Regulation of acetylcholinesterase mRNA stability by calcium during differentiation from myoblasts to myotubes

Tau phosphorylation by glycogen synthase kinase 3β modulates enzyme acetylcholinesterase expression

In Alzheimer's disease (AD), the enzyme acetylcholinesterase (AChE) co‐localizes with hyperphosphorylated tau (P‐tau) within neurofibrillary tangles. Having demonstrated that AChE expression is increased in the transgenic mouse model of tau Tg‐VLW, here we examined whether modulating phosphorylated tau levels by over‐expressing wild‐type human tau and glycogen synthase kinase‐3β (GSK3β) influences AChE expression. In SH‐SY5Y neuroblastoma cells expressing higher levels of P‐tau, AChE activity and protein increased by (20% ± 2%) and (440% ± 150%), respectively. Western blots and qPCR assays showed that this increment mostly corresponded to the cholinergic ACHE‐T variant, for which the protein and transcript levels increased ~60% and ~23%, respectively. Moreover, in SH‐SY5Y cells differentiated into neurons by exposure to retinoic acid (10 µM), over‐expression of GSK3β and tau provokes an imbalance in cholinergic activity with a decrease in the neurotransmitter acetylcholine in the cell (45 ± 10%). Finally, we obtained cerebrospinal fluid (CSF) from AD patients enrolled on a clinical trial of tideglusib, an irreversible GSK3β inhibitor. In CSF of patients that received a placebo, there was an increase in AChE activity (35 ± 16%) respect to basal levels, probably because of their treatment with AChE inhibitors. However, this increase was not observed in tideglusib‐treated patients. Moreover, CSF levels of P‐tau at the beginning measured by commercially ELISA kits correlated with AChE activity. In conclusion, this study shows that P‐tau can modulate AChE expression and it suggests that AChE may possibly increase in the initial phases of AD.

The NMJ as a model synapse: New perspectives on formation, synaptic transmission and maintenance: Acetylcholinesterase at the neuromuscular junction

Acetylcholinesterase (AChE) is an essential enzymatic component of the neuromuscular junction where it is responsible for terminating neurotransmission by the cholinergic motor neurons. The enzyme at the neuromuscular junction (NMJ) is contributed primarily by the skeletal muscle where it is produced at higher levels in the post-synaptic region of the fibers. The major form of AChE at the NMJ is a large asymmetric form consisting of three tetramers covalently attached to a three-stranded collagen-like tail which is responsible for anchoring it to the synaptic basal lamina. Its location and expression is regulated to a large extent by the motor neurons and occurs at the transcriptional, translational and post-translational levels. While its expression can be quite rapid in tissue cultured cells, its half-life in vivo appears to be quite long, about three weeks, although more rapidly turning over pools have been described. Finally the essential nature of this enzyme is underscored by the fact that no naturally occurring null mutations of the catalytic subunit have been described in higher organisms and the few dozen humans carrying mutations in the collagen tail responsible for anchoring the enzyme at the NMJ are severely affected.

Altered expression of brain acetylcholinesterase in FTDP-17 human tau transgenic mice

Pathological hyperphosphorylation and aggregation of the tau protein is associated with dementia and can be the central cause of neurodegeneration. Here, we examined potential alterations in the level of the cholinergic enzyme acetylcholinesterase (AChE) in the brain of transgenic mice (Tg-VLW) expressing human tau mutations. Overexpression of mutant hyperphosphorylated tau (P-tau) led to an increase in the activity of AChE in the brain of Tg-VLW mice, paralleled by an increase in AChE protein and transcripts; whereas the levels of the enzyme choline acetyltransferase remained unaffected. VLW tau overexpression in SH-SY5Y cells also increased AChE activity levels. All major molecular forms of AChE were increased in the Tg-VLW mice, including tetrameric AChE, which is the major species involved in hydrolysis of acetylcholine in the brain. Colocalization of human P-tau and AChE supports the conclusion that P-tau can act to increase AChE. This study is the first direct evidence of a modulatory effect of P-tau on brain AChE expression.

Calcium signaling-induced Smad3 nuclear accumulation induces acetylcholinesterase transcription in apoptotic HeLa cells

Recently, acetylcholinesterase (AChE) has been studied as an important apoptosis regulator. We previously showed that cellular calcium mobilization upregulated AChE expression by modulating promoter activity and mRNA stability. In this study, we have identified a potential Smad3/4 binding element, TGCCAGACA, located within the -601 to -571 bp fragment of the AChE promoter, as an important calcium response motif. Smad2/3 and Smad4 were shown to bind this element. Overexpression of human Smad3 increased AChE transcription activity in a dose-dependent manner in HeLa cells, whereas dominant-negative Smad3 blocked this activation. Upon A23187 and thapsigargin treatment, nuclear Smad3 accumulation was observed, an effect that was blocked by the intracellular Ca(2+) chelator BAPTA-AM. Calcium-induced AChE transcriptional activation was significantly blocked when the nuclear localization signal of Smad3 was destroyed. Taken together, our data suggest Smad3 can regulate AChE transcriptional activation following calcium-induced nuclear accumulation.

Dysregulation of calcium homeostasis in Alzheimer's disease

The accumulation of oligomeric species of beta-amyloid protein in the brain is considered to be a key factor that causes Alzheimer's disease (AD). However, despite many years of research, the mechanism of neurotoxicity in AD remains obscure. Recent evidence strongly supports the theory that Ca2+ dysregulation is involved in AD. Amyloid proteins have been found to induce Ca2+ influx into neurons, and studies on transgenic mice suggest that this Ca2+ influx may alter neuronal excitability. The identification of a risk factor gene for AD that may be involved in the regulation of Ca2+ homeostasis and recent findings which suggest that presenilins may be involved in the regulation of intracellular Ca2+ stores provide converging lines of evidence that support the idea that Ca2+ dysregulation is a key step in the pathogenesis of AD.

Multiple cascade effects of oxidative stress on astroglia

Many neurodegenerative diseases share common underlying features, most prominent of which are dysregulation of calcium homeostasis and reactive astrogliosis, ultimately triggered by oxidative stress. Interestingly, an additional feature of the early response to stress conditions is the upregulation and release of acetylcholinesterase (AChE). This study therefore investigates the link between oxidative stress, calcium influx, gene expression, protein synthesis, and AChE release. We report that, in astroglia and in an immortalized cell line, GH4-halpha7, acute oxidative stress causes influx of extracellular calcium through L-type voltage-gated calcium channels (L-VGCC), followed by increased release of AChE into the extracellular medium. Moreover, rapid and sustained changes in mRNA expression of AChE, L-VGCC, and melastatin-like transient receptor potential 2 (TRPM2) accompany profound suppression of global protein synthesis. Application of L-VGCC blockers selectively reduces stress-induced calcium influx and AChE release, mitigates changes in gene expression, and facilitates recovery from protein synthesis suppression. Although glia exhibit greater sensitivity in their responses, the results are comparable in astroglia and GH4-halpha7 cells, and suggest a generalized and integrated cellular response to stress conditions that characterizes changes observed in neurodegeneration.

The CCAAT-binding factor CBF/NF-Y regulates the human acetylcholinesterase promoter activity during calcium ionophore A23187-induced cell apoptosis

We previously reported that the expression of acetylcholinesterase during A23187-induced apoptosis of HeLa cells is regulated by Ca(2+) mobilization through the modulation of mRNA stability and acetylcholinesterase promoter activity. Transactivation of the human acetylcholinesterase promoter by A23187 was partially mediated by the distal CCAAT motif within the -1270 to -1248 fragment of the human acetylcholinesterase promoter, which was bound by the CCAAT binding factor (CBF/NF-Y). In the present study, we investigated the molecular mechanisms by which CBF/NF-Y regulates A23187-induced activation of the human acetylcholinesterase promoter. The results indicate that CBF/NF-Y binding to the distal CCAAT motif suppresses the promoter activity. Electrophoretic mobility shift assays (EMSAs) demonstrated that binding of CBF/NF-Y to the distal CCAAT motif decreased after A23187 treatment. Our results suggest that acetylcholinesterase promoter activation during A23187-induced HeLa cell apoptosis may result partly from the dissociation of CBF/NF-Y from the distal CCAAT motif in the acetylcholinesterase promoter, reversing this suppression.

Calcineurin mediates acetylcholinesterase expression during calcium ionophore A23187-induced HeLa cell apoptosis

We previously reported that acetylcholinesterase plays a critical role in apoptosis and its expression is regulated by Ca(2+) mobilization. In the present study, we show that activated calpain, a cytosolic calcium-activated cysteine protease, and calcineurin, a calcium-dependent protein phosphatase, regulate acetylcholinesterase expression during A23187-induced apoptosis. The calpain inhibitor, calpeptin, and the calcineurin inhibitors, FK506 and cyclosporine A, inhibited acetylcholinesterase expression at both mRNA and protein levels and suppressed the activity of the human acetylcholinesterase promoter. In contrast, overexpression of constitutively active calcineurin significantly activated the acetylcholinesterase promoter. Furthermore, we identify a role for the transcription factor NFAT (nuclear factor of activated T cells), a calcineurin target, in regulating the acetylcholinesterase promoter during ionophore-induced apoptosis. Overexpression of human NFATc3 and NFATc4 greatly increased the acetylcholinesterase promoter activity in HeLa cells treated with A23187. Overexpression of constitutive nuclear NFATc4 activated the acetylcholinesterase promoter independent of A23187, whereas overexpression of dominant-negative NFAT blocked A23187-induced acetylcholinesterase promoter activation. These results indicate that calcineurin mediates acetylcholinesterase expression during apoptosis.

Regulation of acetylcholinesterase expression by calcium signaling during calcium ionophore A23187- and thapsigargin-induced apoptosis

We have recently reported that acetylcholinesterase expression was induced during apoptosis in various cell types. In the current study we provide evidence to suggest that the induction of acetylcholinesterase expression during apoptosis is regulated by the mobilization of intracellular Ca(2+). During apoptosis, treatment of HeLa and MDA-MB-435s cells with the calcium ionophore A23187 resulted in a significant increase in acetylcholinesterase mRNA and protein levels. Chelation of intracellular Ca(2+) by BAPTA-AM (1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester), an intracellular Ca(2+) chelator, inhibited acetylcholinesterase expression. A23187 also enhanced the stability of acetylcholinesterase mRNA and increased the activity of acetylcholinesterase promoter, effects that were blocked by BAPTA-AM. Perturbations of cellular Ca(2+) homeostasis by thapsigargin resulted in the increase of acetylcholinesterase expression as well as acetylcholinesterase promoter activity during thapsigargin induced apoptosis in HeLa and MDA-MB-435s cells, effects that were also inhibited by BAPTA-AM. We further demonstrated that the transactivation of the human acetylcholinesterase promoter by A23187 and thapsigargin was partially mediated by a CCAAT motif within the -1270 to -1248 fragment of the human acetylcholinesterase promoter. This motif was able to bind to CCAAT binding factor (CBF/NF-Y). These results strongly suggest that cytosolic Ca(2+) plays a key role in acetylcholinesterase regulation during apoptosis induced by A23187 and thapsigargin.

Termination and beyond: acetylcholinesterase as a modulator of synaptic transmission

Termination of synaptic transmission by neurotransmitter hydrolysis is a substantial characteristic of cholinergic synapses. This unique termination mechanism makes acetylcholinesterase (AChE), the enzyme in charge of executing acetylcholine breakdown, a key component of cholinergic signaling. AChE is now known to exist not as a single entity, but rather as a combinatorial complex of protein products. The diverse AChE molecular forms are generated by a single gene that produces over ten different transcripts by alternative splicing and alternative promoter choices. These transcripts are translated into six different protein subunits. Mature AChE proteins are found as soluble monomers, amphipatic dimers, or tetramers of these subunits and become associated to the cellular membrane by specialized anchoring molecules or members of other heteromeric structural components. A substantial increasing body of research indicates that AChE functions in the central nervous system go far beyond the termination of synaptic transmission. The non-enzymatic neuromodulatory functions of AChE affect neurite outgrowth and synaptogenesis and play a major role in memory formation and stress responses. The structural homology between AChE and cell adhesion proteins, together with the recently discovered protein partners of AChE, predict the future unraveling of the molecular pathways underlying these multileveled functions.

The regulation of acetylcholinesterase by cis-elements within intron I in cultured contracting myotubes

The onset of spontaneous contraction in rat primary muscle cultures coincides with an increase in acetylcholinesterase (AChE) activity. In order to establish whether contractile activity modulates the rate of AChE transcript synthesis, and what elements of the gene are determinant, we examined the promoter and intron I in contracting muscle cultures. Ache genomic fragments attached to a luciferase reporter were transfected into muscle cultures that were either electrically stimulated or paralyzed with tetrodotoxin to enhance or inhibit contractions, respectively. Cultures transfected with intron I-containing constructs showed a 2-fold increase in luciferase activity following electrical stimulation, compared to tetrodotoxin treatment, suggesting that this region contains elements responding to contractile activity. Deleting a 780 bp distal region within intron I, containing an N-box element at +890 bp, or introducing a 2-bp mutation within its core sequence, eliminated the contraction-induced response. In contrast, mutating an N-box element at +822 bp had no effect on the response. Furthermore, co-transfecting a dominant negative GA-binding protein (GABP), a transcription factor known to selectively bind N-box elements, reduced the stimulation-mediated increase. Our results suggest that the N-box within intron I at +890 bp is a regulatory element important in the transcriptional response of Ache to contractile activity in muscle.

The RNA-binding protein HuR binds to acetylcholinesterase transcripts and regulates their expression in differentiating skeletal muscle cells

During myogenic differentiation, acetylcholinesterase (AChE) transcript levels are known to increase dramatically. Although this increase can be attributed in part to increased transcriptional activity, posttranscriptional mechanisms have also been implicated in the high levels of AChE mRNA in myotubes. In this study, we observed that transfection of a luciferase reporter construct containing the full-length AChE 3'-untranslated region (UTR) resulted in significantly higher (5-fold) luciferase activity in differentiated myotubes versus myoblasts. RNA-electrophoretic mobility shift assays (REMSAs) performed with a full-length AChE 3'-UTR probe and the AU-rich element revealed that the intensity of RNA-binding protein complexes increased as myogenic differentiation proceeded. Using several complementary approaches including supershift REMSA, mRNA-binding protein pull-down assays, and immunoprecipitation followed by reverse transcription-PCR, we found that the mRNA-stabilizing protein HuR interacts directly with AChE transcripts. Stable overexpression of HuR in C2C12 cells increased the expression of endogenous AChE transcripts as well as that of the luciferase reporter construct containing the AChE 3'-UTR. In vitro stability assays performed with protein extracts from these cells versus controls resulted in a slower rate of AChE mRNA decay. The down-regulation of HuR expression mediated through small interfering RNA further confirmed the role of HuR in the regulation of AChE mRNA levels. Taken together, these studies demonstrate that HuR interacts with the AChE 3'-UTR to regulate posttranscriptionally the expression of AChE mRNA during myogenic differentiation.

Acetylcholinesterase in Hirschsprung's disease

The association between the congenital absence of colonic ganglion cells and an increased acetylcholinesterase (AChE) expression in the affected tissue is of diagnostic importance in Hirschsprung's disease (HSCR). Investigation of AChE's function in development may also help unravel some of the complex pathophysiology in HSCR. Normal nerves do not stain for AChE, but increased AChE expression is associated with the hypertrophied extrinsic nerve fibres of the aganglionic segment in HSCR. Although a high degree of histochemical diagnostic accuracy exists, results are not always uniform, and false positives and false negatives are reported. False negative results are primarily related to age, and an absence of AChE reaction does not exclude HSCR in neonates within the first 3 weeks after birth. AChE staining results may lack uniformity, resulting in a number of technical modifications that have been made to improve standardization of AChE staining. At least two distinct histological patterns are described, types A and B. The interpretation of increased AChE staining patterns in ganglionated bowel at the time of surgical pull-through remains a problem in patients with HSCR. The development of rapid staining techniques has helped to identify normal ganglionated bowel with greater certainty. The presence of fine AChE neurofibrils in the ganglionated segment has contributed to the debate surrounding intestinal neuronal dysplasia. Quantitative assay of cholinesterase activity confirms the pattern of histochemical staining. AChE is particularly increased in relation to butrylcholinesterase, with one molecular form, the G4 tetrameric form, predominating. It is likely that the raised levels of AChE in aganglionic tissue are the transcriptional consequence of the abnormalities in signalling molecules that characterize HSCR. Evidence suggests that this AChE is functioning in a nonenzymatic capacity to promote cell adhesion and differentiation and that the hypertrophied nerves and neurofibrils may be the result of this increased AChE expression.

MuSK is required for anchoring acetylcholinesterase at the neuromuscular junction

At the neuromuscular junction, acetylcholinesterase (AChE) is mainly present as asymmetric forms in which tetramers of catalytic subunits are associated to a specific collagen, collagen Q (ColQ). The accumulation of the enzyme in the synaptic basal lamina strictly relies on ColQ. This has been shown to be mediated by interaction between ColQ and perlecan, which itself binds dystroglycan. Here, using transfected mutants of ColQ in a ColQ-deficient muscle cell line or COS-7 cells, we report that ColQ clusterizes through a more complex mechanism. This process requires two heparin-binding sites contained in the collagen domain as well as the COOH terminus of ColQ. Cross-linking and immunoprecipitation experiments in Torpedo postsynaptic membranes together with transfection experiments with muscle-specific kinase (MuSK) constructs in MuSK-deficient myotubes or COS-7 cells provide the first evidence that ColQ binds MuSK. Together, our data suggest that a ternary complex containing ColQ, perlecan, and MuSK is required for AChE clustering and support the notion that MuSK dictates AChE synaptic localization at the neuromuscular junction.

Analogs of alpha-conotoxin MII are selective for alpha6-containing nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) both mediate direct cholinergic synaptic transmission and modulate synaptic transmission by other neurotransmitters. Novel ligands are needed as probes to discriminate among structurally related nAChR subtypes. Alpha-conotoxin MII, a selective ligand that discriminates among a variety of nAChR subtypes, fails to discriminate well between some subtypes containing the closely related alpha3 and alpha6 subunits. Structure-function analysis of alpha-conotoxin MII was performed in an attempt to generate analogs with preference for alpha6-containing [alpha6(*) (asterisks indicate the possible presence of additional subunits)] nAChRs. Alanine substitution resulted in several analogs with decreased activity at alpha3(*) versus alpha6(*) nAChRs heterologously expressed in Xenopus laevis oocytes. From the initial analogs, a series of mutations with two alanine substitutions was synthesized. Substitution at His9 and Leu15 (MII[H9A;L15A]) resulted in a 29-fold lower IC(50) at alpha6beta4 versus alpha3beta4 nAChRs. The peptide had a 590-fold lower IC(50) for alpha6/alpha3beta2 versus alpha3beta2 and a 2020-fold lower IC(50) for alpha6/alpha3beta2beta3 versus alpha3beta2 nAChRs. MII[H9A;L15A] had little or no activity at alpha2beta2, alpha2beta4, alpha3beta4, alpha4beta2, alpha4beta4, and alpha7 nAChRs. Functional block by MII[H9A;L15A] of rat alpha6/alpha3beta2beta3 nAChRs (IC(50) = 2.4 nM) correlated well with the inhibition constant of MII[H9A;L15A] for [(125)I]alpha-conotoxin MII binding to putative alpha6beta2(*) nAChRs in mouse brain homogenates (K(i) = 3.3 nM). Thus, structure-function analysis of alpha-conotoxin MII enabled the creation of novel selective antagonists for discriminating among nAChRs containing alpha3 and alpha6 subunits.

α7-Nicotinic acetylcholine receptors mediate an Aβ1−42-induced increase in the level of acetylcholinesterase in primary cortical neurones

The beta-amyloid protein (Abeta) is the major protein component of amyloid plaques found in the Alzheimer brain. Although there is a loss of acetylcholinesterase (AChE) from both cholinergic and non-cholinergic neurones in the brain of Alzheimer patients, the level of AChE is increased around amyloid plaques. Previous studies using P19 cells in culture and transgenic mice which overexpress human Abeta have suggested that this increase may be due to a direct action of Abeta on AChE expression in cells adjacent to amyloid plaques. The aim of the present study was to examine the mechanism by which Abeta increases levels of AChE in primary cortical neurones. Abeta1-42 was more potent than Abeta1-40 in its ability to increase AChE in primary cortical neurones. The increase in AChE was unrelated to the toxic effects of the Abeta peptides. The effect of Abeta1-42 on AChE was blocked by inhibitors of alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) as well as by inhibitors of L- or N-type voltage-dependent calcium channels (VDCCs), whereas agonists of alpha7 nAChRs (choline, nicotine) increased the level of AChE. The results demonstrate that the effect of Abeta1-42 on AChE is due to an agonist effect of Abeta1-42 on the alpha7 nAChR.

Chronic high pressure potentiates the antiproliferative effect and abolishes contractile phenotypic changes caused by endothelial cells in cocultured smooth muscle cells

High in vitro pressures have been reported to alter smooth muscle cell (SMC) and endothelial cell (EC) phenotype, while endothelial cells (ECs) can influence the proliferation, phenotype, and contractile features of smooth muscle cells (SMC) in coculture systems. However, little is known about the in vitro effects of pressure on EC/SMC cocultures. We therefore sought to compare SMC proliferation in independent and EC coculture under ambient and high pressure, and identify changes in the contractile phenotype of SMCs by measuring levels of the L-type Ca(2+) channel a(1) subunit (dihydropyridine-DHP receptor) which is critical for Ca(2+) transients, differentiation and contractility in SMC.

METHODS

Rat aortic SMCs in independent culture (SMC/0) and coculture with ECs (SMC/EC) were maintained in 5% CO(2) under either atmospheric or high pressure (130 mmHg). SMC were counted at 0, 1, 3, and 5 days and compared to initial cell counts of day 0 before the exposure to experimental conditions. DHP receptor levels were quantitated by Western blotting (three similar studies).

RESULTS

ECs suppressed SMC proliferation on day 1 of coculture in both atmospheric and high pressure (20% inhibition vs independent culture, P < or = 0.05). By day 3, cocultured SMC under atmospheric pressure displayed no EC-mediated inhibition, and at day 5, atmospheric cocultured SMCs revealed statistically significant enhanced proliferation as compared with SMCs in independent cultures. However, cocultured SMCs exposed to 130 mmHg pressure displayed sustained sensitivity to EC growth inhibition at both days 3 and 5 of the experiment. Coculture decreased SMC DHP-receptor levels under atmospheric pressure. However, this effect was abolished in cocultures exposed to high pressure.

CONCLUSIONS

High pressure substantially alters the regulatory influence of EC on SMC proliferation and contractile potential. This pressure/coculture model should increase our understanding of cellular interaction in hypertensive vasculopathy.

Acetylcholinesterase is increased in mouse neuronal and astrocyte cultures after treatment with beta-amyloid peptides

The cellular origin of the acetylcholinesterase (AChE) associated with amyloid plaques in the Alzheimer's disease (AD) brain is unknown. In this study we report that amyloid beta-peptides (Abeta) increased AChE levels in both neuronal and astrocytic primary cultures, supporting the possibility that both neurons and glia may make a direct contribution to the pool of AChE seen around amyloid deposits in the AD brain.

Post-transcriptional regulation of acetylcholinesterase mRNAs in nerve growth factor-treated PC12 cells by the RNA-binding protein HuD

Expression of acetylcholinesterase (AChE) is greatly enhanced during neuronal differentiation, but the nature of the molecular mechanisms remains to be fully defined. In this study, we observed that nerve growth factor treatment of PC12 cells leads to a progressive increase in the expression of AChE transcripts, reaching approximately 3.5-fold by 72 h. Given that the AChE 3'-untranslated region (UTR) contains an AU-rich element, we focused on the potential role of the RNA-binding protein HuD in mediating the increase in AChE mRNA seen in differentiating neurons. Using PC12 cells engineered to stably express HuD or an antisense to HuD, our studies indicate that HuD can regulate the abundance of AChE transcripts in neuronal cells. Furthermore, transfection of a reporter construct containing the AChE 3'-UTR showed that this 3'-UTR can increase expression of the reporter gene product in cells expressing HuD but not in cells expressing the antisense. RNA gel shifts and Northwestern blots revealed an increase in the binding of several protein complexes in differentiated neurons. Immunoprecipitation experiments demonstrated that HuD can bind directly AChE transcripts. These results show the importance of post-transcriptional mechanisms in regulating AChE expression in differentiating neurons and implicate HuD as a key trans-acting factor in these events.

Localizing synaptic mRNAs at the neuromuscular junction: it takes more than transcription

The neuromuscular junction has been used for several decades as an excellent model system to examine the cellular and molecular events involved in the formation and maintenance of a differentiated chemical synapse. In this context, several laboratories have focused their efforts over the last 15 years on the important contribution of transcriptional mechanisms to the regulation of the development and plasticity of the postsynaptic apparatus in muscle fibers. Converging lines of evidence now indicate that post-transcriptional events, operating at the level of mRNA stability and targeting, are likely to also play key roles at the neuromuscular junction. Here, we present the recent findings highlighting the role of these additional molecular events and extend our review to include data showing that post-transcriptional events are also important in the control of the expression of genes encoding synaptic proteins in muscle cells placed under different conditions. Finally, we discuss the possibility that mis-regulation of post-transcriptional events can occur in certain neuromuscular diseases and cause abnormalities of the neuromuscular junction.

A non-cholinergic, trophic action of acetylcholinesterase on hippocampal neurones in vitro: molecular mechanisms

In this study neurite outgrowth from cultured hippocampal neurones was increased by addition of acetylcholinesterase acting in a non-cholinergic manner. Only monomeric acetylcholinesterase, a form of acetylcholinesterase dominant in development, increased neurite outgrowth (3-10 U/ml); moreover this effect was not blocked by active site blockers (echothiophate and galanthamine) but was sensitive to the addition of peripheral site blockers (fasciculin and BW284c51). It appears therefore that acetylcholinesterase has alternative, non-cholinergic functions, one of which could be in development, via a peripheral site. The possibility of a causal relationship between neurite outgrowth and calcium influx was explored using a spectrum of acetylcholinesterase variants, inhibitors and calcium channel blockers. Acetylcholinesterase regulation of outgrowth was shown to depend on an influx of extracellular calcium specifically via the L-type voltage-gated calcium channel. In summary, we propose that, independent of its catalytic activity, a selective form of acetylcholinesterase has a role in the development of hippocampal neurones via a selective voltage-gated calcium channel.

Multiple regulatory events controlling the expression and localization of utrophin in skeletal muscle fibers: insights into a therapeutic strategy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most prevalent inherited muscle disease and results from mutations/deletions in the X-linked dystrophin gene. Although several approaches have been envisaged to counteract the effects of this progressive disease, there is currently no cure available. One strategy consists in utilizing a protein normally expressed in DMD muscle which, once expressed at appropriate levels and at the correct subcellular location, could compensate for the lack of dystrophin. A candidate for such a role is the dystrophin-related protein now referred to as utrophin. In contrast to dystrophin, which is expressed along the length of healthy muscle fibers, utrophin accumulates at the neuromuscular junction in both normal and DMD fibers. Several years ago, we began a series of experiments to determine the mechanisms responsible for the expression of utrophin at the neuromuscular synapse. Initially, we showed that utrophin transcripts accumulate preferentially within the postsynaptic sarcoplasm. To determine whether selective transcription of the utrophin gene accounts for this synaptic accumulation of utrophin mRNAs, we injected several utrophin promoter-reporter constructs directly into mouse muscle and demonstrated the preferential synaptic expression of the reporter gene. These results suggested that local transcriptional activation of the utrophin gene is responsible for the accumulation of utrophin mRNAs at the neuromuscular junction. In these studies, we also demonstrated that an N-box motif contained within the utrophin promoter plays a critical role in directing the synapse-specific expression of the utrophin gene. Additionally, our studies have shown that the ets-factors GABP alpha and beta are part of a protein complex that can bind to the N-box motif to transactivate the gene in muscle cells in culture and in vivo. In these experiments, we also noted that the nerve-derived trophic factors agrin and ARIA/heregulin regulate expression of utrophin via the activation of GABP alpha and beta which in turn, transactivate the utrophin gene via the N-box motif. Although these studies demonstrate that transcriptional activation can regulate utrophin mRNA levels, it is possible that additional mechanisms are also involved. In particular, the association of mRNAs with cytoskeletal elements and RNA-binding proteins may contribute to the accumulation of utrophin transcripts within the postsynaptic sarcoplasm. In recent studies, we have begun to examine this and we have now identified specific regions within the 3' untranslated region that are necessary for targeting and stabilizing utrophin mRNAs in skeletal muscle cells. A series of in vivo studies have also led us to conclude that post-transcriptional mechanisms are indeed important in regulating the abundance of utrophin transcripts in muscle. Together, these studies should lead to the identification of cis- and trans-acting elements regulating transcription of the utrophin gene as well as the stability and targeting of its mRNA in muscle cells. The results should therefore, identify specific targets that may become important in designing specific pharmacological interventions directed at increasing the expression of utrophin into extrasynaptic regions of DMD muscle fibers. In addition, these findings will contribute to our basic understanding of the cellular and molecular events involved in the formation, maintenance and plasticity of the neuromuscular synapse.

Activity of the nicotinic acetylcholine receptor alpha5 and alpha7 subunit promoters in muscle cells

The acetylcholine receptor alpha5 and alpha7 subunits are components of different nicotinic receptor subtypes expressed in the nervous system. However, they are also present in non-neuronal tissues. We have detected alpha5 and alpha7 transcripts in mouse C2C12 muscle cells. Moreover, on differentiation of myoblasts into myotubes, the amount of alpha7 transcripts increased significantly, whereas alpha5 remained unchanged. In order to explore how the expression of these neuronal genes is regulated in muscle, we have characterized their promoter activities. Deletion and mutagenesis analysis with transfected reporter genes showed that transcriptional activity was controlled by regulatory elements also operative in neuronal-like cells. Thus, the activity of the alpha5 subunit core promoter decreased to approximately 50% on alteration of one, two, or three of the five Sp1 binding sites present in this region and was almost abolished when four or five sites were mutated simultaneously. In the case of the alpha7 subunit promoter, the upstream stimulatory factor and the early growth response gene transcription factor were involved in regulating its transcriptional activity. In addition, the alpha7 promoter was activated during the differentiation process, in a mechanism partially dependent on the mentioned factors.

Molecular mechanisms underlying the activity-linked alterations in acetylcholinesterase mRNAs in developing versus adult rat skeletal muscles

The molecular mechanisms underlying the activity-linked plasticity of acetylcholinesterase (AChE) mRNA levels in mammalian skeletal muscle have yet to be established. Here, we demonstrate that denervation of adult muscle induces a dramatic (up to 90%) and rapid (within 24 h) decrease in the abundance of AChE mRNAs. By contrast, denervation of 14-day-old rats leads to a significantly less pronounced reduction (50% of control) in the expression of AChE mRNAs. Assessment of the transcriptional activity of the AChE gene reveals that it remains essentially unchanged in adult denervated muscles, whereas it displays an approximately two- to three-fold increase (p < 0.05) in denervated muscles from 2- to 14-day-old rats. In addition, we observed a higher rate of degradation of in vitro transcribed AChE mRNAs upon incubation with protein extracts from denervated muscles. Finally, UV-crosslinking experiments reveal that denervation increases the abundance of RNA-protein interactions in the 3' untranslated region of AChE transcripts. Taken together, these data suggest that the abundance of AChE transcripts in mature muscles is controlled primarily via posttranscriptional regulatory mechanisms, whereas in neo- and postnatal muscles, both transcriptional and posttranscriptional regulation appears critical in dictating AChE mRNA levels. Accordingly, the activity-linked transcriptional regulation of the AChE gene appears to demonstrate a high level of plasticity during muscle development when maturation of the neuromuscular junctions is still occurring.

Calcium channel activation stabilizes a neuronal calcium channel mRNA

We have identified a calcium-dependent pathway in neurons that regulates expression levels of the alpha1B subunit and N channel current. When neurons are depolarized and voltage-gated calcium channels activated, the half-life of cellular N channel alpha1B mRNA is prolonged. This stabilizing effect of depolarization is mediated through the 3' untranslated region of a long form of the alpha1B mRNA and may represent a form of modulation of N-channel levels that does not require changes in gene transcription. Increases in N channel expression would affect several key neuronal functions controlled by calcium, including transmitter release and neurite outgrowth.

Denervation of chicken skeletal muscle causes an increase in acetylcholinesterase mRNA synthesis

We have examined the changes in enzymatic activity and the levels of transcripts for AChE following denervation of chicken skeletal muscle. Quantitation of RNA blots indicates that AChE transcripts are increased following denervation. AChE transcripts increased approximately 17-fold in the fast-twitch posterior latissimus dorsi muscle and approximately 4-fold in the tonic anterior latissimus dorsi muscle 10 days after denervation of adult chickens. Both AChE transcript levels and enzyme activity increased in parallel for the two muscles. AChE transcripts also increased approximately 4-fold in the shank muscles of 2-day-old chicks following denervation. Transcript synthesis, measured by run-on transcription, increased approximately 3-fold in these denervated muscles. These results suggest that the increase in AChE transcripts following denervation in the chicken is due, at least in part, to an increase in the rate of its synthesis.

Molecular isoform distribution and glycosylation of acetylcholinesterase are altered in brain and cerebrospinal fluid of patients with Alzheimer's disease

The glycosylation of acetylcholinesterase (AChE) in CSF was analyzed by lectin binding. AChE from Alzheimer's disease (AD) patients was found to bind differently to two lectins, concanavalin A and wheat germ agglutinin, than AChE from controls. As multiple isoforms of AChE are present in both CSF and brain, we examined whether the abnormal glycosylation of AD AChE was due to changes in a specific molecular isoform. Globular amphiphilic dimeric (G2a) and monomeric (G1a) isoforms of AChE were found to be differentially glycosylated in AD CSF. Glycosylation of AChE was also altered in AD frontal cortex but not in cerebellum and was also associated with an increase in the proportion of light (G2 and G1) isoforms. This study demonstrates that the glycosylation of AChE is altered in the AD brain and that changes in AChE glycosylation in AD CSF may reflect changes in the distribution of brain isoforms. The study also suggests that glycosylation of AChE may be a useful diagnostic marker for AD.

Splicing of 5' introns dictates alternative splice selection of acetylcholinesterase pre-mRNA and specific expression during myogenesis

Splicing of alternative exon 6 to invariant exons 2, 3, and 4 in acetylcholinesterase (AChE) pre-mRNA results in expression of the prevailing enzyme species in the nervous system and at the neuromuscular junction of skeletal muscle. The structural determinants controlling splice selection are examined in differentiating C2-C12 muscle cells by selective intron deletion from and site-directed mutagenesis in the Ache gene. Transfection of a plasmid lacking two invariant introns (introns II and III) within the open reading frame of the Ache gene, located 5' of the alternative splice region, resulted in alternatively spliced mRNAs encoding enzyme forms not found endogenously in myotubes. Retention of either intron II or III is sufficient to control the tissue-specific pre-mRNA splicing pattern prevalent in situ. Further deletions and branch point mutations revealed that upstream splicing, but not the secondary structure of AChE pre-mRNA, is the determining factor in the splice selection. In addition, deletion of the alternative intron between the splice donor site and alternative acceptor sites resulted in aberrant upstream splicing. Thus, selective splicing of AChE pre-mRNA during myogenesis occurs in an ordered recognition sequence in which the alternative intron influences the fidelity of correct upstream splicing, which, in turn, determines the downstream splice selection of alternative exons.

L-type calcium channel expression depends on the differentiated state of vascular smooth muscle cells

Despite intensive interest in understanding the differentiation of vascular smooth muscle cells (VSMC), no information is available about differential regulation of ion channels in these cells. Since expression of the L-type Ca2+ channel can be influenced by differentiation in other cell types, we tested the hypothesis that the L-type (C class) channel is a specific differentiation marker of VSMC and that expression of these channels depends on the state of cell differentiation. We used rat aortic (A7r5) VSMC, which express functional L-type Ca2+ channels, and induced dedifferentiation by cell culture in different media. Treatment with retinoic acid was used to redifferentiate the VSMC. We characterized the differentiated state of the cells by using immunohistochemistry and Western blot analysis for smooth muscle (SM) alpha-actin and SM-myosin heavy chain (MHC). The number of functional Ca2+ channels was significantly decreased in dedifferentiated VSMC and increased upon differentiation with retinoic acid. Ca2+ channel function was assessed by whole-cell voltage clamp techniques. Using Western blot and dihydropyridine binding analysis, we found that the expression of the Ca2+ channel alpha1 subunit, and to a lesser extent the beta2 subunit, was directly correlated with the expression of SM alpha-actin and SM-MHC. We conclude that expression of L-type Ca2+ channel alpha1 subunits, and thus a functional Ca2+ channel, is highly coordinated with expression of the SM-specific proteins required for specialized smooth muscle cell functions. Furthermore, our results demonstrate that the L-type Ca2+ channel is a novel marker for differentiation of VSMC. The data suggest that regulation of ion channel expression during differentiation may have physiological importance for normal smooth muscle function and may influence VSMC behavior under pathophysiological conditions.

Increased expression of acetylcholinesterase T and R transcripts during hematopoietic differentiation is accompanied by parallel elevations in the levels of their respective molecular forms

Differentiation of hematopoietic cells is known to be accompanied by profound changes in acetylcholinesterase (AChE) enzyme activity, yet the basic mechanisms underlying this developmental regulation remain unknown. We initiated a series of experiments to examine the molecular mechanisms involved in regulating AChE expression during hematopoiesis. Differentiation of murine erythroleukemia (MEL) cells using dimethyl sulfoxide resulted in a 5- and 10-fold increase in intracellular and secreted AChE enzyme activity, respectively. Interestingly, these increases resulted from a preferential induction of the globular molecular form G1 and a slight increase in G4 instead of an increase in the levels of the G2 membrane-bound form, a molecular form expressed in mature erythrocytes. Concomitantly, expression of the two predominant AChE transcripts (R and T, for read-through and tail, respectively) in MEL cells was induced to a similar extent with differentiation. Nuclear run-on assays performed with nuclei isolated from induced versus uninduced MEL cells revealed that in contrast to the large increases seen in the transcription of the beta-globin gene, the transcriptional activity of the AChE gene remained largely unaffected after differentiation. Determination of the half-lives of the R and T transcripts demonstrated that they both exhibited an increase in stability in induced MEL cells. Taken together, results from these studies indicate that post-transcriptional regulatory mechanisms account for the increased expression of AChE in differentiated hematopoietic cells.

Mechanical stimulation increases expression of acetylcholinesterase in cultured myotubes

We tested the hypothesis that acetylcholinesterase (AChE) expression in skeletal muscle cells is increased by passive mechanical stimulation. To this end, primary cultures of myotubes were subjected to repeated cycles of stretch-relaxation for 5 min, 30 min, 3 h, and 24 h, using the Flexercell FX-2000 strain unit. Although mechanical stimulation did not affect AChE expression at early time points, it led to a significant increase (42%; P < 0.05) in total AChE activity at 24 h. This increase reflected a general elevation in the activity of all AChE molecular forms as opposed to a preferential increase in a specific form. Tetrodotoxin (TTX) treatment did not prevent the increase in AChE expression, whereas nifedipine partially blocked it. These changes in enzyme expression were accompanied by increases in the levels of AChE mRNA, suggesting the involvement of pretranslational regulatory mechanisms. Together, these results illustrate that, in addition to neural activation and trophic factors, passive mechanical forces modulate expression of AChE in skeletal muscle cells. Because TTX did not prevent the increase in AChE expression, it appears that the effects of mechanical stimulation are independent of electrical activity, which further indicates the use of an alternate signaling pathway.

Denervation decreases the ipsilateral expression of AChE in chick lumbaric motor neurons

In vertebrate neuromuscular junctions, acetylcholinesterase (AChE; EC 3.1.1.7) is highly concentrated at the synaptic basal lamina and the postsynaptic muscle fiber. The postsynaptic muscle cell is the primary source of AChE. However, several lines of evidence indicate that the presynaptic motor neuron is able to synthesize and secrete AChE at the neuromuscular junctions. By using anti-AChE monoclonal antibody in immunohistochemical staining, we found that the AChE-positive cells were labeled only at the motor neurons of the chick spinal cords. When the protein extract of chick spinal cords was analyzed by a Western blot analysis, a protein band of approximately 105 kDa was recognized. In denervated chicks, the expression of motor neuron AChE, as recognized on a Western blot, decreased by approximately 50% 4 days after denervation. The AChE expression in denervated chick spinal cords, however, was restored to control level 10 days after denervation. The decreased AChE expression was restricted to the ipsilateral side of the denervated chick spinal cord while the contralateral side was relatively unchanged. In comparison with the contralateral side, the level of AChE protein and enzymatic activity expressed in the ipsilateral spinal cord was approximately 50% lower. This is the first demonstration to show that the ipsilateral and contralateral sides of chick spinal cords respond differently after nerve denervation.

Changes in IP3 metabolism during skeletal muscle development in vivo and in vitro

We have investigated whether IP3 metabolism presents particular changes during critical stages of muscle development. With this aim, we have measured IP3 formation through phospholipase C activity, IP3 removal through IP3 5-phosphatase and IP3 3-kinase activities, as well as IP3 mass, during myogenesis in vivo and in vitro. In developing rat skeletal muscle, both IP3 3-kinase and 5-phosphatase activities were relatively constant from embryonary day 15, the earliest age studied to postnatal day 10; 5-phosphatase decreased upon further development. A transient, major increase in phospholipase C activity was evident at embryonary day 18 while a non-significant increase in IP3 mass was detected at this embrionary age. In rat skeletal muscle in primary culture, all enzyme activities as well as the mass of IP3 increased significantly in myotubes compared to myoblasts. Myotubes incubated with calcitonin gene-related peptide, responded with a transient increase in IP3 mass after 2 to 10 sec; the CGRP-induced increase being completely blocked by U-73122, a phospholipase C inhibitor. Furthermore, IP3 mass increased within 1 hr after exposure to differentiating agents of both RCMH cells, a line derived from normal human skeletal muscle, and C2C12 cells. These results indicate that changes in IP3 metabolism can be correlated to critical stages of muscle development and differentiation, suggesting a possible role for IP3 in these processes.

Regulation of acetylcholinesterase expression during neuronal differentiation

We have examined the developmental expression of acetylcholinesterase (AChE) during the process of neuronal differentiation from a pluripotent stem cell. P19 embryonic carcinoma cells form embryoid bodies, which, when cultured with retinoic acid, are induced to differentiate into neurons and glia. No AChE activity is present in the undifferentiated stem cells, and mRNA protection analyses do not detect AChE mRNA. Commitment to a neuronal differentiation pathway results in increased levels of AChE mRNA, production of a tetrameric form of the enzyme, and secretion of AChE into the culture medium. Concomitant with subsequent morphological differentiation into neurons, enzyme secretion diminishes and AChE becomes largely tethered to the neuronal cell membranes. The enzyme is attached to the cell surface as a globular tetramer. Its hydrodynamic properties are consistent with association through a noncatalytic hydrophobic subunit rather than anchorage by a glycophospholipid tail. No change in the rate of transcription of the Ache gene was detected during the course of differentiation, suggesting that the gene is actively transcribed at very early stages of development. Results suggest that stabilization of a labile mRNA governs the increase in AChE mRNA and gene product. The studies presented indicate that an early event in neuronal differentiation is the stabilization of the mRNA leading to expression of a secreted form of AChE. A subsequent step associated with neurite outgrowth results in a transition from secretion of the tetrameric enzyme to its localization on the cell membrane.

Regulation of dihydropyridine and ryanodine receptor gene expression in skeletal muscle. Role of nerve, protein kinase C, and cAMP pathways

The dihydropyridine (DHP) and ryanodine (RY) receptors play a critical role in depolarization-induced calcium release in skeletal muscle, yet the factors which govern their expression remain unknown. We investigated the roles of electrical activity and trophic factors in the regulation of the genes encoding the alpha 1, alpha 2, and beta subunits of the DHP receptor as well as the RY receptor in rat skeletal muscle in vivo. Muscle paralysis, induced by denervation, had no effect on the DHP receptor mRNA levels while the RY receptor mRNA was decreased. In contrast, chronic superfusion of tetrodotoxin onto the sciatic nerve resulted in a marked increase in mRNA levels and transcriptional activity of both DHP and RY receptor genes. Since nerve can induce changes in second messenger pathways which modulate muscle gene expression, we attempted to identify factors which regulate DHP and RY receptor expression using cultured myotubes. Elevated cAMP levels specifically inhibited the expression of RY receptor mRNA while 12-O-tetradecanoylphorbol-13-acetate, an activator of protein kinase C, increased the transcripts encoding the RY receptor and the alpha 1 subunit of the DHP receptor. Changes in the level of mRNAs were paralleled by altered receptor numbers. Neither cAMP nor protein kinase C altered transcriptional activity of the DHP and RY receptor genes. These results demonstrate that neural factor(s) regulate DHP and RY receptor mRNA levels in vivo via transcriptional mechanisms while protein kinase C and cAMP can modulate DHP and RY receptor transcript levels by a transcription-independent process.

Chronic enhancement of neuromuscular activity increases acetylcholinesterase gene expression in skeletal muscle

We determined levels of mRNA encoding acetylcholinesterase (AChE) in muscles of rats subjected to chronic enhancement of neuromuscular activation. After 8 wk of voluntary wheel running, extensor digitorum longus (EDL) muscles displayed a 72% increase in total AChE activity as a result of a selective threefold increase in the G4 content. Soleus muscles, on the other hand, exhibited a 30% decrease in A12 while displaying a small (33%) increase in total AChE activity. These enzymatic adaptations were paralleled by increases in the levels of AChE mRNAs in both EDL (32%; P < 0.03) and soleus (42%; P < 0.02) muscles. In addition, compensatory hypertrophy of the plantaris muscle increased total AChE activity by 75%. This change was reflected by an elevation in all AChE molecular forms with A12 (89%) and A8 (179%) showing the most prominent increases. Similar to exercise-trained muscles, hypertrophied plantaris muscles displayed an increase in AChE transcripts (25%; P < 0.04). These results indicate that increases in neuromuscular activity modulate expression of the AChE gene in vivo and suggest the involvement of pretranslational regulatory mechanisms in the adaptive response of AChE to enhanced neuromuscular activation.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.