Interactions between p75 and TrkA receptors in differentiation and vulnerability of SN56 cholinergic cells to beta-amyloid

NGF modifies cholinergic neurons through its low-p75 and high affinity-TrkA receptors. Native p75(+)TrkA(-) and trkA-transfected p75(+)TrkA(+) SN56 hybrid cholinergic septal cells were used here to discriminate effects mediated by each receptor. In TrkA(-) cells, NGF (100 ng/ml) affected neither choline acetyltransferase nor morphology but depressed pyruvate dehydrogenase activity by about 30%. Aged 25-35 beta-amyloid (1 microM) caused no changes in choline acetyltransferase and pyruvate dehydrogenase activities in nondifferentiated and differentiated TrkA(-) cells. On the contrary, in nondiferentiated TrkA(+) NGF brought about a 2.5-fold increase of choline acetyltransferase. In differentiated TrkA(+) cells, beta-amyloid resulted in no change in PDH but 65% suppression of choline acetyltransferase activity and reduction of their extensions. Thus, activation of TrkA receptors may overcome p75 receptor-mediated inhibitory effects on pyruvate dehydrogenase expression in cholinergic cells. On the other hand, it would make expression of choline acetyltransferase and cell differentiation more susceptible to suppressory effects of beta-amyloid.

[Disturbances of glucose metabolism in epilepsy and other neurodegenerative diseases]

Disturbances of glucose metabolism in epilepsy and other neurodegenerative diseases. Impairment of glucose and acetyl-CoA metabolism is a characteristic feature of several degenerative brain diseases. Presented paper provides experimental evidences that NO, aluminum and thiamine deficiency result in concordant disturbances in acetyl-CoA campartmentalisation as well as in nonquantal and quantal acetylcholine release in rat brain nerve terminals. Our findings indicate that simultaneous depression of acetyl-CoA synthesis and its increased utilisation for acetylcholine synthesis in the presence of neurotoxic factors is likely to make brain cholinergic neurones particularly prone to neurodegeneration.

Acute and chronic effects of aluminum on acetyl-CoA and acetylcholine metabolism in differentiated and nondifferentiated SN56 cholinergic cells

Mechanisms of preferential loss of cholinergic neurons in the course of neurodegenerative diseases are unknown. Therefore, we investigated whether differentiation-evoked changes in acetyl-CoA and acetylcholine metabolism contribute to the susceptibility of cholinergic neuroblastoma to cytotoxic effects of Al. In SN56 cells differentiated with retinoic acid and dibutyryl cAMP (DC), pyruvate utilization and acetyl-CoA content were lower and acetylcholine level higher than in nondifferentiated cells (NC), respectively. In DC Al and Ca accumulations were 50% and 100%, respectively higher than in NC. Acute Al addition caused inhibition, whereas its chronic application had no effect on pyruvate utilization both in NC and in DC. On the other hand, in both experiments, Al evoked a greater decrease of acetyl-CoA level in DC than in NC. Acute addition of Al depressed acetylcholine release from DC to two times lower values than in NC. On the other hand, chronic addition of Al increased ACh release from DC over twofold, being without effect on its release from NC. These findings indicate that higher accumulation of Ca, along with low levels of acetyl-CoA, could make DC more susceptible to neurotoxic inputs than NC. Excessive acetylcholine release, evoked by Al, is likely to increase acetyl-CoA utilization for resynthesis of the neurotransmitter pool and cause deficit of this metabolite in DC. On the other hand, NC, owing to lower Ca accumulation, slower ACh metabolism, and higher level of acetyl-CoA, would be less prone to these harmful conditions.

Putative significance of shifts in acetyl-CoA compartmentalization in nerve terminals for disturbances of cholinergic transmission in brain

Acetylcholine and acetyl-CoA metabolism in nerve terminals isolated from rat brain were found to be affected by several neurotoxic and neuroprotective agents, such as aluminium, nitric oxide, beta-hydroxybutyrate, verapamil and thiamine deficiency. The changes evoked by these factors in Ca2+-dependent acetylcholine release were highly significantly correlated (r = 0.98) with changes in concentration of synaptoplasmic acetyl-CoA. On the other hand, in the same experimental conditions, no correlation was found between rates of pyruvate oxidation, intramitochondrial acetyl-CoA levels and different pools of releasable acetylcholine. These data indicate that disturbances in the availability of acetyl-CoA in the cytoplasm of nerve terminals may be a key factor in the pathogenesis of several cholinergic encephalopathies.

Acetyl-CoA metabolism in cholinergic neurons and their susceptibility to neurotoxic inputs

Cholinergic neurons, unlike other brain cells utilize acetyl-CoA not only for energy production but also for acetylcholine (ACh) synthesis. Therefore, suppression of acetyl-CoA metabolism by different neurotoxic inputs may be particularly harmful for this group of cells. Differentiation of SN56 cholinergic hybrid cells increased their choline acetyltransferase (ChAT) activity and ACh content but depressed pyruvate dehydrogenase activity and acetyl-CoA content. Differentiated cells were more susceptible to acute and chronic influences of aluminum, NO and amyloid-beta. Al decreased acetyl-CoA content, ACh release and increased Ca accumulation in differentiated cells (DC) to much higher degree than in non-differentiated ones (NC). NO strongly depressed acetyl-CoA level and increased ACh release in DC but did not affect NC. Additive effects of Al and NO were seen in DC but not in NC. Also long term suppressory effects of amyloid-beta, Al and NO on cholinergic phenotype and morphologic maturation were more evident in DC than in NC. Thus, relative shortage of acetyl-CoA in highly differentiated cholinergic neurons could make them particularly susceptible to degenerative insults in the course of different cholinergic encephalopathies.

Acetylcholine and acetyl-CoA metabolism in differentiating SN56 septal cell line

The rate of acetylcholine (ACh) synthesis was found to depend on the activity of choline acetyltransferase (ChAT) and on the concentrations of the two substrates of this enzyme, choline and acetyl-CoA. In SN56 cells treated for 3 days with 1 mM dbcAMP activities of ChAT and acetylcholinesterase (AChE) were elevated. It was accompanied by an increased activity of ATP-citrate lyase (ACL)-an enzyme responsible for provision of part of acetyl-CoA for ACh synthesis in cholinergic neurons. In contrast lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH) activities were reduced by dbcAMP. Treatment with 0.001 mM all-trans retinoic acid (RA) elevated ChAT and LDH activities but reduced the activities of AChE and ACL. The combined treatment with db-cAMP and tRA increased ChAT activity in supra-additive fashion. The effects of these two compounds on the other enzymes were not additive. Neither compound altered the activities of carnitine acetyl-transferase, acetyl-CoA synthase, or acetyl-CoA hydrolase. On the other hand, they decreased acetyl-CoA content and rate of ACh release. Overall, the results indicate that tRA upregulates only ChAT expression, whereas dbcAMP upregulates several features of cholinergic neurons including ChAT, AChE, and ACL. Low levels of acetyl-CoA in differentiated cells may result in a low rate of ACh release and resynthesis during their depolarization.

[Mechanisms of selective vulnerability of cholinergic neurons to neurotoxic stimuli]

Preferential loss of cholinergic neurons in course of several encephalopathies may result from the fact that they utilize acetyl-CoA not only for energy production, but also for acetylcholine synthesis. Changes in activities of acetyl-CoA metabolizing enzymes and shifts in acetyl-CoA compartmentalization, found in different animal models of brain pathologies and in human post mortem brain, are discussed in therms of their impact on cholinergic system integrity.

Pathways of beta-hydroxybutyrate contribution to metabolism of acetyl-CoA and acetylcholine in rat brain nerve terminals

beta-hydroxybutyrate increased concentration of acetyl-CoA in mitochondria of resting and in cytoplasm of Ca-activated rat brain synaptosomes. Adequate rise of Ca-evoked acetylcholine release was also observed. The activation was abolished by verapamil. It indicates that beta-hydroxybutyrate-derived acetyl-CoA is transported from mitochondria to synaptoplasm by direct Ca-dependent transport mechanism. Presented data evidence that level of synaptoplasmic acetyl-CoA plays an important role in the regulation of cholinergic activity in the brain.

Key role of acetyl-CoA in cytoplasm of nerve terminals in disturbances of acetylcholine metabolism in brain

This paper review data indicating that the concentration of acetyl-CoA in cytoplasm of cholinergic terminals is an important factor which may determine the rate and size of quantal acetylcholine in different physiologic and pathologic conditions.

Effects of aluminum and calcium on acetyl-CoA metabolism in rat brain mitochondria

Al complexes are known to accumulate in extra- and intracellular compartments of the brain in the course of different encephalopathies. In this study possible effects of Al accumulation in the cytoplasmic compartment on mitochondrial metabolism were investigated. Al, like Ca, inhibited pyruvate utilization as well as citrate and oxoglutarate accumulation by whole brain mitochondria. Potencies of Ca2+(total) effects were 10-20 times stronger than those of Al. Al decreased mitochondrial acetyl-CoA content in a concentration-dependent manner, along with an equivalent rise of free CoA level, whereas Ca caused loss of both intermediates from mitochondria. In the absence of Pi in the medium, Ca had no effect on mitochondrial metabolism, whereas Al lost its ability to suppress pyruvate utilization and acetyl-CoA content in Ca-free conditions. Verapamil potentiated, whereas ruthenium red reversed, Ca-evoked suppression of mitochondrial metabolism. On the other hand, in Ca-supplemented medium, Al partially overcame the inhibitory influence of verapamil. Accordingly, verapamil increased mitochondrial Ca levels much more strongly than Al. However, Al partially reversed the verapamil-evoked rise of Ca2+(total) level. These data indicate that Al accumulated in cytoplasm in the form of the Al(PO4)OH- complex may inhibit mitochondrial functions by an increase of intramitochondrial [Ca2+]total resulting from the Al-evoked rise of cytoplasmic [Ca2+]free, as well as from inhibitory interference with the verapamil binding site on the Na+/Ca2+ antiporter.

Effect of aluminum on acetyl-CoA and acetylcholine metabolism in nerve terminals

The potential ability of Al to affect cholinergic transmission was studied on synaptosomal fractions of rat brain incubated with pyruvate in depolarizing medium containing 30 mM K+. Addition of 1 mM Ca caused a 266% increase in the acetylcholine (ACh) release despite decreased pyruvate oxidation. Under these conditions, 0.25 mM Al did not affect pyruvate oxidation but raised mitochondrial and decreased synaptoplasmic acetyl-CoA. Simultaneously, a 61% inhibition of Ca-evoked ACh release was observed. Verapamil (0.1 and 0.5 mM) decreased the acetyl-CoA concentration in synaptoplasm and inhibited ACh release. Al (0.012 mM) partially reversed these inhibitory effects. Omission of Pi from the medium abolished suppressive effects of Al on acetyl-CoA content and Ca-evoked transmitter release. We conclude that the Al(PO4)OH- complex may be the active form of Al, which, by interaction with the verapamil binding sites of Ca channels, is likely to restrict the Ca influx to the synaptoplasm. This may inhibit the provision of acetyl-CoA to the synaptoplasm as well as the Ca-evoked ACh release. One may suppose that excessive accumulation of Al in some encephalopathic brains may, by this mechanism, suppress still-surviving cholinergic neurons and exacerbate cognitive deficits caused by already-existing structural losses in the cholinergic system.

Disturbances of acetyl-CoA, energy and acetylcholine metabolism in some encephalopathies

Acetyl-CoA provision to the synaptoplasmic compartment of cholinergic nerve terminals plays a regulatory role in the synthesis of acetylcholine. The disturbances in glucose utilization and in decarboxylation of the end product of its metabolism pyruvate, are considered to be significant factors causing cholinergic deficits in several diseases of the central nervous system. In this article we review data concerning role of acetyl-CoA in patomechanisms of disturbances of cholinergic metabolism in Alzheimers disease, thiamine deficiency, inherited defects of pyruvate dehydrogenase and diabetes.

Acetylcholine synthesis in nerve terminals of diabetic rats

Streprozotocin diabetes and extracerebral insulin affect acetyl-CoA and acetylcholine metabolism in the brain. In the present study we have shown that pyruvate utilization, acetyl-CoA content and ACh synthesis in nerve terminals from diabetic rats were 45, 30 and 50%, respectively, higher than that in healthy animals. Treatment with insulin normalized pyruvate utilization and acetylcholine synthesis but did not decrease the acetyl-CoA level. 3-Hydroxybutyrate did not affect acetyl-CoA and acetylcholine metabolism in control rats. However, in diabetic animals, 3-hydroxybutyrate significantly increased supply of acetyl-CoA for acetylcholine synthesis. These data provide evidence that increased provision of acetyl-CoA is prerequisite for activation of acetylcholine synthesis in diabetic brain.

Effect of angiotensin II and eledoisin on cholinergic neurons in rat hippocampus

Angiotensin II and eledoisin modulate drinking behaviour in rats that is mediated by monoaminergic and cholinergic neurons. In the present study we have shown that combined intracerebroventricular injections of either 0.1 or 1.0 microgram doses of angiotensin and eledoisin resulted in a decrease of about 25-35% in activities of choline acetyltransferase, ATP-citrate lyase in the hippocampus. In addition, 1 microgram quantities of these peptides depressed activity of carnitine acetyltransferase but did not alter activity of acetylcholinesterase. On the other hand, the application of 0.1 microgram of angiotensin caused no change in activity of monoamine oxidase A, while 1.0 microgram dose brought about its 67% activation. Eledoisin abolished this effect of angiotensin II. These data provide evidence that angiotensin II and eledoisin evoke non related adaptive changes in cholinergic and monoaminergic neurons of the hippocampus.

Differential effects of angiotensin II and eledoisin on monoamine oxidase A and B activities in rat brain

Intracerebroventricular injections of angiotensin II caused 108, 62, and 54% increases in monoamine oxidase A activities in rat hippocampus, hypothalamus, and striatum, respectively. These activatory effects were abolished by simultaneous injections of eledoisin. No significant changes of monoamine oxidase B activities were found under the same experimental conditions. Neither angiotensin II nor elodoisin changed substrate/inhibitor affinities of both isoenzymes. These data indicate that angiotensin II and tachykinin transmitter systems may exert opposite, long-term regulatory effects on monoaminergic neurons in rat brain.

Modification of substrate-inhibitor affinities of human platelet monoamine oxidase B in vitro

The rate of benzylamine utilization by monoamine oxidase (MAO)-B from human blood platelets was 2-4 times higher than that for octopamine. Both activities were inhibited 100% by 10(-7) M deprenyl (a specific MAO-B inhibitor) and were not affected by clorgyline (a specific MAO-A inhibitor) or by polyclonal antibodies to MAO-A. The preincubation of platelet MAO-B with purified MAO-A from mitochondrial membranes of human placenta resulted in appearance of excess octopamine activity. This additional activity was not precipitated by antibodies to MAO-A or inhibited by deprenyl but was inhibited by clorgyline. Incubation of the MAO-A preparation from placenta at 45 degrees C for 15 min before its preincubation with MAO-B caused 50% loss of both activities. Protease inhibitors had no effect on the modification of MAO. These data indicate that MAO-A or a factor tightly bound to it can modify MAO-B yielding a form of the enzyme with both MAO-A and MAO-B substrate and inhibitor affinities and MAO-B immunospecificity.