured neuronal promoter activities and found that one promoter region of the gene encoding choline acetyltransferase was up-regulated by more than sevenfold by leukemia inhibitory factor. This method thus provides the means to investigate the function of neuronal genes and the mechanisms that regulate their transcription in cultured sympathetic neurons.

Vitamin A deficiency produces spatial learning and memory impairment in rats

Vitamin A and its derivatives (retinoids) play important roles in many physiological processes. The recent finding of high levels of cellular retinol-binding protein type 1 immunoreactivity, cellular retinoic acid-binding protein type 1 immunoreactivity and the presence of nuclear retinoid receptors in the central nervous system of adult rodents suggests that retinoids may carry out important roles in the adult brain. In consideration of the role of the hippocampus in spatial learning and memory we evaluated the effect of vitamin A deprivation in adult rats on these functions. Following 12 weeks of vitamin A-free diet, rats were trained to acquire a radial-arm maze task. Results show that this diet induced a severe deficit in the spatial learning and memory task. The cognitive impairment was fully restored when vitamin A was replaced in the diet. We also found a significant decrease in hippocampal acetylcholine release induced by scopolamine, assessed using microdialysis technique, and a reduction in the size of hippocampal nuclei of CA1 region in vitamin-deficient rats, compared to rats fed with a vitamin A-sufficient diet. These results demonstrate that vitamin A has a critical role in the learning and memory processes linked to a proper hippocampal functioning.

Regulation of acetylcholine synthesis and storage

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

Synergistic activation of the human choline acetyltransferase gene by c-Myb and C/EBPbeta

To elucidate regulatory mechanisms at the transcriptional level of the human choline acetyltransferase gene (hChAT) we performed cotransfections assays in NG108-15 and SN56 cells using ChAT-CAT reporter plasmids with c-Myb and C/EBPbeta expression plasmids. The hChAT gene has several promoters, one of which (promoter P2 or M-type) is both c-Myb and C/EBPbeta inducible as 3-4-fold trans-activation was obtained in both cell lines when using either c-Myb or C/EBPbeta expression vectors alone. The simultaneous expression of c-Myb and C/EBPbeta in the absence or presence of NGFI-C (egr4) leads respectively to a 15-fold and 32-fold synergistic transcriptional activation of promoter P2. In the region upstream of exon M (P2) we identified a functional composite element including a c-Myb next to a C/EBP binding site. An oligonucleotide containing the composite element confers c-Myb and C/EBPbeta responsiveness to a heterologous promoter which is reduced after mutation of the c-Myb binding site. We also show that the coactivators CBP/p300 are required for c-Myb and C/EBPbeta trans-activation function and that RARalpha, RXRalpha and T3R have an inhibitory action on the synergistic transcriptional activity of c-Myb and C/EBPbeta and propose a model to explain the phenomena. Taken together, the results suggest that the synergistic effect of c-Myb and C/EBPbeta, previously observed in the hematopoietic system, functions equally in the neuronal system.

Acetyl-L-carnitine shows neuroprotective and neurotrophic activity in primary culture of rat embryo motoneurons

We evaluated the role of acetyl-L-carnitine (ALCAR) in protecting primary motoneuron cultures exposed to excitotoxic agents or serum-brain derived neurotrophic factor (BDNF) deprived. To exclude that ALCAR works as a metabolic source, we compared its effects with those of L-carnitine (L-CAR), that seems to have no neurotrophic effect. A concentration of 10 mM ALCAR, but not L-CAR, significantly reduced the toxic effect of 50 microM N-methyl-D-aspartate (NMDA, % viability: NMDA 45.4+/-2.80, NMDA+ALCAR 90.8+/-11.8; P<0.01) and of 5 microM kainate in cultured motoneurons (% viability: kainate 40.66+/-10.73; kainate+ALCAR 63.80+/-13.88; P<0.05). The effect was due to a shift to the right of the dose-response curve for kainate (EC50 for kainate 5.99+/-1.012 microM; kainate+ALCAR 8.62+/-1.13 microM; P<0.05). ALCAR, but not L-CAR, significantly protected against BDNF and serum-deprivation reducing the apoptotic cell death (% viability respect to control: without BDNF/serum 61.8+/-13.3: without BDNF/serum+ALCAR 111.8+/-13.9; P<0.01). Immunocytochemistry showed an increase in choline acethyltransferase and tyrosine kinaseB receptors in motoneurons treated with ALCAR but not with L-CAR. These results suggest that ALCAR treatment improves the motoneurons activity, acting as a neurotrophic factor.

Analysis of uptake and release of newly synthesized acetylcholine in PC12 cells overexpressing the rat vesicular acetylcholine transporter (VAChT)

Rat VAChT cDNA was stably transfected into PC12 cells to generate cell clones overexpressing different quantities of VAChT protein. Membrane fractions prepared from one cell clone (#3) in which VAChT was highly expressed (as determined by Western blot and [(3)H]vesamicol binding analyses) accumulated approximately two and half times as much [(3)H]ACh during incubation as did membrane fractions prepared from control cells. Vesamicol inhibited this ATP-dependent uptake. Membrane fractions isolated from a second cell clone (#6), which contained considerably less VAChT protein than did clone #3, accumulated no more [(3)H]ACh than did control cells. We compared the accumulation of newly synthesized [(14)C]ACh by a particulate fraction prepared from clones #3 and #6 with untransfected cells when these cells were incubated with either labeled acetate or choline. The results indicated that particulate fractions of clones #3 and #6 did not accumulate any more newly synthesized [(14)C]ACh than did the particulate fraction of untransfected cells. Furthermore, vesamicol reduced the filling of a particulate fraction of untransfected cells with newly synthesized ACh better than it reduced the refilling of a particulate fraction of VAChT-transfected cells. High K(+) depolarization did not release any more newly synthesized [(14)C]ACh from VAChT-transfected than it did from untransfected cells. In related studies, overexpression of VAChT in clone #3 induced a slight but significant increase in ChAT activity. Overall, our results indicate that an increase in the amount of VAChT protein associated with a particulate fraction of PC12 cells does not augment the amount of newly synthesized [(14)C]ACh acquired by that particulate fraction. However, it does reduce the effectiveness of vesamicol in blocking the filling of the particulate fraction with newly synthesized [(14)C]ACh. In summary, our results suggest that the vesicular release of ACh from PC12 cells is not regulated at the level of its uptake into synaptic vesicles.

Mitogen-activated protein kinase kinase negatively modulates ciliary neurotrophic factor-activated choline acetyltransferase gene expression

The expression of the choline acetyltransferase (ChAT) enzyme that synthesizes the neurotransmitter acetylcholine (ACh) is upregulated by ciliary neurotrophic factor (CNTF). We studied the involvement of the mitogen-activated protein kinase (MAPK) pathway in regulating ChAT expression in a murine septal cell line. Surprisingly, we found that PD98059 and U0126, two structurally distinct inhibitors of MAPK kinase (MEK1), increased both basal and CNTF-induced ACh production. Transient transfections with ChAT promoter-luciferase reporter construct demonstrated synergy between PD98059 and CNTF at the transcriptional level. Moreover, in cotransfection studies, overexpression of constitutively activated MEK1 completely abrogated the CNTF-mediated induction of the reporter. Blocking MEK1 did not significantly alter CNTF-induced Tyr705 phosphorylation of the principal mediator of the CNTF pathway, the transcription factor Stat3. However, PD98059 inhibited Ser727 phosphorylation of Stat3, demonstrating that the latter is MEK1-dependent. Taken together, these results indicate that activation of the MEK1/MAPK pathway inhibits the CNTF-mediated stimulation of ChAT expression, possibly as a part of a feedback mechanism.

Thyroid hormone and retinoids affect motoneuron phenotype and reaction after axotomy in the spinal cord of adult rats

Motoneuron phenotype in the spinal cord is regulated by an intrinsic genetic program, extrinsic environmental signals and target-derived molecules. Axonal lesions trigger a phenotype switch to foster repair phenomena and axonal re-growth. We have investigated the influence of the long-term treatment with thyroid hormone and all trans retinol palmitate (RA) on motoneuron phenotype and spinal cord reaction to axotomy in adult male rats. Neurochemical markers, investigated by in situ hybridization and immunocytochemistry, included choline acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP) and neurotrophin low affinity receptor p75. Treatment was administered for 56 days and then mid-thigh sciatic axotomy was performed on a number of animals from each experimental groups; the rats were examined 9 days after surgery. The results indicate that: (1) Number and size of ChAT-immunoreactive neurons in the lumbar tract of the spinal cord was reduced in hypothyroid compared to control rats, whereas steady-state level of ChAT mRNA in labelled motoneurons failed to be modified by hypo and hyperthyroidism, but was increased by RA administration; (2) none of the administered treatments did alter CGRP mRNA level, whereas all of them influenced the axotomy-induced changes of motoneuron phenotype; (3) in hyperthyroid rats ChAT mRNA level of lumbar motoneurons not reduced homolateral to lesion while the number of ChAT-IR profiles was pronouncedly reduced; (4) up-regulation of p75 induced by peripheral nerve lesion was reduced in RA-treated rats. These data indicate that the motoneuron phenotype is regulated by transcription factors, which also play a role in phenotype switch regulation after axotomy.

Mutation analysis of the retinoid X receptor beta, nuclear-related receptor 1, and peroxisome proliferator-activated receptor alpha genes in schizophrenia and alcohol dependence: possible haplotype association of nuclear-related receptor 1 gene to alcohol dependence

Because retinoid cascades are involved in the regulation and development of the central nervous system, including dopaminergic neurons, retinoic acid signaling defects may contribute to schizophrenia and substances dependence. Retinoid X receptors (RXRs) form heterodimer complexes with nuclear-related receptor 1 (NURR1) or with peroxisome proliferator-activated receptors (PPARs). We examined 48 Japanese patients with schizophrenia and 32 patients with alcohol dependence to detect mutations in the retinoid X receptor beta gene (RXRB) on chromosome 6p21.3, the NURR1 gene (NR4A2) on chromosome 2q22-q23, and the PPAR alpha gene (PPARA) on chromosome 22q12.2-13.1. A Val95Ala polymorphism of the RXRB gene, a Val227Ala polymorphism in the PPARA gene, and two synonymous single-nucleotide and CA repeat polymorphisms in the 5' region and 3' untranslated region of the NR4A2 gene were identified. Extended case control samples did not suggest an association between the diseases and the RXRB or PPARA polymorphisms. However, they revealed a significant association between the NR4A2 gene haplotype and alcohol dependence, indicating that 2q22-q23 including the NR4A2 gene locus is a possible genomic region contributing to genetic susceptibility to alcohol dependence.

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.

Upregulation of acetylcholine synthesis by bone morphogenetic protein 9 in a murine septal cell line

Previous studies showed that bone morphogenetic protein 9 (BMP-9) induces the expression of choline acetyltransferase and the vesicular acetylcholine (ACh) transporter, and upregulates ACh synthesis in cultured primary neurons from embryonic mouse septum [I. López-Coviella, B. Berse, R. Krauss, R.S. Thies, J.K. Blusztajn, Induction and maintenance of the neuronal cholinergic phenotype in the central nervous system by BMP-9. Science 289 (2000) 313-316]. In the present studies we investigated the effects of BMP-9 on ACh synthesis in the cholinergic mouse SN56T17 septal cell line. BMP-9 increased ACh synthesis in these cells up to 2.5-fold in a time- and dose-dependent, saturable manner. The maximal effect of BMP-9 was observed after a 3-day treatment and the median effective concentration of BMP-9 was 0.5 ng/ml. These data show that SN56T17 cells are a useful model for studies of the effects of BMPs on the cholinergic phenotype.

Generation of choline for acetylcholine synthesis by phospholipase D isoforms

BACKGROUND

In cholinergic neurons, the hydrolysis of phosphatidylcholine (PC) by a phospholipase D (PLD)-type enzyme generates some of the precursor choline used for the synthesis of the neurotransmitter acetylcholine (ACh). We sought to determine the molecular identity of the relevant PLD using murine basal forebrain cholinergic SN56 cells in which the expression and activity of the two PLD isoforms, PLD1 and PLD2, were experimentally modified. ACh levels were examined in cells incubated in a choline-free medium, to ensure that their ACh was synthesized entirely from intracellular choline.

RESULTS

PLD2, but not PLD1, mRNA and protein were detected in these cells and endogenous PLD activity and ACh synthesis were stimulated by phorbol 12-myristate 13-acetate (PMA). Introduction of a PLD2 antisense oligonucleotide into the cells reduced PLD2 mRNA and protein expression by approximately 30%. The PLD2 antisense oligomer similarly reduced basal- and PMA-stimulated PLD activity and ACh levels. Overexpression of mouse PLD2 by transient transfection increased basal- (by 74%) and PMA-stimulated (by 3.2-fold) PLD activity. Moreover, PLD2 transfection increased ACh levels by 26% in the absence of PMA and by 2.1-fold in the presence of PMA. Overexpression of human PLD1 by transient transfection increased PLD activity by 4.6-fold and ACh synthesis by 2.3-fold in the presence of PMA as compared to controls.

CONCLUSIONS

These data identify PLD2 as the endogenous enzyme that hydrolyzes PC to generate choline for ACh synthesis in cholinergic cells, and indicate that in a model system choline generated by PLD1 may also be used for this purpose.

Distribution of the high-affinity choline transporter in the central nervous system of the rat

In cholinergic nerve terminals, Na(+)- and Cl(-)-dependent, hemicholinium-3-sensitive, high-affinity choline uptake is thought to be the rate-limiting step in acetylcholine synthesis. The high-affinity choline transporter cDNA responsible for the activity was recently cloned. Here we report production of a highly specific antibody to the high-affinity choline transporter and distribution of the protein in the CNS of the rat. The antibody stained almost all known cholinergic neurons and their terminal fields. High-affinity choline transporter-immunoreactive cell bodies were demonstrated in the olfactory tubercle, basal forebrain complex, striatum, mesopontine complex, medial habenula, cranial nerve motor nuclei, and ventral horn and intermediate zone of the spinal cord. Noticeably, high densities of high-affinity choline transporter-positive axonal fibers and puncta were encountered in many brain regions such as cerebral cortex, hippocampus, amygdala, striatum, several thalamic nuclei, and brainstem. Transection of the hypoglossal nerve resulted in a loss of high-affinity choline transporter immunoreactivity in neurons within the ipsilateral hypoglossal motor nucleus, which paralleled a loss of immunoreactivity to choline acetyltransferase. The antibody also stained brain sections from human and mouse, suggesting cross-reactivity. These results confirm that the high-affinity choline transporter is uniquely expressed in cholinergic neurons and is efficiently transported to axon terminals. The antibody will be useful to investigate possible changes in cholinergic cell bodies and axon terminals in human and rodents under various pathological conditions.

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.

Elevation of nerve growth factor and antisense knockdown of TrkA receptor during contextual memory consolidation

We report here a series of experiments establishing a role for nerve growth factor and its high-affinity receptor TrkA in contextual memory consolidation. In all experiments, we trained rats in a novel chamber using tone and shock. Our first experiment revealed that endogenous nerve growth factor (NGF) increases in the hippocampus at a critical time during consolidation that occurs 1 week after training. NGF levels at other intervals (24 hr and 2 and 4 weeks after training) did not differ from those of naive control animals. In our second experiment, we blocked effects that NGF has at 1 week after training by infusing antisense TrkA phosphorothioate DNA oligonucleotide. Reduction of septohippocampal TrkA receptor expression selectively impaired memory consolidation for context but not for tone. Animals with antisense TrkA oligonucleotide infused into the medial septal area or CA1 of the hippocampus froze less when placed in the training chamber than did animals infused with inactive randomized oligonucleotide. At 4 weeks after training, antisense TrkA oligonucleotide had no effect on freezing. Third, we correlated levels of freezing with choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) immunohistochemistry. Antisense TrkA infused into CA1 of the hippocampus reduced cell body cross-sectional area for cholinergic cells in the medial septal area and decreased the density of hippocampal terminals labeled for ChAT and VAChT proteins. Cholinergic cell body measurements were significantly correlated with freezing. Taken together, these results indicate a role for nerve growth factor acting via the TrkA receptor on ChAT and VAChT proteins in contextual memory consolidation.

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.

Exogenous testosterone treatment decreases diaphragm neuromuscular transmission failure in male rats

The effect of chronic exogenous testosterone (T) treatment on neuromuscular transmission in the diaphragm (Dia) muscle of adult male rats was determined. The contribution of neuromuscular transmission failure (NTF) to Dia fatigue was evaluated by superimposing intermittent direct muscle stimulation on repetitive nerve stimulation of isometric contraction in vitro. T treatment significantly reduced the contribution of NTF to Dia fatigue by approximately 20% (P < 0.001). Fiber type-specific effects on NTF were determined by measuring Dia fiber glycogen levels subsequent to repetitive nerve or muscle stimulation. T treatment had no effect on glycogen depletion in Dia type I and IIa fibers regardless of stimulation route. In the control group, type IIx fibers demonstrated significantly less glycogen depletion after nerve stimulation compared with direct muscle stimulation (P < 0.05), suggesting the presence of NTF. In contrast, T treatment increased glycogen depletion of type IIx fibers during nerve stimulation to levels similar to those after direct muscle stimulation. These data indicate that testosterone treatment substantially improves neuromuscular transmission in the Dia.

Chemical neuroanatomy of the vesicular amine transporters

Acetylcholine, catecholamines, serotonin, and histamine are classical neurotransmitters. These small molecules also play important roles in the endocrine and immune/inflammatory systems. Serotonin secreted from enterochromaffin cells of the gut epithelium regulates gut motility; histamine secreted from basophils and mast cells is a major regulator of vascular permeability and skin inflammatory responses; epinephrine is a classical hormone released from the adrenal medulla. Each of these molecules is released from neural, endocrine, or immune/inflammatory cells only in response to specific physiological stimuli. Regulated secretion is possible because amines are stored in secretory vesicles and released via a stimulus-dependent exocytotic event. Amine storage-at concentrations orders of magnitude higher than in the cytoplasm-is accomplished in turn by specific secretory vesicle transporters that recognize the amines and move them from the cytosol into the vesicle. Immunohistochemical visualization of specific vesicular amine transporters (VATs) in neuronal, endocrine, and inflammatory cells provides important new information about how amine-handling cell phenotypes arise during development and how vesicular transport is regulated during homeostatic response events. Comparison of the chemical neuroanatomy of VATs and amine biosynthetic enzymes has also revealed cell groups that express vesicular transporters but not enzymes for monoamine synthesis, and vice versa: their function and regulation is a new topic of investigation in mammalian neurobiology. The chemical neuroanatomy of the vesicular amine transporters is reviewed here. These and similar data emerging from the study of the localization of the recently characterized vesicular inhibitory and excitatory amino acid transporters will contribute to understanding chemically coded synaptic circuitry in the brain, and amine-handling neuroendocrine and immune/inflammatory cell regulation.

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.

Cyclic AMP prevents an increase in GAP-43 but promotes neurite growth in cultured adult rat dorsal root ganglion neurons

High expression of the growth-associated protein GAP-43 in neurons is correlated with developmental and regenerative axon growth. It has been postulated that during development and after injury, GAP-43 expression is elevated due to the unavailability of a target-derived repressive signal, but that GAP-43 expression then declines upon target contact. Here we examine the cyclic AMP second messenger signaling pathway to determine if it might mediate retrograde transmission of a signal which represses GAP-43 expression and inhibits growth. Cultures of adult rat dorsal root ganglia were chronically exposed to membrane-permeable analogs of cyclic AMP and activators of adenyl cyclase. These treatments caused GAP-43 protein levels to decrease in a dose-dependent manner, although neuronal survival was not affected. GAP-43 mRNA was also decreases by cyclic AMP. GAP-43 protein levels were not repressed by neurotrophins, cytokines, or other agents. Surprisingly, cyclic AMP caused an increase in the rate of neurite outgrowth, even though the neurons were partially depleted of GAP-43. Growth stimulation was quickly inducible and reversible, could occur in the presence of transcription inhibitors, and did not entail alterations in branching pattern. These findings suggest that axon growth involving high levels of GAP-43 is distinct from the growth stimulation which is rapidly induced by cyclic AMP.

Cholinergic differentiation triggered by blocking cell proliferation and treatment with all-trans-retinoic acid

This study determined whether the effect of all-trans-retinoic acid (t-RA) on markers of cholinergic differentiation in a murine septal cell line, SN56.B5.G4, differed depending upon the cell's proliferative status. To develop a model of non-proliferating cells, aphidicolin, a DNA alpha-polymerase inhibitor, was used. Cessation of proliferation by aphidicolin increased intracellular choline and acetylcholine (ACh) levels in the absence of change to choline acetyltransferase (ChAT) activity and mRNA and vesicular ACh transporter (VAChT) mRNA. Importantly, the response to t-RA differed depending upon proliferative status. Consistent with previous reports, t-RA increased ChAT and VAChT mRNA, ChAT activity and intracellular ACh levels in proliferating SN56 cells with no effect on intracellular choline levels. When cells were treated with t-RA while undergoing proliferative arrest, an additive effect of combined treatment was observed on ACh levels; nevertheless, this was only accompanied by an increase in choline levels, VAChT and ChAT mRNAs, but not ChAT activity. Indeed, aphidicolin treatment completely suppressed the t-RA-induced increase in ChAT activity observed in proliferating cells. To explore the response to t-RA in post-mitotic cells, a sequential treatment of aphidicolin and t-RA was employed. t-RA treatment was ineffective in increasing ACh and choline levels, over and above that observed with the aphidicolin treatment alone. Comparable to the combined treatment, sequential treatment lead to an increase in ChAT mRNA without any increase in ChAT activity. In conclusion, both the magnitude and the mechanism(s) of action whereby t-RA enhances the cholinergic phenotype of SN56 cells is dependent upon the cell's proliferative status.

Induction and maintenance of the neuronal cholinergic phenotype in the central nervous system by BMP-9

Bone morphogenetic proteins (BMPs) have multiple functions in the developing nervous system. A member of this family, BMP-9, was found to be highly expressed in the embryonic mouse septum and spinal cord, indicating a possible role in regulating the cholinergic phenotype. In cultured neurons, BMP-9 directly induced the expression of the cholinergic gene locus encoding choline acetyltransferase and the vesicular acetylcholine transporter and up-regulated acetylcholine synthesis. The effect was reversed upon withdrawal of BMP-9. Intracerebroventricular injection of BMP-9 increased acetylcholine levels in vivo. Although certain other BMPs also up-regulated the cholinergic phenotype in vitro, they were less effective than BMP-9. These data indicate that BMP-9 is a differentiating factor for cholinergic central nervous system neurons.

Functional implications of the noradrenergic-cholinergic switch induced by retinoic acid in NB69 neuroblastoma cells

Some neuroblastoma cell lines change their neurotransmitter phenotype from noradrenergic to cholinergic under retinoic acid treatment. Such "neurotransmitter switch" seems to be a consequence of changes in the expression and activity of the biosynthetic machinery for both neurotransmitters. In this study, we have characterized this "neurotransmitter switch" induced by retinoic acid in a human neuroblastoma cell line (NB69) showing catecholaminergic characteristics. Retinoic acid treatment reduced tyrosine hydroxylase activity and noradrenaline levels in NB69 cells but did not modify the expression of this enzyme. Moreover, the calcium-dependent release of [(3)H]noradrenaline in control cells was highly reduced by retinoic acid treatment. On the other hand, NB69 cells treated with retinoic acid enhanced the expression of choline acetyltransferase and acquired the capability to release [(3)H]acetylcholine in a calcium-dependent way. In addition, we found that the expression of the vesicular monoamine transporter 2 (VMAT2) and the vesicular acetylcholine transporter (VAChT) was increased in those cells treated with retinoic acid. Immunostaining revealed that retinoic acid treatment changed the cellular distribution of both vesicular monoamine transporter 2 and vesicular acetylcholine transporter. In conclusion, retinoic acid induces a noradrenergic to cholinergic switch in NB69 cells by acting at several levels of the neurotransmitter phenotypic expression.

Leptin receptor-mediated regulation of cholinergic neurotransmitter phenotype in cells of central nervous system origin

Leptin is an adipocyte-secreted hormone that regulates body weight and exerts effects on hematopoiesis, reproduction, and immunity. The leptin receptor (OBR) shares sequence similarity and signaling capabilities with receptors for cytokines of the ciliary neurotrophic factor (CNTF) family. Our previous finding that CNTF and leptin exert similar anti-obesity effects and activate common neuronal signaling pathways, prompted us to investigate whether leptin may share with CNTF the ability to regulate the expression of specific neuronal genes. To this end, we established a cell line, derived from the murine septal cholinergic neuronal cell line SN-56, which stably expresses OBR. In this cell line, termed SN-56/OBR, leptin induces STAT transcription factor activation and STAT-dependent reporter gene expression in a manner similar to that of CNTF. Furthermore, in SN-56/OBR cells both CNTF and leptin produce changes in neurotransmitter and neuropeptide phenotype characteristic of cholinergic neurons, such as an increase in choline acetyltransferase and vasoactive intestinal polypeptide, and a decrease in neuropeptide Y expression. SN-56/OBR cells thus constitute an interesting new model system to investigate leptin action in cells of central nervous system origin. Possible physiological implications of OBR's intrinsic ability to regulate cholinergic phenotypic markers are discussed.

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.

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.

Developmental changes in the transmitter properties of sympathetic neurons that innervate the periosteum

During the development of sweat gland innervation, interactions with the target tissue induce a change from noradrenergic to cholinergic and peptidergic properties. To determine whether the change in neurotransmitter properties that occurs in the sweat gland innervation occurs more generally in sympathetic neurons, we identified a new target of cholinergic sympathetic neurons in rat, the periosteum, which is the connective tissue covering of bone, and characterized the development of periosteal innervation of the sternum. During development, sympathetic axons grow from thoracic sympathetic ganglia along rib periosteum to reach the sternum. All sympathetic axons displayed catecholaminergic properties when they reached the sternum, but these properties subsequently disappeared. Many axons lacked detectable immunoreactivities for vesicular acetylcholine transporter and vasoactive intestinal peptide when they reached the sternum and acquired them after arrival. To determine whether periosteum could direct changes in the neurotransmitter properties of sympathetic neurons that innervate it, we transplanted periosteum to the hairy skin, a noradrenergic sympathetic target. We found that the sympathetic innervation of the transplant underwent a noradrenergic to cholinergic and peptidergic change. These results suggest that periosteum, in addition to sweat glands, regulates the neurotransmitter properties of the sympathetic neurons that innervate it.

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.