Complex regulation of acetylcholinesterase gene expression in human brain tumors

Differential localization of acetylcholinesterase in neuronal and non-neuronal cells

Acetylcholinesterase (AChE) expression is regulated in cell types at the transcriptional and translational levels. In this study, we characterized and compared AChE catalytic activity, mRNA, protein expression, and protein localization in a variety of neuronal (SH-SY5Y neuroblastoma and primary cerebellar granule neurons (CGN)) and non-neuronal (LLC-MK2, HeLa, THP-1, and primary astrocytes) cell types. All cell lines expressed AChE catalytic activity; however the levels of AChE-specific activity were higher in neuronal cells than in the non-neuronal cell types. CGN expressed significantly more AChE activity than SH-SY5Y cells. All cell lines analyzed expressed AChE protein at equivalent levels, as well as mRNA splice variants. Localization of AChE was characterized by immunofluorescence and confocal microscopy. SH-SY5Y, CGN, and nerve-growth factor-differentiated PC-12 cells exhibited a pattern of AChE localization characterized as diffuse in the cytoplasm and punctate staining along neurites and on the plasma membrane. The localization in HeLa, LLC-MK2, fibroblasts, and undifferentiated PC-12 cells was significantly different than in neuronal cells-AChE was intensely localized in the perinuclear region, without staining near or on the plasma membrane. Based on the evidence presented here, we hypothesize that the presence of AChE protein doesn't correlate with catalytic activity, and the diffuse cytoplasmic and plasma membrane localization of AChE is a property of neuronal cell types.

Cholinesterase activity of human lung tumours varies according to their histological classification

The probable involvement of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in cancer and the relevance of cholinergic responses for lung cancer growth prompted us to study whether cholinesterase activity of human lung is altered by malignancy. Surgical pieces of non-small lung carcinomas (NSLC) and their adjacent non-cancerous tissues (ANCT) were analysed for AChE and BChE activities. AChE activity in adenocarcinoma (AC) was 7.80 +/- 5.59 nmol of substrate hydrolysed per min and per mg of protein (mU/mg), the same as in their ANCT (8.83 +/- 4.72 mU/mg; P = 0.823); in large cell carcinoma (LCC), 7.52 +/- 3.32 mU/mg, approximately 50% less than in their ANCT (15.39 +/- 5.66 mU/mg; P = 0.043); and in squamous cell carcinoma (SCC), 1.39 +/- 0.58 mU/mg, 80% less than in ANCT (6.08 +/- 2.88 mU/mg; P = 0.003). BChE activity was 5.85 +/- 3.20 mU/mg in AC and 9.56 +/- 3.38 mU/mg in ANCT (P = 0.022); 2.94 +/- 2.01 mU/mg in LCC and 6.50 +/- 6.63 mU/mg in ANCT (P = 0.068); and 4.49 +/- 2.30 mU/mg in SCC and ANCT 6.56 +/- 4.09 mU/mg (P = 0.026). Abundant AChE dimers and fewer monomers were identified in lung and, although their distribution was unaffected by cancer, the binding with concanavalin A revealed changes in AChE glycosylation between SCC and their ANCT. The fall in BChE activity affected all molecules, with a strong decrease of the amphiphilic tetramers. Western blotting revealed protein bands with the expected mass of the principal AChE subunits, and the deeper intensity of the protein signal in SCC than in healthy lung, in lanes loaded with the same units of AChE activity, supported an augment in the amount of AChE protein/unit of AChE activity in SCC. The increased availability of acetylcholine in neoplastic lung, resulting from the fall of cholinesterase activity, may enhance cholinergic signalling and contribute to tumour progression.

SC35 promotes sustainable stress-induced alternative splicing of neuronal acetylcholinesterase mRNA

Long-lasting alternative splicing of neuronal acetylcholinesterase (AChE) pre-mRNA occurs during neuronal development and following stress, altering synaptic properties. To explore the corresponding molecular events, we sought to identify mRNAs encoding for abundant splicing factors in the prefrontal cortex (PFC) following stress. Here we show elevated levels of the splicing factor SC35 in stressed as compared with naïve mice. In cotransfections of COS-1 and HEK293 cells with an AChE minigene allowing 3' splice variations, SC35 facilitated a shift from the primary AChE-S to the stress-induced AChE-R variant, while ASF/SF2 caused the opposite effect. Transfection with chimeric constructs comprising of SC35 and ASF/SF2 RRM/RS domains identified the SC35 RRM as responsible for AChE mRNA's alternative splicing. In poststress PFC neurons, increased SC35 mRNA and protein levels coincided with selective increase in AChE-R mRNA. In the developing mouse embryo, cortical progenitor cells in the ventricular zone displayed transient SC35 elevation concomitant with dominance of AChE-R over AChE-S mRNA. Finally, transgenic mice overexpressing human AChE-R, but not those overexpressing AChE-S, showed significant elevation in neuronal SC35 levels, suggesting a reciprocal reinforcement process. Together, these findings point to an interactive relationship of SC35 with cholinergic signals in the long-lasting consequences of stress on nervous system plasticity and development.

Expression of cholinesterases in brain and non-brain tumours

Although the involvement of cholinesterases (ChEs) in the removal of acetylcholine (ACh) at cholinergic synapses is firmly established, there is evidence to suggest that acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) take part in several cellular processes. The early expression of ChE genes during embryonic development and their role in morphogenesis and apoptosis have been explained on the basis of the non-cholinergic actions of ChEs. In addition, the effects of AChE and BuChE, their inhibitors and antisense oligonucleotides in proliferating cellular systems, together with the mitogenic actions of ACh, support a role for ChEs in cell cycle control. The anomalous expression of ChEs may increase cell proliferation and contribute to cancer growth or development. The aim of this report is to compile the available information on ChEs in cancerous tissues in order to stimulating the research to clarify the molecular mechanisms by which ChEs may participate in cancer. Future investigations may throw light into this intriguing issue which will be of benefit to humankind.

MicroRNA modulation of megakaryoblast fate involves cholinergic signaling

MicroRNAs (miRNAs) are abundant small regulatory RNAs with multiple roles in cell fate determination. The processes regulating cellular miRNA levels are still unclear and experimental oligonucleotide tools to readily mimic their effects are not yet available. Here, we report that thapsigargin-induced intracellular Ca(++) release suppressed pre-miR-181a levels in human promegakaryotic Meg-01 cells, induced differentiation-associated nuclear endoreduplication and caspase-3 activation and replaced the acetylcholinesterase 3' splice variant AChE-S with AChE-R. AChE, PKC and PKA inhibitors all attenuated the pre-miR-181a decline and the induced differentiation. AChmiON, a synthetic 23-mer 2'-oxymethylated oligonucleotide mimicking the miR-181a sequence, blocked the calcium-induced differentiation while elevating cellular pre-miR-181a levels and inducing DNA fragmentation and cell death. Moreover, when added to RW 264.7 macrophages, AChmiON at 100 nM induced nitric oxide production with efficiency close to that of bacterial endotoxin, demonstrating physiologically relevant activities also in blood-born monocytes/macrophages. The stress-induced modulation of hematopoietic miR-181a levels through AChE, PKC and PKA cascade(s) suggests using miRNA mimics for diverting the fate of hematopoietic tumor cells towards differentiation and/or apoptosis.

Role of ELAV-like RNA-binding proteins HuD and HuR in the post-transcriptional regulation of acetylcholinesterase in neurons and skeletal muscle cells

Over the last few years, several laboratories have focused their attention on elucidating the molecular events that control the expression and localization of acetylcholinesterase (AChE) in neurons and skeletal muscle cells. In this context, results from a number of studies have clearly shown the important contribution of transcriptional events in regulating AChE expression. Specifically, these studies have highlighted the roles of several cis- and trans-acting factors that control transcription of the AChE gene in these excitable cells. However, it has also become apparent that changes in the transcriptional activity of the AChE gene cannot fully account for the alterations seen in the overall abundance of AChE transcripts in neurons and muscle cells placed under a variety of experimental conditions. This indicates, therefore, that post-transcriptional mechanisms also play a significant role in controlling AChE mRNA expression. With this in mind, we have recently begun to address this issue in greater detail. Here, we provide a summary of our most recent findings dealing with the post-transcriptional regulation of AChE. Together, our studies have shown so far the important contribution of an AU-rich element located in the 3'UTR of AChE transcripts and of the stabilizing RNA-binding proteins of the ELAV-like family in regulating AChE expression in differentiating neuronal and muscle cells.

Muscular dystrophy by merosin deficiency decreases acetylcholinesterase activity in thymus of Lama2dy mice

Half of congenital muscular dystrophy cases arise from laminin alpha2 (merosin) deficiency, and merosin-deficient mice (Lama2dy) exhibit a dystrophic phenotype. The abnormal development of thymus in Lama2dy mice, the occurrence of acetylcholinesterase (AChE) in the gland and the impaired distribution of AChE molecules in skeletal muscle of the mouse mutant prompted us to compare the levels of AChE mRNAs and enzyme species in thymus of control and Lama2dy mice. AChE activity in normal thymus (mean +/- SD 1.42 +/- 0.28 micromol acetylthiocholine/h/mg protein, U/mg) was decreased by approximately 50% in dystrophic thymus (0.77 +/- 0.23 U/mg) (p = 0.007), whereas butyrylcholinesterase activity was little affected. RT-PCR assays revealed variable levels of R, H and T AChE mRNAs in thymus, bone marrow and spinal cord. Control thymus contained amphiphilic AChE dimers (G2A, 64%) and monomers (G1A, 19%), as well as hydrophilic tetramers (G4H, 9%) and monomers (G1H, 8%). The dimers consisted of glycosylphosphatidylinositol-anchored H subunits. Western blot assays with anti-AChE antibodies suggested the occurrence of inactive AChE in mouse thymus. Despite the decrease in AChE activity in Lama2dy thymus, no differences between thymuses from control and dystrophic mice were observed in the distribution of AChE forms, phosphatidylinositol-specific phospholipase C sensitivity, binding to lectins and size of AChE subunits.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

From brain to blood: alternative splicing evidence for the cholinergic basis of Mammalian stress responses

Three principal features of mammalian stress responses are that they span peripheral and CNS changes, modify blood cell composition and activities, and cover inter-related alterations in a large number of gene products. The finely tuned spatiotemporal regulation of these multiple events suggests the hierarchic involvement of modulatory neurotransmitters and modified process(es) in the pathway of gene expression that together would enable widely diverse stress responses. We report evidence supporting the notion that acetylcholine (ACh) acts as a stress-response-regulating transmitter and that altered ACh levels are variously associated with changes in the alternative splicing of pre-mRNA transcripts in brain neurons and peripheral blood cells. We used acetylcholinesterase (AChE) gene expression as a case study and developed distinct probes for its alternative splice variants at the mRNA and protein levels. In laboratory animals and human-derived cells, we found stress-induced changes in the alternative splicing patterns of AChE pre-mRNA, which attributes to this gene and its different protein products diverse stress responsive functions that are associated with the enzymatic and noncatalytic properties of AChE. Together, these approaches provide a conceptually unified view of the studied pathways for controlling stress responses in brain and blood.

CREB regulates AChE-R-induced proliferation of human glioblastoma cells

The cyclic adenosine monophosphate (AMP) response element-binding protein, CREB, often modulates stress responses. Here, we report that CREB suppresses the glioblastoma proliferative effect of the stress-induced acetylcholinesterase variant, AChE-R. In human U87MG glioblastoma cells, AChE-R formed a triple complex with protein kinase C (PKC) epsilon and the scaffold protein RACK1, enhanced PKCepsilon phosphorylation, and facilitated BrdU incorporation. Either overexpressed CREB, or antisense destruction of AChE-R mRNA, PKC, or protein kinase A (PKA) inhibitors-but not CREB combined with PKC inhibition suppressed-this proliferation, suggesting that CREB's repression of this process involves a PKC-mediated pathway, whereas impaired CREB regulation allows AChE-R-induced, PKA-mediated proliferation of glioblastoma tumors.

Regulation of TGFβ1-mediated growth inhibition and apoptosis by RUNX2 isoforms in endothelial cells

Runx transcription factors regulate viral growth, hematopoiesis, bone formation, angiogenesis, and gastric epithelial development through specific DNA-binding motifs on target gene promoters. Vascular endothelial cells (ECs) express RUNX genes that are activated by angiogenic factors. The RUNX2 gene also activates the vascular endothelial growth factor promoter. Alternatively spliced forms of RUNX genes have been described, but their functions in angiogenesis have not been elucidated. In this study, expression of a novel alternatively spliced variant of RUNX2 (RUNX2Delta8), lacking the region encoded by exon 8, was detected in aortic tissue undergoing angiogenesis in vitro and in ECs. Expression of RUNX2 and RUNX2Delta8 increased in vascular sprouts concomitant with expression of cellular proteases and cytokines known to mediate angiogenesis. RUNX2 DNA-binding activity was expressed in proliferating but not quiescent ECs. Ectopic expression of RUNX2 in ECs increased cell sprouting, cell proliferation, DNA synthesis, and phosphorylation of phosphorylated retinoblastoma relative to control transfectants while RUNX2, but not RUNX2Delta8 transfectants, acquired resistance to growth inhibition by transforming growth factor (TGFbeta1). Furthermore, RUNX2Delta8-transfected cells were more sensitive to TGFbeta1-induced apoptosis than RUNX2 transfectants. Consistent with these data, the RUNX2 gene was a strong repressor of the promoter of the cyclin-dependent kinase inhibitor, p21(CIP1), while RUNX2Delta8 was a competitive inhibitor of RUNX2 and exhibited weak repression activity. These results support the hypothesis that ECs regulate growth and apoptosis, in part, by alternative splicing events in the RUNX2 transcription factor to affect the TGFbeta1 signaling pathway. The exon 8 domain of RUNX2 may contribute to the strong repression activity of RUNX2 for some target gene promoters.

The C-terminal t peptide of acetylcholinesterase forms an α helix that supports homomeric and heteromeric interactions

Acetylcholinesterase subunits of type T (AChET) possess an alternatively spliced C-terminal peptide (t peptide) which endows them with amphiphilic properties, the capacity to form various homo-oligomers and to associate, as a tetramer, with anchoring proteins containing a proline rich attachment domain (PRAD). The t peptide contains seven conserved aromatic residues. By spectroscopic analyses of the synthetic peptides covering part or all of the t peptide of Torpedo AChET, we show that the region containing the aromatic residues adopts an alpha helical structure, which is favored in the presence of lipids and detergent micelles: these residues therefore form a hydrophobic cluster in a sector of the helix. We also analyzed the formation of disulfide bonds between two different AChET subunits, and between AChET subunits and a PRAD-containing protein [the N-terminal fragment of the ColQ protein (QN)] possessing two cysteines upstream or downstream of the PRAD. This shows that, in the complex formed by four T subunits with QN (T4-QN), the t peptides are not folded on themselves as hairpins but instead are all oriented in the same direction, antiparallel to that of the PRAD. The formation of disulfide bonds between various pairs of cysteines, introduced by mutagenesis at various positions in the t peptides, indicates that this complex possesses a surprising flexibility.

Overexpression of transcription factor BACH1 in fetal Down syndrome brain

There is a series of about 12 transcription factors expressed on chromosome 21. These transcription factors (TFs) are major candidates for playing a pathogenetic role for the abnormal wiring of the brain in fetal Down Syndrome (DS) as approximately 5,000 TFs are developmentally involved in the complex architecture of the human brain. TF derangement in DS has been already reported and we decided to contribute to the problem by studying four TFs encoded on chromosome 21 in fetal DS brain. We used fetal cortex of 8 DS fetuses and 6 controls (females) from the 18-19th week of gestation. Brain homogenates were subject to immunoblotting using goat-anti-BACH1, rabbit anti-heme oxygenase 1 (HO1), rabbit anti-ERG, rabbit anti-RUNX1 and goat anti-SIM2 l. Antibodies against beta-actin were used to normalise cell loss and antibodies against neuron-specific enolase were used to compensate neuronal loss. BACH1 was significantly overexpressed in fetal DS (p < 0.008) as compared to controls whereas RUNX1 and ERG proteins were comparable between groups, and SIM2 l was not detectable in any specimen. BACH1 was even significantly increased in the DS panel when normalised versus the housekeeping protein beta-actin (p < 0.01) or the neuron specific enolase (p < 0.01). HO-1 was found comparable between groups. BACH1, a member of the family of BTB-basic leucine zipper transcription factors, regulates gene expression through the NF-E2 site. More specifically, BACH1 suppresses expression of HO1. Increased BACH1, however, did not lead to decreased HO1, which would have explained oxidative stress observed in fetal DS.

Aberrant protein expression of transcription factors BACH1 and ERG, both encoded on chromosome 21, in brains of patients with Down syndrome and Alzheimer’s disease

Down syndrome (DS; trisomy 21) is a genetic disorder associated with early mental retardation and patients inevitably develop Alzheimer's disease (AD)-like neuropathological changes. The molecular defects underlying the DS-phenotype may be due to overexpression of genes encoded on chromosome 21. This so-called gene dosage hypothesis is still controversial and demands systematic work on protein expression. A series of transcription factors (TF) are encoded on chromosome 21 and are considered to play a pathogenetic role in DS. We therefore decided to study brain expression of TF encoded on chromosome 21 in patients with DS and AD compared to controls: Frontal cortex of 6 male DS patients, 6 male patients with AD and 6 male controls were used for the experiments. Immunoblotting was used to determine protein levels of TF BACH1, ERG, SIM2 and RUNX1. SIM2 and RUNX1 were comparable between groups, while BACH1 was significantly reduced in DS, and ERG was increased in DS and AD as compared to controls. These findings may indicate that DS pathogenesis cannot be simply explained by the gene dosage effect hypothesis and that results of ERG expression in DS were paralleling those in AD probably reflecting a common pathogenetic mechanism possibly explaining why all DS patients develop AD like neuropathology from the fourth decade. We conclude that TF derangement is not only due to the process of neurodegeneration and propose that TFs BACH1 and ERG play a role for the development of AD-like neuropathology in DS and pathogenesis of AD per se and the manifold increase of ERG in both disorders may form a pivotal pathogenetic link.