Acetylcholinesterase activity of Xenopus laevis oocytes

On the excitation of action potentials by protons and its potential implications for cholinergic transmission

One of the most conserved mechanisms for transmission of a nerve pulse across a synapse relies on acetylcholine (ACh). Ever since the Nobel Prize-winning works of Dale and Loewi, it has been assumed that ACh-subsequent to its action on a postsynaptic cell-is split into inactive by-products by acetylcholinesterase (AChE). Herein, the widespread assumption of inactivity of ACh's hydrolysis products is falsified. Excitable cells (Chara braunii internodes), which had previously been unresponsive to ACh, became ACh-sensitive in the presence of AChE. The latter was evidenced by a striking difference in cell membrane depolarization upon exposure to 10 mM intact ACh (∆V = -2 ± 5 mV) and its hydrolysate (∆V = 81 ± 19 mV), respectively, for 60 s. This pronounced depolarization, which also triggered action potentials, was clearly attributed to one of the hydrolysis products: acetic acid (∆V = 87 ± 9 mV at pH 4.0; choline ineffective in the range 1-10 mM). In agreement with our findings, numerous studies in the literature have reported that acids excite gels, lipid membranes, plant cells, erythrocytes, as well as neurons. Whether excitation of the postsynaptic cell in a cholinergic synapse is due to protons or due to intact ACh is a most fundamental question that has not been addressed so far.

The acetylcholinesterase inhibitor BW284c51 is a potent blocker of Torpedo nicotinic AchRs incorporated into the Xenopus oocyte membrane

This work was aimed to determine if 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284c51), the most selective acetylcholinesterase inhibitor (AchEI), affects the nicotinic acetylcholine (Ach) receptor (AchR) function. Purified Torpedo nicotinic AchRs were injected into Xenopus laevis oocytes and BW284c51 effects on Ach- and carbamylcholine (Cch)-elicited currents were assessed using the voltage-clamp technique.BW284c51 (up to 1 mM) did not evoke any change in the oocyte membrane conductance. When BW284c51 (10 pM-100 microM) and Ach were co-applied, Ach-evoked currents (I(Ach)) were reversibly inhibited in a concentration-dependent manner (Hill coefficient, 1; IC(50), 0.2-0.5 muM for 0.1-1000 microM Ach). Cch-elicited currents showed a similar inhibition by BW284c51.I(Ach) blockade by BW284c51 showed a strong voltage dependence, being only apparent at hyperpolarising potentials. BW284c51 also enhanced I(Ach) desensitisation.BW284c51 changed the Ach concentration-dependence curve of Torpedo AchR response from two-site to single-site kinetics, without noticeably affecting the EC(50) value. The BW284c51 blocking effect was highly selective for nicotinic over muscarinic receptors. BW284c51 inhibition potency was stronger than that of tacrine, and similar to that of d-tubocurarine (d-TC). Coapplication of BW284c51 with either tacrine or d-TC revealed synergistic inhibitory effects. Our results indicate that BW284c51 antagonises nicotinic AchRs in a noncompetitive way by blocking the receptor channel, and possibly by other, yet unknown, mechanisms. Therefore, besides acting as a selective AchEI, BW284c51 constitutes a powerful and reversible blocker of nicotinic AchRs that might be used as a valuable tool for understanding their function.

Modulation of alpha2beta4 neuronal nicotinic acetylcholine receptors by zinc

A study was made of the modulation of nicotinic acetylcholine receptors by the divalent cation zinc. Rat neuronal nicotinic receptors (alpha2beta4) were expressed in Xenopus oocytes and membrane currents evoked by acetylcholine (ACh currents) were recorded using a two microelectrode voltage clamp. In non-injected oocytes, or in oocytes expressing alpha2beta4 receptors, Zn2+ by itself (1 microM-4 mM) generated only very small membrane currents. In contrast, in oocytes expressing alpha2beta4 receptors, Zn2+ greatly and reversibly increased the ACh current, without affecting considerably its time course. The ACh current potentiation by Zn2+ was weakly dependent on the membrane potential (2.33+/-0.10 times the control current at -100 mV vs 2.04+/-0.06 at -60 mV, suggesting that Zn2+ interacts with the receptor in the vestibule of the ion channel or at an external domain of the protein. The inward rectification of control and Zn2+-potentiated ACh-currents was similar. We conclude that Zn2+ positively and reversibly modulates neuronal nicotinic receptors in a practically voltage-independent manner and without affecting their rate of desensitization. These results will help to understand better the roles played by Zn2+ in brain functions.

A comparison of the Xenopus laevis oocyte acetylcholinesterase with the muscle and brain enzyme suggests variations at the post-translational level

1. Xenopus laevis oocytes express endogenously two components of the cholinergic system: the muscarinic receptors and the acetylcholinesterase (AChE). 2. A biochemical characterization of this enzyme was carried out. 3. The results established that the activity found in the oocytes correspond to 'true' AChE with a molecular weight of 65,000 Da and a sedimentation coefficient of 3-4 S. 4. The enzyme aggregates in the absence of detergent suggesting that it possess an hydrophobic character; despite that, it is not sensitive to PIPLC. 5. A comparison with the Xenopus brain and muscle AChE shows different post-translational modifications and catalytic properties with the oocyte AChE.

Biogenic monoamines in oocytes of echinoderms and bivalve molluscs. A formation of intracellular regulatory systems in oogenesis

1. Identification of catecholamines and indolylakylamines in the fully grown oocytes or mature eggs of the representatives of echinoderms and bivalve molluscs was made using the fluorometric method. 2. Dopamine is the main catecholamine in the oocytes of all investigated animals. Tryptamine was the main indolylalkylamine. 3. The data presented confirm the presence of the main components of regulatory systems (transmitters, enzymes of their exchange, intracellular messengers, intracellular receptive systems) in the fully grown oocytes. 4. A hypothesis is proposed of the formation of intracellular regulatory systems (monoaminergic, cholinergic, peptidergic and steroidal) in oogenesis.

Cholinoceptive properties of human primordial, preantral, and antral oocytes: in situ hybridization and biochemical evidence for expression of cholinesterase genes

In addition to their well-known involvement in neuromuscular junctions and in brain cholinergic synapses, cholinergic mechanisms have been implicated in the growth and maturation of oocytes in various species. Functional acetylcholine receptors were electrophysiologically demonstrated in amphibian and mammalian oocyte membranes, and activity of the acetylcholine-hydrolyzing enzyme, acetylcholinesterase (AChE), was biochemically measured in the exceptionally big oocytes of the frog Xenopus laevis. However, biochemical methods could not reveal whether AChE was produced within the oocytes themselves or in the surrounding follicle cells. Furthermore, this issue is particularly important for understanding growth and fertilization processes in the much smaller human oocytes, in which the sensitivity of AChE biochemical measurements is far too low to be employed. To resolve this question, a molecular biology approach was combined with biochemical measurements on ovarian extracts and sections. To directly determine whether the human cholinesterase (ChE) genes are transcriptionally active in oocytes, and, if so, at what stages in their development, the presence of ChE mRNA was pursued. For this purpose frozen ovarian sections were subjected to in situ hybridization using 35S-labeled human ChE cDNA. Highly pronounced hybridization signals were localized within oocytes in primordial, preantral, and antral follicles, but not in other ovarian cell types, demonstrating that within the human ovary ChE mRNA is selectively synthesized in viable oocytes at different developmental stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the properties of synaptic channels opened by acetylcholine in denervated frog muscle

Acetylcholine (ACh)-activated channels in end-plates of frog sartorius muscle were studied at various times after denervation. Mean open times of the synaptic membrane channels were derived from the time constant of decay of miniature end-plate currents (tau MEPC) evoked by ACh quanta released from Schwann cells, which replace the motor nerve terminals after these degenerate. Membrane current noise, elicited by iontophoretic application of ACh to voltage-clamped end-plates, was also used to determine mean open time (tau noise) and conductance of the ion channels. About 1 week after denervation, soon after Schwann cell MEPCs appeared, they had a tau similar to that of the neural MEPC in innervated end-plates. However, 5-6 weeks after denervation tau MEPC was increased by a factor of about 5. Circa 4 weeks after denervation, cholinesterase activity of the denervated muscle decreased to 76% of that in the contralateral, innervated muscle, and even 4 months after the operation it was still 64%. Thus, it is unlikely that a change in acetylcholinesterase activity is the main factor responsible for the increase in tau of Schwann cell MEPC. About 1 week after denervation tau noise was close to that in innervated end-plates (about 2 ms). Twelve to 24 days after denervation the average channel open time was 4.5 +/- 1.0 ms, with some end-plates still showing normal 'fast' channels. However, in muscles denervated for 47-113 days the open time was 12.9 +/- 1.9 ms. In the early and intermediate periods, ACh-induced noise spectra with two components were obtained from many end-plates, indicating the simultaneous activation of two different types of channels. At some end-plates during the early and intermediate periods after denervation, but not after about 5 weeks, neostigmine caused the appearance of a component, which was as fast as that of normal end-plate channels. In other experiments small doses of alpha-bungarotoxin were applied in order to predominantly block extra-junctional receptors. In the early period of denervation, when two components were present in the noise spectra, alpha-bungarotoxin eliminated the slow component leaving channels as fast as in innervated end-plates. After prolonged denervation, a component with tau of about 5.5 ms was occasionally disclosed by application of alpha-bungarotoxin. tau noise and tau MEPC from the same end-plate closely agreed. Our results indicate that at frog end-plates the open time of the majority of the synaptic channels opened by ACh becomes longer with increasing time after denervation.

Effect of collagenase treatment and subsequent culture on rat muscle fiber acetylcholinesterase activity

After collagenase treatment and mechanical disruption, acetylcholinesterase (AChE) activity on the surface of individual flexor brevis muscle fibers fell by 88%. During the next 48 hr in culture, surface AChE activity continued to decline, while intracellular activity changed little. After 1 week in culture total muscle fiber AChE activity fell to very low levels and intracellular AChE activity could no longer be detected, probably as a result of reduced synthesis and rapid externalization of intracellular AChE. Apart from the removal of most of the surface activity, cultured muscle fibers had similar AChE activity to muscle fibers that had been denervated in vivo, suggesting that the changes observed in culture reflect the loss of neuromuscular interaction and not to any contributory effects of the dissociation process. It is to be hoped that these results, along with the published results of Bekoff and Betz [J. Physiol, 271:25-40, 537-547], will serve as useful background data for those continuing to use adult dissociated muscle fibers in their studies.

Choline acetyltransferase and acetylcholine in Xenopus oocytes injected with mRNA from the electric lobe of Torpedo

Xenopus oocytes were injected with poly(A)+ mRNA obtained from the electric lobes of Torpedo marmorata and Torpedo ocellata, which contain the cell bodies of the neurons that innervate the electric organs. The electric lobe mRNA preparation induces the oocytes to synthesize a catalytically active form of the enzyme choline acetyltransferase (EC 2.3.1.6). Enzymatic activity is found almost exclusively in the cytoplasmic fraction of injected, but not control, oocytes. Evidence is presented that distinguishes between the induced choline acetyltransferase activity and an intrinsic carnitine acetyltransferase activity present in the oocytes. This latter enzyme is associated principally with particulate fractions of the oocyte. The level of acetylcholine, which accumulates in mRNA-injected oocytes, is relatively insensitive to pharmacological manipulations that alter the acetylcholine content of other cells. These results show that Xenopus oocytes may be used advantageously to study functional properties of polypeptides associated with presynaptic elements in the nervous system.

The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

Lineage segregation and developmental autonomy in expression of functional muscle acetylcholinesterase mRNA in the ascidian embryo

Acetylcholinesterase is a histospecific marker of cell differentiation occurring only in the muscle and mesenchyme tissues of the ascidian embryo. The distribution of functional mRNA coding for this enzyme has been investigated and it is shown here that only cells of muscle and mesenchyme lineages possess such a template. Blastomeres of four cell lineage quadrants were separated microsurgically from eight-cell-stage embryos of Ciona intestinalis and raised in isolation until muscle development was well advanced. Measurement of enzyme activity in the resulting partial embryos revealed that acetylcholinesterase was limited to descendants of one blastomere pair, the B4.1 blastomeres containing muscle and mesenchyme lineages. To study the tissue distribution of acetylcholinesterase mRNA, RNA from partial embryos was translated in Xenopus laevis oocytes. When oocytes were injected with an appropriate template, they synthesized a biologically active acetylcholinesterase that could be selectively immunopurified with an antiserum to the ascidian enzyme. Under the conditions used the quantity of acetylcholinesterase mRNA was directly related to the enzyme activity in immunoprecipitates. Acetylcholinesterase mRNA was found only in B4.1 lineage partial embryos where it occurred in approximately the same amount as in whole embryos of the same age. Since there is a limited period from gastrulation until the middle tail-formation stage when functional acetylcholinesterase mRNA accumulates, the results of our mRNA distribution experiments strongly suggest that the gene for ascidian acetylcholinesterase is active only in muscle and mesenchyme tissues. The histospecific occurrence of this enzyme apparently does not involve selective, cell-specific control of translation.