Promoter elements of the rat choline acetyltransferase gene allowing nerve growth factor inducibility in transfected primary cultured cells

Effects of repeated administration of rolipram, a cAMP-specific phosphodiesterase inhibitor, on acetylcholinergic indices in the aged rat brain

The effects of repeated treatment of rolipram, a cAMP specific phosphodiesterase inhibitor (0.1 mg/kg/day i.p., 14 days), on several neuronal cholinergic indices, especially on those in aged rats were examined. In young-adult rats, rolipram treatment increased choline acetyltransferase (ChAT) activity (V(max)value) in the striatum as well as in thalamus + midbrain, whereas it decreased choline esterase activity in the hippocampus. The ChAT activity (V(max)value) and the M1-R binding (B(max)value) in the aged control rats were significantly reduced in all the brain regions examined, compared with the young-adult rats, but consecutive rolipram treatment ameliorated the reductions of both indices in the frontal cortex and the hippocampus to approximately the young-adult control levels. Since high membrane binding site concentrations for rolipram itself were revealed in the frontal cortex and the hippocampus, where the rolipram treatment showed ameliorating effects on the ChAT activity and the M1-R binding, the present findings indicate that repeated rolipram administration easily affects these two brain regions. Thus, repeated rolipram administration could restore both the presynaptic ChAT activity and the postsynaptic muscarinic cholinergic M1-R binding which are decreased with aging.

Regulation of cholinergic gene expression by nerve growth factor depends on the phosphatidylinositol-3'-kinase pathway

Nerve growth factor (NGF) exerts anti-apoptotic, trophic and differentiating actions on sympathetic neurons and cholinergic cells of the basal forebrain and activates the expression of genes regulating the synthesis and storage of the neurotransmitter acetylcholine (ACh). We have been studying the intracellular signaling pathways involved in this process. Although, in the rat pheochromocytoma cell line PC12, NGF strongly activates the mitogen-activated protein kinase (MAPK) pathway, prolonged inhibition of MAPK kinase (MEK) activity by PD98059 or U0126 did not affect the ability of NGF to up-regulate choline acetyltransferase (ChAT) or to increase intracellular ACh levels. In contrast, the treatment with the phosphatidylinositol 3'-kinase (PI3K) inhibitor LY294002, but not with its inactive analogue LY303511, completely abolished the NGF-induced production of ACh. Inhibition of PI3K also eliminated the NGF effect on the intracellular ACh level in primary cultures of septal neurons from E18 mouse embryos. Blocking the PI3K pathway prevented the activation of cholinergic gene expression, as demonstrated in RT/PCR assays and in transient transfections of PC12 cells with cholinergic locus promoter-luciferase reporter constructs. These results indicate that the PI3K pathway, but not the MEK/MAPK pathway, is the mediator of NGF-induced cholinergic differentiation.

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.

Lack of association between a single nucleotide polymorphism within the choline acetyltransferase gene and patients with Alzheimer's disease

Alterations of the cholinergic system may account for typical clinical and pathophysiological disturbances of Alzheimer's disease (AD). In particular, a marked decline of choline acetyltransferase activity (CHAT) and as a consequence of acetylcholine during the course of the disease has been described. Due to the chromosomal localization of CHAT at 10q11.23 and its possible role in the pathophysiology of AD, CHAT may represent an appropriate candidate gene conferring risk to AD. In fact, a recent study identified a functional single nucleotide polymorphism (SNP) within the first common exon of CHAT, which was associated with AD giving an odds ratio of 3.8 (Neurosci. Lett. 333 (2002) 9). Because of the potential importance of this finding we analyzed this SNP and another functional SNP within exon 9 (rs868749) of the CHAT gene using a German case control sample consisting of 242 patients with AD and 143 cognitively healthy controls. No statistically significant differences were obtained for the previously described polymorphism. In addition, the exon 9 SNP (rs868749) was not polymorphic in the studied population. We conclude that the previously identified polymorphism is not associated with AD.

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.

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.

Nerve growth factor, but not epidermal growth factor, increases Fra-2 expression and alters Fra-2/JunD binding to AP-1 and CREB binding elements in pheochromocytoma (PC12) cells

In pheochromocytoma (PC12) cells nerve growth factor (NGF) and epidermal growth factor (EGF) activate similar receptor tyrosine kinase signaling pathways but evoke strikingly different biological outcomes: NGF induces differentiation and EGF acts as a mitogen. A novel approach was developed for identifying transcription factor activities associated with NGF-activated, but not EGF-activated, signaling, using random oligonucleotide clones from a DNA recognition library to isolate specific DNA binding proteins from PC12 nuclear extracts. A protein complex from NGF-treated, but not EGF-treated, cells was identified that exhibits increased mobility and DNA binding activity in gel mobility shift assays. The binding complex was identified in supershift assays as Fra-2/JunD. The clones used as probes contain either AP-1 or cAMP response element binding (CREB) recognition elements. Time course experiments revealed further differences in NGF and EGF signaling in PC12 cells. NGF elicits a more delayed and sustained ERK phosphorylation than EGF, consistent with previous reports. Both growth factors transiently induce c-fos, but NGF evokes a greater response than EGF. NGF specifically increases Fra-1 and Fra-2 levels at 4 and 24 hr. The latter is represented in Western blots by bands in the 40-46 kDa range. NGF, but not EGF, enhances the upper bands, corresponding to phosphorylated Fra-2. These findings suggest that prolonged alterations in Fra-2 and subsequent increases in Fra-2/JunD binding to AP-1 and CREB response elements common among many gene promoters could serve to trigger broadly an NGF-specific program of gene expression.

Nuclear factor kappaB/p49 is a negative regulatory factor in nerve growth factor-induced choline acetyltransferase promoter activity in PC12 cells

Anovel nuclear factor kappaB (NF-kappaB) binding site has been identified within the promoter region of the mouse gene encoding choline acetyltransferase (ChAT), the enzyme that synthesizes acetylcholine and has been implicated in the cognitive deficits associated with aging and Alzheimer's disease. This binding site, which is located within the nerve growth factor (NGF)-responsive enhancer element, was recognized by the NF-kappaB protein p49 but not p65 or p50. p49 from both basal forebrain and PC12 nuclear extracts interacted with this specific sequence in electrophoretic mobility shift assays. Mutation of the NF-kappaB site caused an increase in NGF-induced promoter activation, whereas overexpression of p49 in NGF-differentiated PC12 cells caused a decrease in endogenous ChAT enzyme activity and a decrease in promoter activity that was specifically mediated through this NF-kappaB binding site. Treatment of PC12 cells with NGF resulted in a drastic reduction in nuclear p49 binding to the ChAT NF-kappaB site after 24 h, but nuclear p49 levels were not altered, suggesting that late NGF-mediated events prevent binding of p49 to the ChAT promoter by an unknown mechanism other than nuclear translocation. Decreased ChAT expression and increased NF-kappaB activity in the brain are associated with aging and Alzheimer's disease. These data indicate that p49 is a negative regulator of ChAT expression and suggest a possible mechanism for aging-associated declines in cholinergic function.

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.

NGF-mediated alteration of NF-kappaB binding activity after partial immunolesions to rat cholinergic basal forebrain neurons

There are age-associated cognitive and cholinergic deficits in the neurotrophin-dependent cholinergic basal forebrain neurons (CBFNs). There are also increases in the activity of the transcription factor NF-kappaB in the aged rodent brain that may reflect chronic enhancement of stress response signaling. We used partial immunolesions (PIL) to CBFN to examine the role of endogenous NGF on choline acetyltransferase (ChAT) activity and NGF-mediated NF-kappaB alteration after cholinergic deafferentation. We injected 192 IgG-saporin, an immunotoxin selectively taken up by neurotrophin receptor p75(NTR)-bearing neurons, into lateral ventricles, followed by infusions of anti-NGF to assess NF-kappaB, ChAT and NGF responses to PIL after anti-NGF infusion. Treatment with anti-NGF decreased ChAT activity by 17-34% in the cortex, hippocampus, and olfactory bulb and PIL decreased ChAT activity by 47-73%. Changes in AChE activity levels paralleled those observed for ChAT after PIL. NGF protein levels in the olfactory bulb, but not the cortex or hippocampus, increased significantly after PIL treatment. Infusion of anti-NGF abolished the PIL-induced eight-fold NGF increase in CNS. NF-kappaB binding activity to the IgG-kappaB and ChAT specific NF-kappaB consensus sequences, increased in the cortex but not hippocampus after PIL followed by anti-NGF infusion. It is likely that immunolesion-induced changes in ambient NGF levels may perturb NF-kappaB activity.

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.

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.

NGF-induction of the expression of ChAT mRNA in PC12 cells and primary cultures of embryonic rat basal forebrain

The objective of this study was to examine the role of nerve growth factor (NGF) in regulation of expression of the cholinergic phenotype. NGF was administered to PC12 cells or primary cultures of embryonic (E17) rat basal forebrain for 2 days, then steady-state levels of choline acetyltransferase (ChAT) mRNA was monitored. Expression of ChAT mRNA isoforms was investigated using reverse transcription-polymerase chain reaction (RT-PCR) to amplify different upstream regions of the ChAT transcripts, and Southern blot analysis was used to verify identity of the PCR products. An NGF-induced increase of 1.8- and 1.5-fold in steady-state level of the ChAT transcript containing the M-exon (M-ChAT) was observed in PC12 cells and embryonic rat basal forebrain neurons, respectively. Also, a 2-fold increase in ChAT protein as determined by western blot analysis was associated with an NGF-mediated increase of 1.7-fold in ChAT activity in rat basal forebrain neurons within the same cultures following 4 days of NGF treatment.

Molecular mechanisms regulating NGF-mediated enhancement of cholinergic neuronal phenotype: c-fos trans-activation of the choline acetyltransferase gene

Nerve growth factor (NGF) enhances expression of the cholinergic phenotype observed as increased choline acetyltransferase (ChAT) activity, immunoreactivity, and mRNA. In the present study, treatment of cultured rat embryonic basal forebrain neurons with anti-c-fos, prior to administering NGF, blocked NGF-mediated increases in ChAT activity by 67%; basal ChAT activity was not affected by the antisense oligonucleotide treatment. Reverse transcription-polymerase chain reaction (RT-PCR) revealed that anti-c-fos treatment resulted in not only blockade but enhancement of steady-state ChAT mRNA at different time points. These data suggest that c-fos is an important component in NGF-mediated changes in the cholinergic phenotype and support the hypothesis that c-fos plays a role in the regulation of transcription of the ChAT gene. Elucidation of mechanisms underlying this regulation may aid drug development in neurodegenerative disease.

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 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.

Upstream sequencing and functional characterization of the human cholinergic gene locus

The 5' flanking region of the human VAChT gene was sequenced to approx 5350 bases upstream of the initiating methionine codon of the VAChT open reading frame (orf). The 5' flanks of the human and rat cholinergic gene loci were compared to identify regions of local sequence conservation, and therefore of potential regulatory importance. Several discrete domains of high homology, including a cluster of far-upstream cis-active consensus motifs, a neuronally restrictive silencer element consensus sequence, and additional conserved sequences within the putative nerve growth factor response domain of the locus, were identified. The probable start of transcription of the VAChT gene was deduced from mapping of sequences of rat and human VAChT cDNAs onto the 5' flanking regions of the human and rat cholinergic gene loci. The actual utilization of a putative 5' VAChT exon in rat central nervous system (CNS) tissue was assessed by in situ hybridization histochemistry. RNA transcripts containing both VAChT and ChAT protein-coding sequences were abundant in spinal cord motoneurons, sympathetic preganglionic cells, basal forebrain, striatum, and cranial motor nuclei. R-exon-containing transcripts could be detected only at low levels in these cell groups, implying that most transcription of VAChT proceeds from a promoter downstream of the R-exon. To assess the structural requirements for expression of the VAChT gene without bias regarding the actual start of transcription, a 5' fragment of the human gene corresponding to approximately 3 kb of sequence extending upstream from within the presumed 5' untranslated region of VAChT itself was fused to a luciferase-encoding reporter and transfected into VAChT-expressing and nonexpressing human and rat cell lines. This portion of the VAChT gene provided strong promoter expression in both cholinergic and noncholinergic cell lines. Deletion of the putative neuronally restrictive silencer element (NRSE) resulted in enhanced transcription in all cell lines. Lack of differential expression of VAChT transcription in VAChT-expressing vs non-VAChT-expressing cell lines suggested that additional enhancer elements controlling cell-specific expression of the VAChT gene exist further upstream in the cholinergic locus 5' flank. Conservation of potential cis-active elements within a 1.4 kb sequence immediately upstream of the NRSE in both rat and human cholinergic gene loci suggests that this domain is required for cholinergic-specific regulation of VAChT and ChAT gene transcription.

Transcriptional activation of human choline acetyltransferase by AP2- and NGF-induced factors

ChAT (choline acetyltransferase) is the enzyme responsible for acetylcholine synthesis and is specifically expressed in cholinergic neurons. To further characterize the transcriptional regulation of the hCHAT (human ChAT) gene by NGF, we examined the effects upon ChAT promoter activity of a family of transcription factors which are activated by NGF and several extracellular stimuli and encoded by immediate-early genes. These include NGFI-A (Egr1, zif268), NGFI-C (Egr2), Krox-20 and NGFI-B (Nurr77). Two fragments of the hChAT gene were used for functional analysis carrying 944 bp (P1) and 4000 bp (P1 + P2) of the 5' flanking region in front of the chloramphenicol acetyltransferase (CAT) reporter gene. They were transiently co-transfected with NGFI-A, NGFI-C, Krox-20 and NGFI-B expression vectors in NG108-15, SN6 and COS-1 cells. CAT activity after transfection of the p4000 ChAT-CAT reporter into both neuronal cell lines (NG108-15 and SN6 cells) was increased up to 5-fold in the presence of co-transfected NGFI-A and up to 5- and 12-fold after co-transfection of NGFI-C expression vector in NG108-15 and SN6 cells, respectively. In NG108-15 cells, dbcAMP excerted a strong enhancing activity on the transactivation properties of NGFI-C while this was not observed when cells were transfected with NGFI-A. These trans-activation effects were specific for neuronal cells. When NG108-15 cells were treated with dbcAMP in the presence of H89, a specific PKA inhibitor, the increase of transcriptional activity of NGFI-C was abolished, indicating that a signalling transduction mechanism through PKA plays a role in NGFI-C-induced trans-activation. Electrophoretic mobility-shift assays showed that the sequence GCCCGGGGAG (NGFRE) located 1205 bp upstream of the first coding ATG (E1) can bind NGFI-A but not NGFI-C. Several possibilities explaining the observed results are discussed. Finally, transfections of ChAT-CAT reporters including the P1 + P2 region or a minimal ChAT enhancer present in the P2 region in front of a heterologous promoter indicated the presence of a regulatory element which conferred AP2-dependent trans-activation with homologous as well as with heterologous promoter constructs.

NGF and BDNF increase the immunoreactivity of vesicular acetylcholine transporter in cultured neurons from the embryonic rat septum

The expression of vesicular acetylcholine transporter (VAChT), which transports ACh into synaptic vesicles, is coregulated with choline acetyltransferase (ChAT). Therefore, the effects of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) on the levels of VAChT in cultured neurons from the septum of embryonic rats were investigated by immunocytochemistry. NGF and BDNF increased the number of VAChT-immunoreactive neurons by approximately 1.5-fold and enhanced the immunoreactivity in each positive cell. These results suggest that the neurotrophins enhance not only synthesis but also storage of ACh in septal neurons.

Cell type-specific regulation of choline acetyltransferase gene expression. Role of the neuron-restrictive silencer element and cholinergic-specific enhancer sequences

This study demonstrates the presence of positive and negative regulatory elements within a 2336-base pair-long region of the rat choline acetyltransferase (ChAT) gene promoter that cooperate to direct cell type-specific expression in cholinergic cells. A 21-base pair-long neuron-restrictive silencer element (NRSE) was identified in the proximal part of this region. This element was recognized by the neuron-restrictive silencer factor (NRSF), previously shown to regulate expression of other neuron-specific genes. The ChAT NRSE was inactive in both cholinergic and non-cholinergic neuronal cells, but repressed expression from a heterologous promoter in non-neuronal cells. Specific deletion of this element allowed ChAT gene promoter activity in non-neuronal cells, and overexpression of NRSF repressed ChAT gene promoter activity in cholinergic cells. The distal part of the ChAT gene promoter showed cholinergic-specific enhancing activity, which stimulated promoter activity in cholinergic cells, but was inactive in non-cholinergic neuronal and non-neuronal cells. This enhancer region suppressed the activity of the ChAT NRSE in cholinergic cells, even after NRSF overexpression. Thus, at least two kinds of regulatory elements cooperate to direct ChAT gene expression to cholinergic neurons, namely a neuron-restrictive silencer element and a cholinergic-specific enhancer.

Alterations of cAMP response element-binding activity in the aged rat brain in response to administration of rolipram, a cAMP-specific phosphodiesterase inhibitor

Transcription factor, cAMP response element-binding protein (CREB), which is phosphorylated by cAMP-dependent kinase via an increase in cAMP, and regulates gene transcription by binding to the cAMP response element (CRE) on target genes. We examined age-dependent alterations in the DNA-binding activity of CREB in rat brain regions, and the effects of rolipram, a cAMP-specific phosphodiesterase (PDE) inhibitor on the CRE-binding activity by electrophoretic mobility-shift assay (EMSA). A marked age-dependent decrease in the CRE-binding activity was shown in all brain regions examined, especially in the basal forebrain, the striatum and the hippocampus. Furthermore, CRE-binding activities in the basal forebrain of both young-adult and aged rats significantly increased 2 h after rolipram administration (1 mg/kg, i.p.), and the rolipram treatment recovered the decreased CRE-binding activity in the aged rats. The saturation experiment in EMSA also revealed that rolipram reversed the decrease in the maximum CRE-bindings in the basal forebrain with aging. Since the 5' upstream region of the rat choline acetyltransferase (ChAT) gene contains CRE, and ChAT-positive neurons in the basal forebrain project to the frontal cortex and the hippocampus, rolipram may exert its previously reported ameliorating effect on the age-related reductions of ChAT activities in the frontal cortex and the hippocampus by phosphorylating CREB in the basal forebrain with activation of cAMP-dependent protein kinase via inhibition of PDE.

Developmental age-dependent upregulation of choline acetyltransferase and vesicular acetylcholine transporter mRNA expression in neonatal rat septum by nerve growth factor

We examined the effect of intraventricular injection of nerve growth factor (NGF) on the choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) mRNA expression in the septa of neonatal rats. Rat pups were injected with 2.5 S NGF or cytochrome-c (control) on postnatal days (PN) 4 and 18, and sacrificed 3 days after injections for analysis of ChAT and VAChT mRNA levels by dot-blot hybridization of total septal RNA. In the NGF-treated pups, the ChAT and VAChT mRNA levels were elevated 3- and 2-fold, respectively, at PN7, and 1.8- and 1.3-fold at PN21. These results indicate that (1) NGF upregulates the expression of both ChAT and VAChT genes, (2) NGF has a greater effect on the expression of ChAT mRNA than VAChT mRNA, and (3) the effect of exogenous NGF on the expression of both genes diminishes with developmental age.

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

The proteins responsible for acetylcholine (ACh) synthesis (choline acetyltransferase, ChAT) and storage (vesicular ACh transporter, VAChT) are encoded by two closely linked genes in vertebrates, with the VAChT coding sequence contained within the first intron of the ChAT gene. This unusual genomic organization suggests that the transcription of these two genes is coordinately regulated. Using Northern analysis we studied the modulation of ChAT and VAChT expression in a murine septal cell line (SN56) by three groups of agents: retinoids, trophic factors belonging to the leukemia inhibitory factor/ciliary neurotrophic factor (LIF/CNTF) family, and cAMP. All-trans-retinoic acid increased both ChAT and VAChT mRNA levels in SN56 cells up to 3.5-fold, and elevated intracellular ACh levels by 2.5-fold. This effect was mimicked by a retinoic acid receptor alpha (RAR alpha) agonist (Ro 40-6055) and prevented by a specific antagonist (Ro 41-5253), indicating that it was mediated by RAR alpha. ChAT- and VAChT-specific transcripts were also induced (up to 3-fold) by treatment with CNTF or LIF (20 ng/ml, 48 h), as well as by dibutyryl cAMP (1 mM). All these agents increased the ACh level in the cells (up to 2.5-fold). Dibutyryl cAMP had a greater effect on the level of VAChT mRNA (4-fold induction) than on the level of ChAT mRNA (2-fold induction), suggesting a quantitatively differential transcriptional regulation of the two genes by the cAMP pathway. The effects of the three groups of agents studied on ChAT and VAChT mRNA levels were additive, pointing to several independent mechanisms by which the cholinergic properties of septal neurons can be modulated.

Molecular biology of the vesicular ACh transporter

The cholinergic synapse has long been a model for biochemical studies of neurotransmission. The molecules that are responsible for synaptic transmission are being identified rapidly. The vesicular transporter for ACh, which is responsible for the concentration of ACh within synaptic vesicles, has been characterized recently, both at the molecular and functional level. Definitive identification of the cloned gene involved genetics of Caenorhabditis elegans, the specialized Torpedo electromotor system, and expression in mammalian tissue culture. Comparison of the vesicular transporter for ACh with the vesicular transporters for monoamines demonstrates a new gene family. Gene mapping has demonstrated a unique relationship between the genes for the vesicular ACh transporter and for choline acetyltransferase.

Human choline acetyltransferase gene: localization of alternative first exons

Two overlapping cosmids containing the 5' end of human choline acetyltransferase (ChAT) gene have been cloned. Using heterologous probes, we localized two alternative first exons homologous to rodent ChAT exons R and M (Misawa et al.: J Biol Chem 267:20392-20399, 1992). The sequence of rodent exon N was not conserved in the human gene. Northern blot analysis of mRNA purified from the human neuroepithelioma cell lines LA-N2 and MC-I-XC revealed that both exons R and M were transcribed in mRNA species of 6.0 and 2.5 kb. Only the 6-kb species was detected with both R- and M-specific probes in the neuroepithelioma cell line CHP126. Reverse transcription-polymerase chain reaction (RT-PCR) analysis suggested that the major mRNA species in MC-I-XC and CHP126 cells contained the proximal part of exon M spliced to exon 1, which contains the alternative ACG initiation codon. RT-PCR also allowed the characterization of a mRNA species containing exon R spliced to exon 1, but no species containing both exon R and the distal part of exon M could be detected. RT-PCR was also used to evidence an alternative exon (tentatively numbered exon 8) in the coding sequence.

Positive and negative effects of nuclear receptors on transcription activation by AP-1 of the human choline acetyltransferase proximal promoter

We have examined the 5'-flanking region (944 bp) of the human choline acetyltransferase (hChAT) gene for sequences that modulate its transcriptional activity and identified a sequence 5'-TGACCCA-3' which confers c-Jun/c-Fos (AP-1) inducibility of homologous and heterologous promoters. Using transient transfections in neuroblastoma NE-1-115 and COS-1 cells, we show that ligand-activated estrogen receptor (HEGo) represses the transcriptional activation by c-Fos/c-Jun. Testing HEGo mutants in transfection assays reveals that the ligand-binding domain is crucial for this repression, whereas the N-terminal (A/B) region and the DNA-binding domain are not essential. Gel retardation assays show that the hChAT AP-1 recognition sequence binds in vitro baculovirus-produced c-Jun/c-Fos proteins. This binding is inhibited by addition of baculovirus-produced HEGo. In contrast to HEGo, ligand-activated glucocorticoid, androgen, and retinoic acid receptors (RARs) enhance the transcription activation induced by c-Jun/c-Fos. All three types of RARs--RAR alpha, beta, gamma--and RXR alpha are able to stimulate AP-1 activity on the proximal hChAT promoter. Several mechanism possibilities involving protein-protein interaction are discussed to explain the phenomena.

The use of neuronal networks on multielectrode arrays as biosensors

Mammalian spinal neuronal networks growing on arrays of photoetched electrodes in culture provide a highly stable system for the long-term monitoring of multichannel, spontaneous or evoked electrophysiological activity. In the absence of the homeostatic control mechanisms of the central nervous system, these networks show remarkable sensitivities to minute chemical changes and mimic some of the properties of sensory tissue. These sensitivities could be enhanced by receptor up-regulation and altered by the expression of unique receptors. The fault-tolerant spontaneous network activity is used as a dynamic platform on which large changes in activity signify detection of chemical substances. We present strategies for the expression of novel supersensitivities to foreign molecules via genetic engineering that involves the grafting of ligand binding cDNA onto truncated native receptor DNA and the subsequent expression of such chimeric receptors.

Choline acetyltransferase: celebrating its fiftieth year

It is well known that the regulation of choline acetyltransferase (ChAT) activity under physiological and pathological conditions is important for the development and neuronal activities of cholinergic systems involved in many fundamental brain functions. This review focuses on recent progress in understanding the regulation of ChAT at the levels of both the protein and the mRNA. A deficiency in ChAT activity has been reported for neurodegenerative conditions such as Alzheimer's disease, amyotrophic lateral sclerosis, and schizophrenia. Although a major feature of ChAT regulation is likely to involve the spatial and temporal control of transcription, regulation of expression can also be at the level of RNA processing, transport/translocation, turnover, or translation. In addition, there is increasing evidence that ChAT might be regulated at the posttranslational level by compartmentation and/or covalent modification, i.e., phosphorylation, as well as noncovalent modification (protein-protein interaction, etc.). Synaptic activity and the state of neuronal transmission may also involve the regulation of ChAT at different levels via both positive and negative feedback loops, as was demonstrated in the characterization of two ChAT mutant Drosophila strains. Clearly, identification of cholinergic-specific elements and the characterization of the trans-acting factors that bind to them represent an important area of future research. Equally important is research on the mechanisms governing ChAT as an enzymatic entity. The future should be an exciting time during which we look forward to the elucidation of the cholinergic signal and its regulation as well as the determination of the three-dimensional structure of the enzyme.

Trans-activation by thyroid hormone receptors of the 5' flanking region of the human ChAT gene

Fusion gene constructs containing the human choline acetyltransferase 5' flanking region are stimulated by thyroid hormone (T3) in neuronal NG108-15 and NE1-115 cells but not in non neuronal COS-1 and JEG-3 cells. To identify potential T3 receptor binding elements (T3RE), chimeric plasmids containing various lengths of the 5' end of the hChAT gene linked to the CAT reporter gene were assayed by transient transfections into NG108-15, NE1-115 and COS-1 cells. We show that regulation is T3 specific as estrogen, dexamethasone, dihydrotestosterone, all-trans-retinoic acid and 9-cis-retinoic acid have no effect. We localized several potential T3REs and characterized the most proximal T3RE (position 3280-3291) which contains two hexameric half-sites arranged as a direct repeat without a base pair spacer. An oligonucleotide containing this sequence confers T3 responsiveness to a heterologous promoter. The transcriptional response of this T3RE is markedly reduced after mutation of the first or second half-site indicating that both half-sites are required for a maximal T3 response. We have found that RAR alpha, RXR alpha and COUP-TF do not enhance T3 responsiveness and therefore they may not interact with T3R alpha in NG108-15 cells on this regulatory sequence. T3R monomer and dimer specific binding to the proximal T3RE is demonstrated by gel-retardation DNA binding assays and by methylation interference experiments. In COS-1 cells, T3R inhibits transcriptional activation by the transcription factor AP-1 whereas in NE1-115 cells T3R enhances AP-1 mediated activation in a T3 dependant fashion. It is likely that these effects involve protein-protein interactions. These results suggest that the T3 receptor can act as a positive transcriptional regulatory factor on the hChAT gene.

Distribution of the vesicular acetylcholine transporter (VAChT) in the central and peripheral nervous systems of the rat

Expression of the acetylcholine biosynthetic enzyme choline acetyltransferase (ChAT), the vesicular acetylcholine transporter (VAChT), and the high-affinity plasma membrane choline transporter uniquely defines the cholinergic phenotype in the mammalian central (CNS) and peripheral (PNS) nervous systems. The distribution of cells expressing the messenger RNA encoding the recently cloned VAChT in the rat CNS and PNS is described here. The pattern of expression of VAChT mRNA is consistent with anatomical, pharmacological, and histochemical information on the distribution of functional cholinergic neurons in the brain and peripheral tissues of the rat. VAChT mRNA-containing cells are present in brain areas, including neocortex and hypothalamus, in which the existence of cholinergic neurons has been the subject of debate. The demonstration that VAChT is a completely adequate marker for cholinergic neurons should allow the systematic delineation of cholinergic synapses in the rat nervous system when antibodies directed to this protein are available.

Multiple promoters of human choline acetyltransferase and aromatic L-amino acid decarboxylase genes

The promoter regions of human choline acetyltransferase (ChAT) and aromatic L-amino acid decarboxylase (AADC) genes have been analyzed by transient transfection assays. AADC gene is transcribed from two alternative noncoding first exons, 1N and 1NN, expressed in pheochomocytoma and hepatoma cells, respectively. 5' flanking sequences of exon 1 N (from 9000 to 147 bp) display promoter activity in SK-N-BE neuroblastoma cells, but not in MC-I-XC cholinergic neuroepithelioma cells, and in AADC-rich non-neuronal cells. On the contrary, 5' flanking sequences of exon 1 NN (from 1117 to 119 bp) display high promoter activity in human hepatoma cells HepG2, but not in SK-N-BE cells, suggesting high degrees of specificity of promoters N and NN for AADC-expressing neuronal and non-neuronal cells, respectively. Preliminary evidence suggests that leukemia inhibitory factor suppresses the activity of the neuronal promoter in cultured sympathetic neurons. Two alternative first exons, R and M, have been localized in human ChAT gene, and the corresponding promoters characterized in cholinergic PC12 and NG-108-15 cells, and in non-cholinergic neuro2A cells. Several positively or negatively acting cis elements have been localized in the two promoters, as well as a cAMP-inducible, enhancer-like element in the second intron. Among the various cell lines studied, there was no correlation between promoter activities and the expression of the endogenous ChAT gene, suggesting that the fine-tuning of ChAT gene expression is controlled by silencer elements which remain to be localized.

Comparison of the promoter region of the human and porcine choline acetyltransferase genes: localization of an important enhancer region

Genomic clones of human and porcine choline acetyltransferase were obtained by screening genomic libraries with synthetic oligonucleotides. The human and porcine genes exhibit significant conservation of both their intron/exon structure and the nucleotide sequence in their 5' flanking regions. However, the two genes differ in several respects, including the absence of a "TATA" box in the human gene and differences in the position of the methionine start codon. Analysis of the promoter region of the two genes has led to the localization of an enhancer element that appears necessary for efficient transcription of the gene.

Retinoic acid induces cholinergic differentiation of cultured newborn rat sympathetic neurons

Many studies provide evidence that retinoic acid (RA), an endogenous derivative of vitamin A, plays a role in the development of the nervous system. We now report that RA controls the neurotransmitter phenotype of post-mitotic rat sympathetic neurons in cell culture. RA added to the culture medium increased the specific activity of choline acetyltransferase (ChAT) and the level of acetylcholine (ACh). Concomitantly, RA reduced the specific activities of two catecholamine synthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) and the level of norepinephrine (NE). After a 2 week treatment with 5 microM RA, ChAT was increased by 5-10 fold, whereas TH and DBH were decreased by 10-15 fold and 2-3 fold, respectively, as compared to sympathetic neurons grown in the absence of RA. The modulation of the activity of the three enzymes was dose-dependent and followed a similar time course. The decrease of TH expression was demonstrated to be due to a decreased number of TH molecules.

Multiple mRNA species of choline acetyltransferase from rat spinal cord

A cDNA library directed by a specific primer was constructed from the rat spinal cord and screened with 32P-labeled rat choline acetyltransferase cDNA which was recently isolated in this laboratory. Sequence analysis of 29 clones indicated that there are four types of cDNA (R1-, R2-, N1- and M-types). The nucleotide sequences in these cDNAs were identical in the coding region and the first 38 bp of the 5'-noncoding region, but differed in the 5'-noncoding region upstream of -38 bp. The R1-type was identical to the cDNA previously cloned from the rat spinal cord. The M and N1-type cDNAs both had sequences homologous to that of the cDNA previously obtained from the mouse spinal cord. Polymerase chain reaction analysis confirmed the presence of these 4 types of mRNA and found another type (N2-type) of transcript. The numbers of cDNA clones isolated and the relative amounts of polymerase chain reaction products for each type of mRNA suggested that the most abundant transcript was M-type. Sequencing of the genomic clone containing the 5'-region of choline acetyltransferase mRNA revealed that these five types of mRNA species were transcribed from three different promoter regions and produced by differential splicing of the 5'-noncoding exons.

Partial cloning of the rat choline acetyltransferase gene and in situ localization of its transcripts in the cell body of cholinergic neurons in the brain stem and spinal cord

We have isolated recombinant lambda (lambda) phages which contain a part of the rat choline acetyltransferase (ChAT) gene. Restriction and Southern blot analyses using synthetic oligonucleotides indicate that these clones overlap one another and contain at least four exons which reside in 16.4 kb of sequence encoding from the middle to the 3' end, but not the 5'-region, of the rat ChAT gene. Partial sequence analyses revealed that the clones contain an exon whose nucleotide sequence corresponds to a highly conserved region of ChAT during evolution. RNase protection mapping experiments show that sequences represented by this exon are expressed at high levels in the spinal cord of adult rats and at low but detectable levels in PC12 cells. By using the genomic sequences, including the exon, as a hybridization probe, we have detected ChAT mRNAs in situ in rat tissues. In situ hybridization experiments using radioactive and non-radioactive probes revealed that cholinergic motoneurons in the spinal cord, the laterodorsal tegmental nucleus as well as the hypoglossal nucleus in the brain stem were labeled, suggesting that the genomic sequence can be used as a probe to measure the ChAT mRNA levels in those cholinergic neurons. The results also indicate that the non-radioactive method gives a better resolution in localizing the expression of ChAT transcripts in the cytoplasm of cholinergic neurons.