A model for dynamic regulation of choline acetyltransferase by phosphorylation

Emerging phosphodiesterase inhibitors for treatment of neurodegenerative diseases

Neurodegenerative diseases (NDs) cause progressive loss of neuron structure and ultimately lead to neuronal cell death. Since the available drugs show only limited symptomatic relief, NDs are currently considered as incurable. This review will illustrate the principal roles of the signaling systems of cyclic adenosine and guanosine 3',5'-monophosphates (cAMP and cGMP) in the neuronal functions, and summarize expression/activity changes of the associated enzymes in the ND patients, including cyclases, protein kinases, and phosphodiesterases (PDEs). As the sole enzymes hydrolyzing cAMP and cGMP, PDEs are logical targets for modification of neurodegeneration. We will focus on PDE inhibitors and their potentials as disease-modifying therapeutics for the treatment of Alzheimer's disease, Parkinson's disease, and Huntington's disease. For the overlapped but distinct contributions of cAMP and cGMP to NDs, we hypothesize that dual PDE inhibitors, which simultaneously regulate both cAMP and cGMP signaling pathways, may have complementary and synergistic effects on modifying neurodegeneration and thus represent a new direction on the discovery of ND drugs.

Analysis of C. elegans acetylcholine synthesis mutants reveals a temperature-sensitive requirement for cholinergic neuromuscular function

In Caenorhabditis elegans, the cha-1 gene encodes choline acetyltransferase (ChAT), the enzyme that synthesizes the neurotransmitter acetylcholine. We have analyzed a large number of cha-1 hypomorphic mutants, most of which are missense alleles. Some homozygous cha-1 mutants have approximately normal ChAT immunoreactivity; many other alleles lead to consistent reductions in synaptic immunostaining, although the residual protein appears to be stable. Regardless of protein levels, neuromuscular function of almost all mutants is temperature sensitive, i.e., neuromuscular function is worse at 25° than at 14°. We show that the temperature effects are not related to acetylcholine release, but specifically to alterations in acetylcholine synthesis. This is not a temperature-dependent developmental phenotype, because animals raised at 20° to young adulthood and then shifted for 2 hours to either 14° or 25° had swimming and pharyngeal pumping rates similar to animals grown and assayed at either 14° or 25°, respectively. We also show that the temperature-sensitive phenotypes are not limited to missense alleles; rather, they are a property of most or all severe cha-1 hypomorphs. We suggest that our data are consistent with a model of ChAT protein physically, but not covalently, associated with synaptic vesicles; and there is a temperature-dependent equilibrium between vesicle-associated and cytoplasmic (i.e., soluble) ChAT. Presumably, in severe cha-1 hypomorphs, increasing the temperature would promote dissociation of some of the mutant ChAT protein from synaptic vesicles, thus removing the site of acetylcholine synthesis (ChAT) from the site of vesicular acetylcholine transport. This, in turn, would decrease the rate and extent of vesicle-filling, thus increasing the severity of the behavioral deficits.

Scopolamine promotes neuroinflammation and delirium-like neuropsychiatric disorder in mice

Postoperative delirium is a common neuropsychiatric syndrome resulting a high postsurgical mortality rate and decline in postdischarge function. Extensive research has been performed on both human and animal delirium-like models due to their clinical significance, focusing on systematic inflammation and consequent neuroinflammation playing a key role in the pathogenesis of postoperative cognitive dysfunctions. Since animal models are widely utilized for pathophysiological study of neuropsychiatric disorders, this study aimed at examining the validity of the scopolamine-induced delirium-like mice model with respect to the neuroinflammatory hypothesis of delirium. Male C57BL/6 mice were treated with intraperitoneal scopolamine (2 mg/kg). Neurobehavioral tests were performed to evaluate the changes in cognitive functions, including learning and memory, and the level of anxiety after surgery or scopolamine treatment. The levels of pro-inflammatory cytokines (IL-1β, IL-18, and TNF-α) and inflammasome components (NLRP3, ASC, and caspase-1) in different brain regions were measured. Gene expression profiles were also examined using whole-genome RNA sequencing analyses to compare gene expression patterns of different mice models. Scopolamine treatment showed significant increase in the level of anxiety and impairments in memory and cognitive function associated with increased level of pro-inflammatory cytokines and NLRP3 inflammasome components. Genetic analysis confirmed the different expression patterns of genes involved in immune response and inflammation and those related with the development of the nervous system in both surgery and scopolamine-induced mice models. The scopolamine-induced delirium-like mice model successfully showed that analogous neuropsychiatric changes coincides with the neuroinflammatory hypothesis for pathogenesis of delirium.

A multidimensional computational exploration of congenital myasthenic syndrome causing mutations in human choline acetyltransferase

Missense mutations of human choline acetyltransferase (CHAT) are mainly associated with congenital myasthenic syndrome (CMS). To date, several pathogenic mutations have been reported, but due to the rarity and genetic complexity of CMS and difficult genotype-phenotype correlations, the CHAT mutations, and their consequences are underexplored. In this study, we systematically sift through the available genetic data in search of previously unreported pathogenic mutations and use a dynamic in silico model to provide structural explanations for the pathogenicity of the reported deleterious and undetermined variants. Through rigorous multiparameter analyses, we conclude that mutations can affect CHAT through a variety of different mechanisms: by disrupting the secondary structure, by perturbing the P-loop through long-range allosteric interactions, by disrupting the domain connecting loop, and by affecting the phosphorylation process. This study provides the first dynamic look at how mutations affect the structure and catalytic activity in CHAT and highlights the need for further genomic research to better understand the pathology of CHAT.

Diagnoses of Pathological States Based on Acetylcholinesterase and Butyrylcholinesterase

Two cholinesterases exist: Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). While AChE plays a crucial role in neurotransmissions, BChE has no specific function apart from the detoxification of some drugs and secondary metabolites from plants. Thus, both AChE and BChE can serve as biochemical markers of various pathologies. Poisoning by nerve agents like sarin, soman, tabun, VX, novichok and overdosing by drugs used in some neurodegenerative disorders like Alzheimer´s disease and myasthenia gravis, as well as poisoning by organophosphorus pesticides are relevant to this issue. But it appears that changes in these enzymes take place in other processes including oxidative stress, inflammation, some types of cancer and genetically conditioned diseases. In this review, the cholinesterases are introduced, the mechanism of inhibitors action is explained and the relations between the cholinesterases and pathologies are explained.

Increased Cholinergic Response in α-Synuclein Transgenic Mice (h-α-synL62)

Pathological accumulation of misfolded α-synuclein (α-syn) in the brain plays a key role in the pathogenesis of Parkinson's disease, leading to neuronal dysfunction and motor disorders. The underlying mechanisms linking α-syn aggregations with neurotransmitter disturbance in Parkinson's brains are not well characterized. In the present study, we investigated transgenic mice expressing an aggregation-prone form of full-length human α-syn (h-α-synL62) linked to a signal sequence. These mice display dopamine depletion and progressive motor deficits. We detected accumulation of α-syn in cholinergic interneurons where they are colocalized with choline acetyltransferase. Using microdialysis, we measured acetylcholine levels in the striatum at baseline and during stimulation in the open field and with scopolamine. While no difference between wild-type and transgenic mice was detected in 3 month old mice, striatal acetylcholine levels at 9 months of age were significantly higher in transgenic mice. Concomitantly, high-affinity choline uptake was also increased while choline acetyltransferase and acetylcholine esterase activities were unchanged. The results suggest a disinhibition of acetylcholine release in α-syn transgenic mice.

Mutation of a serine near the catalytic site of the choline acetyltransferase a gene almost completely abolishes motility of the zebrafish embryo

In zebrafish, the gene choline acetyltransferase a (chata) encodes one of the two ChAT orthologs responsible for the synthesis of acetylcholine. Acetylcholine (ACh) is essential for neuromuscular transmission and its impaired synthesis by ChAT can lead to neuromuscular junction disorders such as congenital myasthenic syndromes in humans. We have identified a novel mutation in the chata gene of zebrafish, chata^tk64, in a collection of uncharacterised ENU-induced mutants. This mutant carries a missense mutation in the codon of a highly conserved serine changing it to an arginine (S102R). This serine is conserved among ChATs from zebrafish, rat, mice and chicken to humans. It resides within the catalytic domain and in the vicinity of the active site of the enzyme. However, it has not been reported so far to be required for enzymatic activity. Modelling of the S102R variant change in the ChAT protein crystal structure suggests that the change affects protein structure and has a direct impact on the catalytic domain of the protein which abolishes embryo motility almost completely.

Acetylcholine signaling system in progression of lung cancers

The neurotransmitter acetylcholine (ACh) acts as an autocrine growth factor for human lung cancer. Several lines of evidence show that lung cancer cells express all of the proteins required for the uptake of choline (choline transporter 1, choline transporter-like proteins) synthesis of ACh (choline acetyltransferase, carnitine acetyltransferase), transport of ACh (vesicular acetylcholine transport, OCTs, OCTNs) and degradation of ACh (acetylcholinesterase, butyrylcholinesterase). The released ACh binds back to nicotinic (nAChRs) and muscarinic receptors on lung cancer cells to accelerate their proliferation, migration and invasion. Out of all components of the cholinergic pathway, the nAChR-signaling has been studied the most intensely. The reason for this trend is due to genome-wide data studies showing that nicotinic receptor subtypes are involved in lung cancer risk, the relationship between cigarette smoke and lung cancer risk as well as the rising popularity of electronic cigarettes considered by many as a "safe" alternative to smoking. There are a small number of articles which review the contribution of the other cholinergic proteins in the pathophysiology of lung cancer. The primary objective of this review article is to discuss the function of the acetylcholine-signaling proteins in the progression of lung cancer. The investigation of the role of cholinergic network in lung cancer will pave the way to novel molecular targets and drugs in this lethal malignancy.

CHAT gene polymorphism rs3810950 is associated with the risk of Alzheimer's disease in the Czech population

Background

Cholinergic hypothesis of Alzheimer’s disease (AD) is based on the findings that a reduced and/or perturbed cholinergic activity in the central nervous system correlates with cognitive decline in patients with Alzheimer’s disease. The hypothesis resulted in the development of centrally-acting agents potentiating cholinergic neurotransmission; these drugs, however, only slowed down the cognitive decline and could not prevent it. Consequently, the perturbation of the central cholinergic signalling has been accepted as a part of the Alzheimer’s aetiology but not necessarily the primary cause of the disease. In the present study we have focused on the rs3810950 polymorphism of ChAT (choline acetyltransferase) gene that has not been studied in Czech population before.

Methods

We carried out an association study to test for a relationship between the rs3810950 polymorphism and Alzheimer’s disease in a group of 1186 persons; 759 patients with Alzheimer’s disease and 427 control subjects. Furthermore, we performed molecular modelling of the terminal domain (1st-126th amino acid residue) of one of the ChAT isoforms (M) to visualise in silico whether the rs3810950 polymorphism (A120T) can change any features of the tertiary structure of the protein which would have a potential to alter its function.

Results

The AA genotype of CHAT was associated with a 1.25 times higher risk of AD (p <  0.002) thus demonstrating that the rs3810950 polymorphism can have a modest but statistically significant effect on the risk of AD in the Czech population. Furthermore, the molecular modelling indicated that the polymorphism is likely to be associated with significant variations in the tertiary structure of the protein molecule which may impact its enzyme activity.

Conclusions

Our findings are consistent with the results of the meta-analytical studies of the relationship between rs3810950 polymorphism and AD and provide further material evidence for a direct (primary) involvement of cholinergic mechanisms in the etiopathogenesis of AD, particularly as a factor in cognitive decline and perturbed conscious awareness commonly observed in patients with AD.

Phosphorylation at serine 31 targets tyrosine hydroxylase to vesicles for transport along microtubules

Tyrosine hydroxylase (TH) catalyzes the conversion of l-tyrosine into l-DOPA, which is the rate-limiting step in the synthesis of catecholamines, such as dopamine, in dopaminergergic neurons. Low dopamine levels and death of the dopaminergic neurons are hallmarks of Parkinson's disease (PD), where α-synuclein is also a key player. TH is highly regulated, notably by phosphorylation of several Ser/Thr residues in the N-terminal tail. However, the functional role of TH phosphorylation at the Ser-31 site (THSer(P)-31) remains unclear. Here, we report that THSer(P)-31 co-distributes with the Golgi complex and synaptic-like vesicles in rat and human dopaminergic cells. We also found that the TH microsomal fraction content decreases after inhibition of cyclin-dependent kinase 5 (Cdk5) and ERK1/2. The cellular distribution of an overexpressed phospho-null mutant, TH1-S31A, was restricted to the soma of neuroblastoma cells, with decreased association with the microsomal fraction, whereas a phospho-mimic mutant, TH1-S31E, was distributed throughout the soma and neurites. TH1-S31E associated with vesicular monoamine transporter 2 (VMAT2) and α-synuclein in neuroblastoma cells, and endogenous THSer(P)-31 was detected in VMAT2- and α-synuclein-immunoprecipitated mouse brain samples. Microtubule disruption or co-transfection with α-synuclein A53T, a PD-associated mutation, caused TH1-S31E accumulation in the cell soma. Our results indicate that Ser-31 phosphorylation may regulate TH subcellular localization by enabling its transport along microtubules, notably toward the projection terminals. These findings disclose a new mechanism of TH regulation by phosphorylation and reveal its interaction with key players in PD, opening up new research avenues for better understanding dopamine synthesis in physiological and pathological states.

Development of a model system for neuronal dysfunction in Fabry disease

Fabry disease is a glycosphingolipid storage disorder that is caused by a genetic deficiency of the enzyme alpha-galactosidase A (AGA, EC 3.2.1.22). It is a multisystem disease that affects the vascular, cardiac, renal, and nervous systems. One of the hallmarks of this disorder is neuropathic pain and sympathetic and parasympathetic nervous dysfunction. The exact mechanism by which changes in AGA activity result in change in neuronal function is not clear, partly due to of a lack of relevant model systems. In this study, we report the development of an in vitro model system to study neuronal dysfunction in Fabry disease by using short-hairpin RNA to create a stable knock-down of AGA in the human cholinergic neuronal cell line, LA-N-2. We show that gene-silenced cells show specifically reduced AGA activity and store globotriaosylceramide. In gene-silenced cells, release of the neurotransmitter acetylcholine is significantly reduced, demonstrating that this model may be used to study specific neuronal functions such as neurotransmitter release in Fabry disease.

Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations

Choline acetyltransferase catalyzes the synthesis of acetylcholine at cholinergic nerves. Mutations in human CHAT cause a congenital myasthenic syndrome due to impaired synthesis of ACh; this severe variant of the disease is frequently associated with unexpected episodes of potentially fatal apnea. The severity of this condition varies remarkably, and the molecular factors determining this variability are poorly understood. Furthermore, genotype-phenotype correlations have been difficult to establish in patients with biallelic mutations. We analyzed the protein expression of phosphorylated ChAT of seven CHAT mutations, p.Val136Met, p.Arg207His, p.Arg186Trp, p.Val194Leu, p.Pro211Ala, p.Arg566Cys, and p.Ser694Cys, in HEK-293 cells to phosphorylated ChAT, determined their enzyme kinetics and thermal stability, and examined their structural changes. Three mutations, p.Arg207His, p.Arg186Trp, and p.Arg566Cys, are novel, and p.Val136Met and p.Arg207His are homozygous in three families and associated with severe disease. The characterization of mutants showed a decrease in the overall catalytic efficiency of ChAT; in particular, those located near the active-site tunnel produced the most seriously disruptive phenotypic effects. On the other hand, p.Val136Met, which is located far from both active and substrate-binding sites, produced the most drastic reduction of ChAT expression. Overall, CHAT mutations producing low enzyme expression and severe kinetic effects are associated with the most severe phenotypes.

Changes in CSF cholinergic biomarkers in response to cell therapy with NGF in patients with Alzheimer's disease

INTRODUCTION

The extensive loss of central cholinergic functions in Alzheimer's disease (AD) brain is linked to impaired nerve growth factor (NGF) signaling. The cardinal cholinergic biomarker is the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT), which has recently been found in cerebrospinal fluid (CSF). The purpose of this study was to see if EC-NGF therapy will alter CSF levels of cholinergic biomarkers, ChAT, and acetylcholinesterase.

METHOD

Encapsulated cell implants releasing NGF (EC-NGF) were surgically implanted bilaterally in the basal forebrain of six AD patients for 12 months and cholinergic markers in CSF were analyzed.

RESULTS

Activities of both enzymes were altered after 12 months. In particular, the activity of soluble ChAT showed high correlation with cognition, CSF tau and amyloid-β, in vivo cerebral glucose utilization and nicotinic binding sites, and morphometric and volumetric magnetic resonance imaging measures.

DISCUSSION

A clear pattern of association is demonstrated showing a proof-of-principle effect on CSF cholinergic markers, suggestive of a beneficial EC-NGF implant therapy.

Neuronal AChE splice variants and their non-hydrolytic functions: redefining a target of AChE inhibitors?

AChE enzymatic inhibition is a core focus of pharmacological intervention in Alzheimer's disease (AD). Yet, AChE has also been ascribed non-hydrolytic functions, which seem related to its appearance in various isoforms. Neuronal AChE presents as a tailed form (AChE-T) predominantly found on the neuronal synapse, and a facultatively expressed readthough form (AChE-R), which exerts short to medium-term protective effects. Notably, this latter form is also found in the periphery. While these non-hydrolytic functions of AChE are most controversially discussed, there is evidence for them being additional targets of AChE inhibitors. This review aims to provide clarification as to the role of these AChE splice variants and their interplay with other cholinergic parameters and their being targets of AChE inhibition: AChE-R is particularly involved in the mediation of (anti-)apoptotic events in cholinergic cells, involving adaptation of various cholinergic parameters and a time-dependent link to the expression of neuroprotective factors. The AChE-T C-terminus is central to AChE activity regulation, while isolated AChE-T C-terminal fragments mediate toxic effects via the α7 nicotinic acetylcholine receptor. There is direct evidence for roles of AChE-T and AChE-R in neurodegeneration and neuroprotection, with these roles involving AChE as a key modulator of the cholinergic system: in vivo data further encourages the use of AChE inhibitors in the treatment of neurodegenerative conditions such as AD since effects on both enzymatic activity and the enzyme's non-hydrolytic functions can be postulated. It also suggests that novel AChE inhibitors should enhance protective AChE-R, while avoiding the concomitant up-regulation of AChE-T.

Altered neurochemical levels in the rat brain following chronic nicotine treatment

Converging evidence shows that neurochemical systems are crucial mediators of nicotine dependence. Our present study evaluates the effect of 3-month chronic nicotine treatment on the levels of multiple quaternary ammonium compounds as well as glutamate and gamma aminobutyric acid in the rat prefrontal cortex, dorsal striatum and hypothalamus. We observed a marked decrease of acetylcholine levels in the dorsal striatum (22.88%, p<0.01), reflecting the impact of chronic nicotine in local interneuron circuits. We found decreases of carnitine in the dorsal striatum and prefrontal cortex (19.44%, p<0.01; 13.58%, p<0.01, respectively), but robust enhancements of carnitine in the hypothalamus (26.59%, p<0.01), which may reflect the alterations in food and water intake during chronic nicotine treatment. Finally, we identified an increase of prefrontal cortex glutamate levels (8.05%, p<0.05), supporting previous studies suggesting enhanced prefrontal activity during chronic drug use. Our study shows that quaternary ammonium compounds are regulated in a highly brain region specific manner during chronic nicotine treatment, and provides novel insights into neurochemical regulation during nicotine use.

Regulated Extracellular Choline Acetyltransferase Activity- The Plausible Missing Link of the Distant Action of Acetylcholine in the Cholinergic Anti-Inflammatory Pathway

Acetylcholine (ACh), the classical neurotransmitter, also affects a variety of nonexcitable cells, such as endothelia, microglia, astrocytes and lymphocytes in both the nervous system and secondary lymphoid organs. Most of these cells are very distant from cholinergic synapses. The action of ACh on these distant cells is unlikely to occur through diffusion, given that ACh is very short-lived in the presence of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), two extremely efficient ACh-degrading enzymes abundantly present in extracellular fluids. In this study, we show compelling evidence for presence of a high concentration and activity of the ACh-synthesizing enzyme, choline-acetyltransferase (ChAT) in human cerebrospinal fluid (CSF) and plasma. We show that ChAT levels are physiologically balanced to the levels of its counteracting enzymes, AChE and BuChE in the human plasma and CSF. Equilibrium analyses show that soluble ChAT maintains a steady-state ACh level in the presence of physiological levels of fully active ACh-degrading enzymes. We show that ChAT is secreted by cultured human-brain astrocytes, and that activated spleen lymphocytes release ChAT itself rather than ACh. We further report differential CSF levels of ChAT in relation to Alzheimer's disease risk genotypes, as well as in patients with multiple sclerosis, a chronic neuroinflammatory disease, compared to controls. Interestingly, soluble CSF ChAT levels show strong correlation with soluble complement factor levels, supporting a role in inflammatory regulation. This study provides a plausible explanation for the long-distance action of ACh through continuous renewal of ACh in extracellular fluids by the soluble ChAT and thereby maintenance of steady-state equilibrium between hydrolysis and synthesis of this ubiquitous cholinergic signal substance in the brain and peripheral compartments. These findings may have important implications for the role of cholinergic signaling in states of inflammation in general and in neurodegenerative disease, such as Alzheimer's disease and multiple sclerosis in particular.

Chromatographic separation of reaction products from the choline acetyltransferase and carnitine acetyltransferase assay: differential ChAT and CrAT activity in brain extracts from Alzheimer's disease versus controls

Choline acetyltransferase (ChAT) catalyzes the reaction between choline and acetylcoenzyme A (AcCoA) to form acetylcholine (ACh) in nerve terminals. ACh metabolism has implications in numerous aspects of physiology and varied disease states, such as Alzheimer's disease. Therefore a specific, sensitive, and reliable method for detecting ChAT enzyme activity is of great utility in a number of situations. Using an existing radionuclide-based enzyme activity assay, we have observed detectable ChAT signals from non-cholinergic cells, suggesting a contaminant in the assay producing an artifactual signal. Previous reports have suggested that L-acetylcarnitine (LAC) contaminates many assays of ChAT activity, because of difficulties in separating LAC from ACh by organic extraction. To determine the source of this hypothesized artifact and to rectify the problem, we have developed a paper chromatography-based assay for the detection of acetylcholine and other contaminating reaction products of this assay, including LAC. Our first goal was to develop a simple and economical method for resolving and verifying the identities of various reaction products or contaminants that could be performed in most laboratories without specialized equipment. Our second goal was to apply this separation method in postmortem human brain tissue samples. Our assay successfully detected several contaminants, especially in assays using brain tissue, and allowed the separation of the intended ACh product from these contaminants. We further demonstrate that this assay can be used to measure carnitine acetyltransferase (CrAT) activity in the same samples, and assays comparing ChAT and CrAT show that CrAT is highly active in neuronal tissues and in neuronal cell cultures relative to ChAT. Thus, the simple chromatography-based assay we describe allows the measurement of specific reaction products separated from contaminants using commonly available and inexpensive materials. Further, we show that ChAT activity is significantly reduced in brain extracts from Alzheimer's disease compared to controls.

Vesicular and plasma membrane transporters for neurotransmitters

The regulated exocytosis that mediates chemical signaling at synapses requires mechanisms to coordinate the immediate response to stimulation with the recycling needed to sustain release. Two general classes of transporter contribute to release, one located on synaptic vesicles that loads them with transmitter, and a second at the plasma membrane that both terminates signaling and serves to recycle transmitter for subsequent rounds of release. Originally identified as the target of psychoactive drugs, these transport systems have important roles in transmitter release, but we are only beginning to understand their contribution to synaptic transmission, plasticity, behavior, and disease. Recent work has started to provide a structural basis for their activity, to characterize their trafficking and potential for regulation. The results indicate that far from the passive target of psychoactive drugs, neurotransmitter transporters undergo regulation that contributes to synaptic plasticity.

Replication and extension of association of choline acetyltransferase with nicotine dependence in European and African American smokers

Choline acetyltransferase is critical in the synthesis of acetylcholine and regulation of cholinergic neuron functions. We recently reported association of the encoding gene ChAT with both smoking cessation and nicotine dependence (ND) in two independent European American (EA) samples; however, in the replication sample, only limited SNPs partially covering the gene were examined. In this study, we examined the association of 14 SNPs, which cover the entire gene, with ND, assessed by smoking quantity (SQ), heaviness of smoking index (HSI), and Fagerström Test for ND (FTND), in 2,037 subjects from 602 families of African American (AA) or EA origin. Individual SNP-based association analysis revealed that five SNPs showed nominal association with at least one ND measure in one of the samples (P = 0.022-0.042); none remained significant after correction for multiple testing. Haplotype-based association analysis revealed that haplotypes G-G-A-C, formed by rs1880676-rs3810950-rs10082479-rs8178990 (P = 0.005-0.0178), and G-G-T-C-G-C, formed by rs1880676-rs3810950-rs10082479-rs8178990-rs3793790-rs12266458 (P = 0.00247-0.00468), displayed significant association with all three ND measures in the AA sample, as did haplotype T-C-G-A-T, formed by rs12266458-rs11101191-rs8178991-rs4838544-rs4838547 (P = 0.00741-0.0103), in the EA sample. All these detected haplotype-based associations remained significant after correction for all major haplotypes for a given SNP combination. Together, our findings, in conjunction with the previous report of the association, warrant further investigation of ChAT in ND.

The vesicular acetylcholine transporter is required for neuromuscular development and function

The vesicular acetylcholine (ACh) transporter (VAChT) mediates ACh storage by synaptic vesicles. However, the VAChT-independent release of ACh is believed to be important during development. Here we generated VAChT knockout mice and tested the physiological relevance of the VAChT-independent release of ACh. Homozygous VAChT knockout mice died shortly after birth, indicating that VAChT-mediated storage of ACh is essential for life. Indeed, synaptosomes obtained from brains of homozygous knockouts were incapable of releasing ACh in response to depolarization. Surprisingly, electrophysiological recordings at the skeletal-neuromuscular junction show that VAChT knockout mice present spontaneous miniature end-plate potentials with reduced amplitude and frequency, which are likely the result of a passive transport of ACh into synaptic vesicles. Interestingly, VAChT knockouts exhibit substantial increases in amounts of choline acetyltransferase, high-affinity choline transporter, and ACh. However, the development of the neuromuscular junction in these mice is severely affected. Mutant VAChT mice show increases in motoneuron and nerve terminal numbers. End plates are large, nerves exhibit abnormal sprouting, and muscle is necrotic. The abnormalities are similar to those of mice that cannot synthesize ACh due to a lack of choline acetyltransferase. Our results indicate that VAChT is essential to the normal development of motor neurons and the release of ACh.

Reduced expression of the vesicular acetylcholine transporter causes learning deficits in mice

Storage of acetylcholine in synaptic vesicles plays a key role in maintaining cholinergic function. Here we used mice with a targeted mutation in the vesicular acetylcholine transporter (VAChT) gene that reduces transporter expression by 40% to investigate cognitive processing under conditions of VAChT deficiency. Motor skill learning in the rotarod revealed that VAChT mutant mice were slower to learn this task, but once they reached maximum performance they were indistinguishable from wild-type mice. Interestingly, motor skill performance maintenance after 10 days was unaffected in these mutant mice. We also tested whether reduced VAChT levels affected learning in an object recognition memory task. We found that VAChT mutant mice presented a deficit in memory encoding necessary for the temporal order version of the object recognition memory, but showed no alteration in spatial working memory, or spatial memory in general when tested in the Morris water maze test. The memory deficit in object recognition memory observed in VAChT mutant mice could be reversed by cholinesterase inhibitors, suggesting that learning deficits caused by reduced VAChT expression can be ameliorated by restoring ACh levels in the synapse. These data indicate an important role for cholinergic tone in motor learning and object recognition memory.

Nutritional factors in a mouse model of Rett syndrome

Environmental factors such as nutrition and housing can influence behavioral and anatomical characteristics of several neurological disorders, including Rett syndrome (RTT). RTT is associated with mutations in the X-linked gene encoding MeCP2, a transcriptional repressor that binds methylated DNA. While direct genetic intervention in humans is impossible at this time, motor and cognitive deficits in RTT may be ameliorated through manipulations of epigenetic/environmental factors. For example, studies in rodents suggest that choline nutrient supplementation during critical periods of brain development enhances cholinergic neurotransmission, alters neuronal size and distribution, and facilitates performance of memory and motoric tasks. Recent work in a mouse model of RTT shows that enhancing maternal nutrition through choline supplementation improves both anatomical and behavioral symptoms in the mutant offspring. We describe here cellular and molecular mechanisms that may underlie this specific enhancement and may provide more general insights into mechanisms underlying gene-environment interactions in neurological disorders.

The length of hippocampal cholinergic fibers is reduced in the aging brain

Cholinergic deficits occur in the aged hippocampus and they are significant in Alzheimer's disease. Using stereological and biochemical approaches, we characterized the cholinergic septohippocampal pathway in old (24 months) and young adult (3 months) rats. The total length of choline acetyltransferase (ChAT)-positive fibers in the dorsal hippocampus was significantly decreased by 32% with aging (F((1,9))=20.94, p=0.0014), along with the levels of synaptophysin, a presynaptic marker. No significant changes were detected in ChAT activity or in the amounts of ChAT protein, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tropomyosin related kinase receptor (Trk) A, TrkB, or p75 neurotrophin receptor (p75(NTR)) in the aged dorsal hippocampus. The number and size of ChAT-positive neurons and the levels of ChAT activity, NGF and BDNF were not statistically different in the septum of aged and young adult rats. This study suggests that substantial synaptic loss and cholinergic axonal degeneration occurs during aging and reinforces the importance of therapies that can protect axons and promote their growth in order to restore cholinergic neurotransmission.

Substrate binding and catalytic mechanism of human choline acetyltransferase

Choline acetyltransferase (ChAT) catalyzes the synthesis of the neurotransmitter acetylcholine from choline and acetyl-CoA, and its presence is a defining feature of cholinergic neurons. We report the structure of human ChAT to a resolution of 2.2 A along with structures for binary complexes of ChAT with choline, CoA, and a nonhydrolyzable acetyl-CoA analogue, S-(2-oxopropyl)-CoA. The ChAT-choline complex shows which features of choline are important for binding and explains how modifications of the choline trimethylammonium group can be tolerated by the enzyme. A detailed model of the ternary Michaelis complex fully supports the direct transfer of the acetyl group from acetyl-CoA to choline through a mechanism similar to that seen in the serine hydrolases for the formation of an acyl-enzyme intermediate. Domain movements accompany CoA binding, and a surface loop, which is disordered in the unliganded enzyme, becomes localized and binds directly to the phosphates of CoA, stabilizing the complex. Interactions between this surface loop and CoA may function to lower the KM for CoA and could be important for phosphorylation-dependent regulation of ChAT activity.

Polysialic acid limits choline acetyltransferase activity induced by brain-derived neurotrophic factor

Choline acetyltransferase (ChAT), the enzyme synthesizing acetylcholine, is known to be activated by brain derived neurotrophic factor (BDNF). We found that the specific removal of the carbohydrate polysialic acid (PSA) significantly increased BDNF-induced ChAT-activity in embryonic septal neurons. Using a p75 neurotrophin receptor (p75(NTR)) function-blocking antibody and K252a, a-pan tropomyosin related kinase (Trk) inhibitor, we demonstrate that BDNF-induced ChAT activity requires the stimulation of p75(NTR) and TrkB. PSA removal drastically increased radioactive iodinated ([(125)I])BDNF's maximal binding capacity (Bmax), derived from concentrations of [(125)I]BDNF ranging from 1 pM to 3.2 nM. In the presence of unlabeled nerve growth factor to prevent the binding of [(125)I]BDNF to p75(NTR) sites, the impact of PSA removal on the binding capacity of [(125)I]BDNF was greatly reduced. In conclusion, PSA limits BDNF-induced ChAT activity and BDNF-receptor interactions. BDNF-induced ChAT activity is TrkB and p75(NTR) dependent, and upon PSA removal the additional binding of BDNF to its receptors, especially p75(NTR), likely contributes to the maximal ChAT activity observed. In vivo, the ontogenetic loss of PSA in the postnatal period may allow more interactions between BDNF and its receptors to increase ChAT activity and assure the proper development of the cholinergic septal neurons.