Short-term effects of zinc on acetylcholine metabolism and viability of SN56 cholinergic neuroblastoma cells

Effects of Marginal Zn Excess and Thiamine Deficiency on Microglial N9 Cell Metabolism and Their Interactions with Septal SN56 Cholinergic Cells

Mild thiamine deficiency aggravates Zn accumulation in cholinergic neurons. It leads to the augmentation of Zn toxicity by its interaction with the enzymes of energy metabolism. Within this study, we tested the effect of Zn on microglial cells cultivated in a thiamine-deficient medium, containing 0.003 mmol/L of thiamine vs. 0.009 mmol/L in a control medium. In such conditions, a subtoxic 0.10 mmol/L Zn concentration caused non-significant alterations in the survival and energy metabolism of N9 microglial cells. Both activities of the tricarboxylic acid cycle and the acetyl-CoA level were not decreased in these culture conditions. Amprolium augmented thiamine pyrophosphate deficits in N9 cells. This led to an increase in the intracellular accumulation of free Zn and partially aggravated its toxicity. There was differential sensitivity of neuronal and glial cells to thiamine-deficiency-Zn-evoked toxicity. The co-culture of neuronal SN56 with microglial N9 cells reduced the thiamine-deficiency-Zn-evoked inhibition of acetyl-CoA metabolism and restored the viability of the former. The differential sensitivity of SN56 and N9 cells to borderline thiamine deficiency combined with marginal Zn excess may result from the strong inhibition of pyruvate dehydrogenase in neuronal cells and no inhibition of this enzyme in the glial ones. Therefore, ThDP supplementation can make any brain cell more resistant to Zn excess.

Phosphonate Inhibitors of Pyruvate Dehydrogenase Perturb Homeostasis of Amino Acids and Protein Succinylation in the Brain

Mitochondrial pyruvate dehydrogenase complex (PDHC) is essential for brain glucose and neurotransmitter metabolism, which is dysregulated in many pathologies. Using specific inhibitors of PDHC in vivo, we determine biochemical and physiological responses to PDHC dysfunction. Dose dependence of the responses to membrane-permeable dimethyl acetylphosphonate (AcPMe2) is non-monotonous. Primary decreases in glutathione and its redox potential, methionine, and ethanolamine are alleviated with increasing PDHC inhibition, the alleviation accompanied by physiological changes. A comparison of 39 brain biochemical parameters after administration of four phosphinate and phosphonate analogs of pyruvate at a fixed dose of 0.1 mmol/kg reveals no primary, but secondary changes, such as activation of 2-oxoglutarate dehydrogenase complex (OGDHC) and decreased levels of glutamate, isoleucine and leucine. The accompanying decreases in freezing time are most pronounced after administration of methyl acetylphosphinate and dimethyl acetylphosphonate. The PDHC inhibitors do not significantly change the levels of PDHA1 expression and phosphorylation, sirtuin 3 and total protein acetylation, but increase total protein succinylation and glutarylation, affecting sirtuin 5 expression. Thus, decreased production of the tricarboxylic acid cycle substrate acetyl-CoA by inhibited PDHC is compensated by increased degradation of amino acids through the activated OGDHC, increasing total protein succinylation/glutarylation. Simultaneously, parasympathetic activity and anxiety indicators decrease.

Metabolic and Cellular Compartments of Acetyl-CoA in the Healthy and Diseased Brain

The human brain is characterised by the most diverse morphological, metabolic and functional structure among all body tissues. This is due to the existence of diverse neurons secreting various neurotransmitters and mutually modulating their own activity through thousands of pre- and postsynaptic interconnections in each neuron. Astroglial, microglial and oligodendroglial cells and neurons reciprocally regulate the metabolism of key energy substrates, thereby exerting several neuroprotective, neurotoxic and regulatory effects on neuronal viability and neurotransmitter functions. Maintenance of the pool of mitochondrial acetyl-CoA derived from glycolytic glucose metabolism is a key factor for neuronal survival. Thus, acetyl-CoA is regarded as a direct energy precursor through the TCA cycle and respiratory chain, thereby affecting brain cell viability. It is also used for hundreds of acetylation reactions, including N-acetyl aspartate synthesis in neuronal mitochondria, acetylcholine synthesis in cholinergic neurons, as well as divergent acetylations of several proteins, peptides, histones and low-molecular-weight species in all cellular compartments. Therefore, acetyl-CoA should be considered as the central point of metabolism maintaining equilibrium between anabolic and catabolic pathways in the brain. This review presents data supporting this thesis.

Protection of Cholinergic Neurons against Zinc Toxicity by Glial Cells in Thiamine-Deficient Media

Brain pathologies evoked by thiamine deficiency can be aggravated by mild zinc excess. Cholinergic neurons are the most susceptible to such cytotoxic signals. Sub-toxic zinc excess aggravates the injury of neuronal SN56 cholinergic cells under mild thiamine deficiency. The excessive cell loss is caused by Zn interference with acetyl-CoA metabolism. The aim of this work was to investigate whether and how astroglial C6 cells alleviated the neurotoxicity of Zn to cultured SN56 cells in thiamine-deficient media. Low Zn concentrations did not affect astroglial C6 and primary glial cell viability in thiamine-deficient conditions. Additionally, parameters of energy metabolism were not significantly changed. Amprolium (a competitive inhibitor of thiamine uptake) augmented thiamine pyrophosphate deficits in cells, while co-treatment with Zn enhanced the toxic effect on acetyl-CoA metabolism. SN56 cholinergic neuronal cells were more susceptible to these combined insults than C6 and primary glial cells, which affected pyruvate dehydrogenase activity and the acetyl-CoA level. A co-culture of SN56 neurons with astroglial cells in thiamine-deficient medium eliminated Zn-evoked neuronal loss. These data indicate that astroglial cells protect neurons against Zn and thiamine deficiency neurotoxicity by preserving the acetyl-CoA level.

The Multifaceted Roles of Zinc in Neuronal Mitochondrial Dysfunction

Zinc is a highly abundant cation in the brain, essential for cellular functions, including transcription, enzymatic activity, and cell signaling. However, zinc can also trigger injurious cascades in neurons, contributing to the pathology of neurodegenerative diseases. Mitochondria, critical for meeting the high energy demands of the central nervous system (CNS), are a principal target of the deleterious actions of zinc. An increasing body of work suggests that intracellular zinc can, under certain circumstances, contribute to neuronal damage by inhibiting mitochondrial energy processes, including dissipation of the mitochondrial membrane potential (MMP), leading to ATP depletion. Additional consequences of zinc-mediated mitochondrial damage include reactive oxygen species (ROS) generation, mitochondrial permeability transition, and excitotoxic calcium deregulation. Zinc can also induce mitochondrial fission, resulting in mitochondrial fragmentation, as well as inhibition of mitochondrial motility. Here, we review the known mechanisms responsible for the deleterious actions of zinc on the organelle, within the context of neuronal injury associated with neurodegenerative processes. Elucidating the critical contributions of zinc-induced mitochondrial defects to neurotoxicity and neurodegeneration may provide insight into novel therapeutic targets in the clinical setting.

The Impact of Acetyl-CoA and Aspartate Shortages on the N-Acetylaspartate Level in Different Models of Cholinergic Neurons

N-acetylaspartate is produced by neuronal aspartate N-acetyltransferase (NAT8L) from acetyl-CoA and aspartate. In cholinergic neurons, acetyl-CoA is also utilized in the mitochondrial tricarboxylic acid cycle and in acetylcholine production pathways. While aspartate has to be shared with the malate–aspartate shuttle, another mitochondrial machinery together with the tricarboxylic acid cycle supports the electron transport chain turnover. The main goal of this study was to establish the impact of toxic conditions on N-acetylaspartate production. SN56 cholinergic cells were exposed to either Zn²⁺ overload or Ca²⁺ homeostasis dysregulation and male adult Wistar rats’ brains were studied after 2 weeks of challenge with streptozotocin-induced hyperglycemia or daily theophylline treatment. Our results allow us to hypothesize that the cholinergic neurons from brain septum prioritized the acetylcholine over N-acetylaspartate production. This report provides the first direct evidence for Zn²⁺-dependent suppression of N-acetylaspartate synthesis leading to mitochondrial acetyl-CoA and aspartate shortages. Furthermore, Zn²⁺ is a direct concentration-dependent inhibitor of NAT8L activity, while Zn²⁺-triggered oxidative stress is unlikely to be significant in such suppression.

Aggravated effects of coexisting marginal thiamine deficits and zinc excess on SN56 neuronal cells

Objectives: Zinc excitotoxicity and thiamine pyrophosphate deficiency (TD) are known pathogenic signals contributing to mechanism of different encephalopathies through inhibition of enzymes responsible for energy metabolism such as pyruvate dehydrogenase, aconitase or ketoglutarate dehydrogenase. The aim of this work was to investigate whether subclinical Zn excess and TD, frequent in aging brain, may combine yielding overt neuronal impairment.Results: Clonal SN56 cholinergic neuronal cells of septal origin were used as the model of brain cholinergic neurons, which are particularly susceptible to neurodegeneration in the course of Alzheimer's disease, hypoxia and other dementia-linked brain pathologies. Neither subtoxic concentration of Zn (0.10 mM) nor mild 20-25% TD deficits alone caused significant negative changes in cultured cholinergic neurons viability and their acetyl-CoA/acetylcholine metabolism. However, cells with mild TD accumulated Zn in excess, which impaired their energy metabolism causing a loss of neurons viability and their function as neurotransmitters. These negative effects of Zn were aggravated by amprolium which is an inhibitor of thiamine intracellular transport.Conclusion: Our data indicate that TD may amplify otherwise non-harmful border-line Zn excitotoxic signals yielding progress of neurodegeneration.

Protective effects of voltage-gated calcium channel antagonists against zinc toxicity in SN56 neuroblastoma cholinergic cells

One of the pathological site effects in excitotoxic activation is Zn²⁺ overload to postsynaptic neurons. Such an effect is considered to be equivalent to the glutamate component of excitotoxicity. Excessive uptake of Zn²⁺ by active voltage-dependent transport systems in these neurons may lead to significant neurotoxicity. The aim of this study was to investigate whether and which antagonists of the voltage gated calcium channels (VGCC) might modify this Zn²⁺-induced neurotoxicity in neuronal cells. Our data demonstrates that depolarized SN56 neuronal cells may take up large amounts of Zn²⁺ and store these in cytoplasmic and mitochondrial sub-fractions. The mitochondrial Zn²⁺ excess suppressed pyruvate uptake and oxidation. Such suppression was caused by inhibition of pyruvate dehydrogenase complex, aconitase and NADP-isocitrate dehydrogenase activities, resulting in the yielding of acetyl-CoA and ATP shortages. Moreover, incoming Zn²⁺ increased both oxidized glutathione and malondialdehyde levels, known parameters of oxidative stress. In depolarized SN56 cells, nifedipine treatment (L-type VGCC antagonist) reduced Zn²⁺ uptake and oxidative stress. The treatment applied prevented the activities of PDHC, aconitase and NADP-IDH enzymes, and also yielded the maintenance of acetyl-CoA and ATP levels. Apart from suppression of oxidative stress, N- and P/Q-type VGCCs presented a similar, but weaker protective influence. In conclusion, our data shows that in the course of excitotoxity, impairment to calcium homeostasis is tightly linked with an excessive neuronal Zn²⁺ uptake. Hence, the VGCCs types L, N and P/Q share responsibility for neuronal Zn²⁺ overload followed by significant energy-dependent neurotoxicity. Moreover, Zn²⁺ affects the target tricarboxylic acid cycle enzymes, yields acetyl-CoA and energy deficits as well.

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

Phenotype-Dependent Interactions between N-acetyl-L-Aspartate and Acetyl-CoA in Septal SN56 Cholinergic Cells Exposed to an Excess of Zinc

Pyruvate dehydrogenase reaction utilizing glucose-derived pyruvate is an almost exclusive source of acetyl-CoA in different cell mitochondrial compartments of the brain. In neuronal mitochondria, the largest fraction of acetyl-CoA is utilized for energy production and the much smaller one for N-acetyl-L-aspartate (NAA) synthesis. Cholinergic neurons, unlike others, require additional amounts of acetyl-CoA for acetylcholine synthesis. Therefore, several neurotoxic signals, which inhibit pyruvate dehydrogenase, generate deeper shortages of acetyl-CoA and greater mortality of cholinergic neurons than noncholinergic ones. NAA is considered to be a marker of neuronal energy status in neuropathic brains. However, there is no data on putative differential fractional distribution of the acetyl-CoA pool between energy producing and NAA or acetylcholine synthesizing pathways in noncholinergic and cholinergic neurons, respectively. Therefore, the aim of this study was to investigate whether zinc-excess, a common excitotoxic signal, may evoke differential effects on the NAA metabolism in neuronal cells with low and high expression of the cholinergic phenotype. Differentiated SN56 neuronal cells, displaying a high activity of choline acetyltransferase and rates of acetylcholine synthesis, contained lower levels of acetyl-CoA and NAA, being more susceptible to ZnCl2 exposition that the nondifferentiated SN56 or differentiated dopaminergic SHSY5Y neuronal and astroglial C6 cells. Differentiated SN56 accumulated greater amounts of Zn2 + from extracellular space than the other ones, and displayed a stronger suppression of pyruvate dehydrogenase complex activity and acetyl-CoA, NAA, ATP, acetylcholine levels, and loss of viability. These data indicate that the acetyl-CoA synthesizing system in neurons constitutes functional unity with energy generating and NAA or acetylcholine pathways of its utilization, which are uniformly affected by neurotoxic conditions.

Early and Late Pathomechanisms in Alzheimer's Disease: From Zinc to Amyloid-β Neurotoxicity

There are several systemic and intracerebral pathologic conditions, which limit provision and utilization of energy precursor metabolites in neuronal cells. Energy deficits cause excessive depolarization of neuronal cells triggering glutamate-zinc evoked excitotoxic cascade. The intracellular zinc excess hits several intraneuronal targets yielding collapse of energy balance and impairment functional and structural impairments cholinergic neurons. Disturbances in metabolism of acetyl-CoA, which is a direct precursor for energy, acetylcholine, N-acetyl-L-aspartate and acetylated proteins synthesis, play an important role in these pathomechanisms. Disruption of brain homeostasis activates slow accumulation of amyloid-β 1-42 , which extra and intracellular oligomeric deposits disrupt diverse transporting and signaling processes in all membrane structures of the cell. Both neurotoxic signals may combine aggravating detrimental effects on neuronal cell. Different neuroglial and neuronal cell types may display differential susceptibility to similar pathogenic insults depending on specific features of their energy and functional parameters. This review, basing on findings gained from cellular and animal models of Alzheimer's disease, discusses putative energy/acetyl-CoA dependent mechanism in early and late stages of neurodegeneration.

AβPP-Transgenic 2576 Mice Mimic Cell Type-Specific Aspects of Acetyl-CoA-Linked Metabolic Deficits in Alzheimer's Disease

The pyruvate-derived acetyl-CoA is a principal direct precursor substrate for bulk energy synthesis in the brain. Deficits of pyruvate dehydrogenase in the neocortex are common features of Alzheimer's disease and other age-related encephalopathies in humans. Therefore, amyloid-β overload in brains of diverse transgenic Alzheimer's disease model animals was investigated as one of neurotoxic compounds responsible for pyruvate dehydrogenase inhibition yielding deficits of cholinergic neurotransmission and cognitive functions. Brains of aged, 14-16-month-old Tg2576 mice contained 0.6 μmol/kg levels of amyloid-β1 - 42. Activities of pyruvate dehydrogenase complex, choline acetyltransferase, and several enzymes of acetyl-CoA and energy metabolism were found to be unchanged in both forebrain mitochondria and synaptosomes of Tg2576 mice, indicating preservation of structural integrity at least in cholinergic neuronal cells. However, in transgenic brain synaptosomes, pyruvate utilization, mitochondrial levels, and cytoplasmic acetyl-CoA levels, as well as acetylcholine content and its quantal release, were all found to be decreased by 25-40% . On the contrary, activation of pyruvate utilization was detected and no alterations in acetyl-CoA content and citrate or α-ketoglutarate accumulation were observed in transgenic whole brain mitochondria. These data indicate that amyloid-β evoked deficits in acetyl-CoA are confined to mitochondrial and cytoplasmic compartments of Tg2576 nerve terminals, becoming early primary signals paving the path for further stages of neurodegeneration. On the other hand, acetyl-CoA synthesis in mitochondrial compartments of glial cells seems to be activated despite amyloid-β accumulated in transgenic brains.

Efficacy of Zinc Sulfate as an Add-on Therapy to Risperidone Versus Risperidone Alone in Patients With Schizophrenia: A Double-Blind Randomized Placebo-Controlled Trial

BACKGROUND

Zinc can modulate fast-excitatory transmission, facilitate the release of amino butyric acid and potentiate nicotinic acetylcholine receptors. There are also emerging evidences discussing the implication of these neurotransmitters in pathophysiology of schizophrenia.

OBJECTIVES

The purpose of this study was to evaluate the efficacy of Zn sulfate as an add-on therapy in the treatment of schizophrenia in a 6-week, double-blind and placebo-controlled trial.

PATIENTS AND METHODS

Eligible participants were 30 inpatients with schizophrenia according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision criteria. Patients were randomly allocated into two equal groups; one group of patients received risperidone 6 mg/day plus capsules of Zn sulfate (each containing 50 mg elemental Zn) three times a day and another group received risperidone 6 mg/day plus placebo. The Positive and Negative Syndrome Scale (PANSS) was applied to assess the psychotic symptoms and aggression risk at baseline, week 2, 4, and 6 of the study.

RESULTS

The results of this study showed that both protocols significantly decreased the scores on all subscales of the PANSS and supplemental aggression risk subscale as well as PANSS total score over the study. However, this improvement was significantly higher in Zn sulfate receiving group compared to the placebo group. No major clinical side-effects were detected.

CONCLUSIONS

It may be concluded that Zn is an effective adjuvant agent in the management of patients with schizophrenia.

Intracellular redistribution of acetyl-CoA, the pivotal point in differential susceptibility of cholinergic neurons and glial cells to neurodegenerative signals

Intramitochondrial decarboxylation of glucose-derived pyruvate by PDHC (pyruvate dehydrogenase complex) is a principal source of acetyl-CoA, for mitochondrial energy production and cytoplasmic synthetic pathways in all types of brain cells. The inhibition of PDHC, ACO (aconitase) and KDHC (ketoglutarate dehydrogenase complex) activities by neurodegenerative signals such as aluminium, zinc, amyloid β-peptide, excess nitric oxide (NO) or thiamine pyrophosphate deficits resulted in much deeper losses of viability, acetyl-CoA and ATP in differentiated cholinergic neuronal cells than in non-differentiated cholinergic, and cultured microglial or astroglial cell lines. In addition, in cholinergic cells, such conditions caused inhibition of ACh (acetylcholine) synthesis and its quantal release. Furthermore, cholinergic neuronal cells appeared to be resistant to high concentrations of LPS (lipopolysaccharide). In contrast, in microglial cells, low levels of LPS caused severalfold activation of NO, IL-6 (interleukin 6) and TNFα (tumour necrosis factor α) synthesis/release, accompanied by inhibition of PDHC, KDHC and ACO activities, and suppression of acetyl-CoA, but relatively small losses in their ATP contents and viability parameters. Compounds that protected these enzymes against inhibitory effects of neurotoxins alleviated acetyl-CoA and ATP deficits, thereby maintaining neuronal cell viability. These data indicate that preferential susceptibility of cholinergic neurons to neurodegenerative insults may result from competition for acetyl-CoA between mitochondrial energy-producing and cytoplasmic ACh-synthesizing pathways. Such a hypothesis is supported by the existence of highly significant correlations between mitochondrial/cytoplasmic acetyl-CoA levels and cell viability/transmitter functions respectively.

Retinoic acid as a therapeutic option in Alzheimer's disease: a focus on cholinergic restoration

Retinoic acid is a potent cell differentiating factor, which through its nuclear receptors affects a vast range of promoter sites in brain neuronal and glial cells in every step of embryonic and postnatal life. Its capacities, facilitating maturation of neurotransmitter phenotype in different groups of neurons, pave the way for its application as a potential therapeutic agent in neurodegenerative diseases including Alzheimer's disease. Retinoic acid was found to exert particularly strong enhancing effects on acetylcholine transmitter functions in brain cholinergic neurons, loss of which is tightly linked to the development of cognitive and memory deficits in course of different cholinergic encephalopathies. Here, we review cholinotrophic properties of retinoic acid and its derivatives, which may justify their application in the management of Alzheimer's disease and the related neurodegenerative conditions.

Differential effects of lipopolysaccharide on energy metabolism in murine microglial N9 and cholinergic SN56 neuronal cells

There are significant differences between acetyl-CoA and ATP levels, enzymes of acetyl-CoA metabolism, and toll-like receptor 4 contents in non-activated microglial N9 and non-differentiated cholinergic SN56 neuroblastoma cells. Exposition of N9 cells to lipopolysaccharide caused concentration-dependent several-fold increases of nitrogen oxide synthesis, accompanied by inhibition of pyruvate dehydrogenase complex, aconitase, and α-ketoglutarate dehydrogenase complex activities, and by nearly proportional depletion of acetyl-CoA, but by relatively smaller losses in ATP content and cell viability (about 5%). On the contrary, SN56 cells appeared to be insensitive to direct exposition to high concentration of lipopolysaccharide. However, exogenous nitric oxide resulted in marked inhibition pyruvate dehydrogenase and aconitase activities, depletion of acetyl-CoA, along with respective loss of SN56 cells viability. These data indicate that these two common neurodegenerative signals may differentially affect energy-acetyl-CoA metabolism in microglial and cholinergic neuronal cell compartments in the brain. Moreover, microglial cells appeared to be more resistant than neuronal cells to acetyl-CoA and ATP depletion evoked by these neurodegenerative conditions. Together, these data indicate that differential susceptibility of microglia and cholinergic neuronal cells to neurotoxic signals may result from differences in densities of toll-like receptors and degree of disequilibrium between acetyl-CoA provision in mitochondria and its utilization for energy production and acetylation reactions in each particular group of cells. There are significant differences between acetyl-CoA and ATP levels and enzymes of acetyl-CoA metabolism in non-activated microglial N9 and non-differentiated cholinergic SN56 neuroblastoma cells. Pathological stimulation of microglial toll-like receptors (TLRs) triggered excessive synthesis of microglia-derived nitric oxide (NO)/NOO radicals that endogenously inhibited pyruvate dehydrogenase complex (PDHC), aconitase, and α-ketoglutarate dehydrogenase complex. However, it caused none or small suppressions of acetyl-CoA and microglial viability, respectively. Microglia-derived NO inhibited same enzymes in cholinergic neuronal cells causing marked viability loss because of acetyl-CoA deficits evoked by its competitive consumption by energy producing and acetylcholine/N-acetyl-l-aspartate (NAA) synthesizing pathways.

Acetyl-CoA the key factor for survival or death of cholinergic neurons in course of neurodegenerative diseases

Glucose-derived pyruvate is a principal source of acetyl-CoA in all brain cells, through pyruvate dehydogenase complex (PDHC) reaction. Cholinergic neurons like neurons of other transmitter systems and glial cells, utilize acetyl-CoA for energy production in mitochondria and diverse synthetic pathways in their extramitochondrial compartments. However, cholinergic neurons require additional amounts of acetyl-CoA for acetylcholine synthesis in their cytoplasmic compartment to maintain their transmitter functions. Characteristic feature of several neurodegenerating diseases including Alzheimer’s disease and thiamine diphosphate deficiency encephalopathy is the decrease of PDHC activity correlating with cholinergic deficits and losses of cognitive functions. Such conditions generate acetyl-CoA deficits that are deeper in cholinergic neurons than in noncholinergic neuronal and glial cells, due to its additional consumption in the transmitter synthesis. Therefore, any neuropathologic conditions are likely to be more harmful for the cholinergic neurons than for noncholinergic ones. For this reason attempts preserving proper supply of acetyl-CoA in the diseased brain, should attenuate high susceptibility of cholinergic neurons to diverse neurodegenerative conditions. This review describes how common neurodegenerative signals could induce deficts in cholinergic neurotransmission through suppression of acetyl-CoA metabolism in the cholinergic neurons.

Pathophysiological amyloid concentrations induce sustained upregulation of readthrough acetylcholinesterase mediating anti-apoptotic effects

Cholinergically differentiated SH-SY5Y neuroblastoma cells were treated with a pathophysiologically relevant, low (300 nM), and a high (3 μM) dose of amyloid beta 1-42 (Abeta) or 42-1 (revAbeta). At early (1 and 4h) and late (24h) time points, the pro- and anti-apoptotic factors--caspase-3 and p53, and B-cell lymphoma 2 protein (Bcl-2), respectively--were assessed together with lactate dehydrogenase (LDH) release as measure of cell viability and ATP levels as marker of mitochondrial activity. The low peptide dose significantly increased Bcl-2 and, time-delayed, caspase-3 and ATP levels, but barely impacted on LDH release, while the high concentration remarkably depressed Bcl-2 levels, depleted ATP and led to increased LDH release. We also monitored acetylcholinesterase (AChE) enzymatic activity and splice variant levels (tailed and readthrough AChE; AChE-T and AChE-R), and assessed choline acetyltransferase (ChAT) and high-affinity choline uptake (HACU). The low Abeta concentration drastically upregulated AChE-R and increased both ChAT and HACU, while the high dose caused cholinergic toxicity. We believe this study offers the first insight into the highly concentration-dependent effects of Abeta on cholinergic dynamics. In particular, it highlights the rescuing role of AChE-R as being, together with mitochondrial activity, involved in cholinergic adaptation to low doses of Abeta.

Acetyl-CoA metabolism in amprolium-evoked thiamine pyrophosphate deficits in cholinergic SN56 neuroblastoma cells

Inhibition of pyruvate (PDHC) and ketoglutarate (KDHC) dehydrogenase complexes induced by thiamine pyrophosphate deficits is known cause of disturbances of cholinergic transmission in the brain, yielding clinical symptoms of cognitive, vegetative and motor deficits. However, particular alterations in distribution of key acetylcholine precursor, acetyl-CoA, in the cholinergic neuron compartment of thiamine pyrophosphate-deficient brain remain unknown. Therefore, the aim of our work was to find out how amprolium-induced thiamine pyrophosphate deficits (TD) affect distribution of acetyl-CoA in the compartment of pure cholinergic neuroblastoma SN56 cells originating from murine septum. Amprolium caused similar concentration-dependent decreases in thiamine pyrophosphate levels in nondifferentiated (NC) and differentiated (DC) cells cultured in low thiamine medium. In such conditions DC displayed significantly greater loss of viability than the NC ones, despite of lesser suppressions of PDHC activities and tetrazolium salt reduction rates in the former. On the other hand, intramitochondrial acetyl-CoA levels in DC were 73% lower than in NC, which explains their greater susceptibility to TD. Choline acetyltransferase activity and acetylcholine content in DC were two times higher than in NC. TD caused 50% decrease of cytoplasmic acetyl-CoA levels that correlated with losses of acetylcholine pool in DC but not in NC. These data indicate that particular sensitivity of DC to TD may result from relative shortage of acetyl-CoA due to its higher utilization in acetylcholine synthesis.

Acetyl-CoA and acetylcholine metabolism in nerve terminal compartment of thiamine deficient rat brain

The decrease of pyruvate and ketoglutarate dehydrogenase complex activities is the main cause of energy and acetyl-CoA deficits in thiamine deficiency-evoked cholinergic encephalopathies. However, disturbances in pathways of acetyl-CoA metabolism leading to appearance of cholinergic deficits remain unknown. Therefore, the aim of this work was to investigate alterations in concentration and distribution of acetyl-CoA and in acetylcholine metabolism in brain nerve terminals, caused by thiamine deficits. They were induced by the pyrithiamine, a potent inhibitor of thiamine pyrophosphokinase. The thiamine deficit reduced metabolic fluxes through pyruvate and ketoglutarate dehydrogenase steps, yielding deficits of acetyl-CoA in mitochondrial and cytoplasmic compartments of K-depolarized nerve terminals. It also inhibited indirect transport of acetyl-CoA though ATP-citrate lyase pathway being without effect on its direct Ca-dependent transport to synaptoplasm. Resulting suppression of synaptoplasmic acetyl-CoA correlated with inhibition of quantal acetylcholine release (r = 0.91, p = 0.012). On the other hand, thiamine deficiency activated non-quantal acetylcholine release that was independent of shifts in intraterminal distribution of acetyl-CoA. Choline acetyltransferase activity was not changed by these conditions. These data indicate that divergent alterations in the release of non-quantal and quantal acetylcholine pools from thiamine deficient nerve terminals could be caused by the inhibition of acetyl-CoA and citrate synthesis in their mitochondria. They in turn, caused inhibition of acetyl-CoA transport to the synaptoplasmic compartment through ATP-citrate lyase pathway yielding deficits of cholinergic functions.