Molecular forms of the cholinesterases inside and outside muscle endplates

The synaptic acetylcholinesterase tetramer assembles around a polyproline II helix

Functional localization of acetylcholinesterase (AChE) in vertebrate muscle and brain depends on interaction of the tryptophan amphiphilic tetramerization (WAT) sequence, at the C-terminus of its major splice variant (T), with a proline-rich attachment domain (PRAD), of the anchoring proteins, collagenous (ColQ) and proline-rich membrane anchor. The crystal structure of the WAT/PRAD complex reveals a novel supercoil structure in which four parallel WAT chains form a left-handed superhelix around an antiparallel left-handed PRAD helix resembling polyproline II. The WAT coiled coils possess a WWW motif making repetitive hydrophobic stacking and hydrogen-bond interactions with the PRAD. The WAT chains are related by an approximately 4-fold screw axis around the PRAD. Each WAT makes similar but unique interactions, consistent with an asymmetric pattern of disulfide linkages between the AChE tetramer subunits and ColQ. The P59Q mutation in ColQ, which causes congenital endplate AChE deficiency, and is located within the PRAD, disrupts crucial WAT-WAT and WAT-PRAD interactions. A model is proposed for the synaptic AChE(T) tetramer.

Existence of an inactive pool of acetylcholinesterase in chicken brain

We analyzed acetylcholinesterase (AcChoEase; EC 3.1.1.7) activity and AcChoEase immunoreactive protein in chicken brain by using five monoclonal antibodies raised against chicken AcChoEase. Four of them specifically recognized AcChoEase catalytic subunits in Western blots and one, C-131, recognized only enzymatically active AcChoEase. We observed considerable differences in the ratio of immunoreactive protein to catalytic activity in various fractions, indicating the existence of inactive AcChoEase protein. This inactive AcChoEase component was more abundant in a low-salt-soluble extract than in a subsequent detergent-soluble extract. On the basis of the ratio between activity and immunoreactivity, we calculated that the inactive component represents about 30% of the total AcChoEase subunits in chicken brain. The immunoreactive AcChoEase protein sedimented in sucrose gradients like the active molecular forms; the G1 and G2 peaks contained inactive molecules, whereas the G4 peak appeared to contain only active AcChoEase. The bulk of inactive AcChoEase reacted with the organophosphate cholinesterase inhibitor O-ethyl S-[2-(diisopropylamino)ethyl]methylphosphonothioate (MTP) but was found to bind the active site affinity ligand N-methylacridinium poorly and was not recognized by the active-form-specific monoclonal antibody, C-131. In addition, most of this fraction is sensitive to endoglycosidase H and binds the lectin wheat germ agglutinin poorly, suggesting that it was not processed in the Golgi apparatus. From these observations, we propose that the active and inactive AcChoEase components are differently folded.

Effects of electrical stimulation on molecular forms of butyrylcholinesterase in denervated fast and slow latissimus dorsi muscles of newly hatched chicken

The effects of denervation and direct electrical stimulation upon the activity and the molecular form distribution of butyrylcholinesterase (BuChE) were studied in fast-twitch posterior latissimus dorsi (PLD) and in slow-tonic anterior latissimus dorsi (ALD) muscles of newly hatched chicken. In PLD muscle, denervation performed at day 2 substantially reduced the rate of rapid decrease of BuChE specific activity which takes place during normal development, whereas in the case of ALD muscle little change was observed. Moreover, the asymmetric forms which were dramatically reduced in denervated PLD muscle were virtually absent in denervated ALD muscle at day 14. Denervated PLD and ALD muscles were stimulated from day 4 to day 14 of age. Two patterns of stimulation were applied, either 5-Hz frequency (slow rhythm) or 40-Hz frequency (fast rhythm). Both patterns of stimulation provided the same number of impulses per day (about 61,000). In PLD muscle, electrical stimulation almost totally prevented the postdenervation loss in asymmetric forms and led to a decrease in BuChE specific activity. In ALD muscle, electrical stimulation partially prevented the asymmetric form loss which occurs after denervation. This study emphasizes the role of evoked muscle activity in the regulation of BuChE asymmetric forms in the fast PLD muscle and the differential response of denervated slow and fast muscles to electrical stimulation.

Acetylcholinesterase in chick embryo latissimus dorsii muscles: effects of curarization and electrical stimulation

The accumulation of acetylcholinesterase (AChE), the changes in AChE-specific activity and in AChE molecular form distribution were studied in slow-tonic anterior latissimus dorsi (ALD) and in fast-twitch posterior latissimus dorsi (PLD) muscles of the chick embryo. From stage 36 (day 11) to stage 42 (day 17) of Hamburger and Hamilton, the AChE-specific activity decreased, while the relative proportion of asymmetric A 12 and A 8 forms increased. Repetitive injection of curare resulted at stage 42 (day 17) in a decrease in AChE-specific activity, in the accumulation of the synaptic AChE and in the expression of AChE asymmetric forms. Electrical stimulation at a relatively high frequency (40 Hz) of curarized ALD and PLD muscles resulted in a normal increase in AChE asymmetric forms, whereas a lower frequency (5 Hz) resulted in a dominance of globular forms. Both patterns of stimulation partly prevented the loss in synaptic AChE accumulations. These results suggest that in chick embryo muscles, muscle activity and its rhythms are involved in the normal evolution of AChE.

An asymmetric form of muscle acetylcholinesterase contains three subunit types and two enzymic activities in one molecule

We have purified completely the principal asymmetric ("heavy") form of acetylcholinesterase (Ac-ChoEase; EC 3.1.1.7) from chick muscle (i.e., the synaptic form in the twitch muscle fibers) by using a monoclonal antibody that recognizes AcChoEase but not pseudocholinesterase (ChoEase; cholinesterase, EC 3.1.1.8). The purified protein exhibits catalytic and inhibition properties characteristic of AcChoEase and ChoEase and contains three distinct subunits of apparent sizes 110 kDa, 72 kDa, and 58 kDa in the ratio 2:2:1. The discovery of an AcChoEase/ChoEase hybrid asymmetric form has been further supported by (i) the identification of active site properties of AcChoEase in the 110-kDa subunit and of ChoEase in the 72-kDa subunit, (ii) the purification or precipitation of both activities together by, also, a ChoEase-specific monoclonal antibody, and (iii) evidence that all subunits are bound in the asymmetric forms by disulfide bonds. The 58-kDa subunit is the only one that is sensitive to digestion with purified collagenase; it carries the collagenous "tail" of the asymmetric form. A model is proposed for this form of AcChoEase.

Modes of attachment of acetylcholinesterase to the surface membrane

Acetylcholinesterase (AChE) occurs in multiple molecular forms differing in their quaternary structure and mode of anchoring to the surface membrane. Attachment is achieved by post-translational modification of the catalytic subunits. Two such mechanisms are described. One involves attachment to catalytic subunit tetramers, via disulfide bridges, of a collagen-like fibrous tail. This, in turn, interacts, primarily via ionic forces, with a heparin-like proteoglycan in the extracellular matrix. A second such modification involve the covalent attachment of a single phosphatidylinositol molecule at the carboxyl-terminus of each catalytic subunit polypeptide; the diacylglycerol moiety of the phospholipid serves to anchor the modified enzyme hydrophobically to the lipid bilayer of the plasma membrane. The detailed molecular structure of these two classes of acetylcholinesterase are discussed, as well as their biosynthesis and mode of anchoring.

Neural control of the forms of acetylcholinesterase in slow mammalian muscles

The 'heavy', collagen-tailed form of acetylcholinesterase (AChE), having a s(0)20,w of 16S in mammals, occurs at vertebrate muscle endplates and has been widely regarded as a marker of neuronal influence on muscle in vivo. However, an interesting exception has been described by Bacou et al., in a previous report in Nature. They found, in a slow-twitch muscle of the rabbit, that after denervation the 16S form of AChE increases markedly, rather than disappearing. Such a phenomenon would modify current concepts of neuromuscular regulation. We report here, however, that this exception is apparent rather than real in terms of endplate AChE regulation.

Polymorphism of pseudocholinesterase in Torpedo marmorata tissues: comparative study of the catalytic and molecular properties of this enzyme with acetylcholinesterase

We report the existence, in Torpedo marmorata tissues, of a cholinesterase species (sensitive to 10(-5) M eserine) that differs from acetylcholinesterase (AChE, EC 3.1.1.7) in several respects: (a) The enzyme hydrolyzes butyrylthiocholine (BuSCh) at about 30% of the rate at which it hydrolyzes acetylthiocholine (AcSCh), whereas Torpedo AChE does not show any activity on BuSCh. (b) It is not inhibited by 10(-5) M BW 284C51, but rapidly inactivated by 10(-8) M diisopropylfluorophosphonate. (c) It does not exhibit inhibition by excess substrate up to 5 X 10(-3) M AcSCh. (d) It does not cross-react with anti-AChE antibodies raised against purified Torpedo AChE. This enzyme is obviously homologous to the "nonspecific" or pseudocholinesterase (pseudo-ChE, EC 3.1.1.8) that exists in other species, although it is closer to "true" AChE than classic pseudo-ChE in several respects. Thus, it shows the highest Vmax with acetyl-, and not propionyl- or butyrylthiocholine, and it is not specifically sensitive to ethopropazine. Pseudo-ChE is apparently absent from the electric organs, but represents the only cholinesterase species in the heart ventricle. Pseudo-ChE and AChE coexist in the spinal cord and in blood plasma, where they contribute to AcSCh hydrolysis in comparable proportions. Pseudo-ChE exists in several molecular forms, including collagen-tailed forms, which can be considered as homologous to those of AChE. In the heart the major component of pseudo-ChE appears to be a soluble monomeric form (G1). This form is inactivated by Triton X-100 within days.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of activities and forms of cholinesterases in human primary brain tumors

The activities and molecular forms of cholinesterases were studied in a collection of primary brain tumors consisting of primarily gliomas and meningiomas, together with samples of forebrain taken postmortem from patients suffering from diseases unrelated to the nervous system. Both types of tumors, as well as normal forebrain, contained substantial amounts of cholinesterase activity and some gliomas contained exceptionally high levels. In both normal forebrain and meningiomas, acetylcholinesterase (acetylcholine hydrolase; EC 3.1.1.7) accounted for almost all the cholinesterase activity, but in almost all gliomas elevated pseudocholinesterase (acylcholine acylhydrolase; EC 3.1.1.8) could be detected. The cholinesterase activity of both normal forebrain and gliomas migrated on sucrose gradients as a major component of 10-11 S together with a minor component of 4-5 S. In meningiomas a light (4.5 S) form was the principal component.

Molecular forms of acetylcholinesterase in synaptic and extrasynaptic regions of avian tonic muscle

Molecular forms of acetylcholinesterase and pseudocholinesterase were analyzed directly in the micro-dissected individual endplates of a slow-tonic chicken muscle. The major form in the endplate is the L2(6.5 S) form, while the collagen-tailed H2c (20 S) form, normally considered to be the synaptic form, is a very minor component, in contrast to its predominance at the chicken fast-twitch fibre endplate. The same is true for pseudocholinesterase at these endplates. Outside the tonic fibre endplates the same forms occur as at the endplates, but at a very much lower concentration. The enzyme at the tonic fibre endplate cannot be attached to the basal lamina by a collagen tail, but appears to have a hydrophobic attachment. Acetylcholinesterase is functional at tonic fibre endplates, but the absence of the collagen-tailed form may account for the lower efficiency of the enzymic removal of acetylcholine there.

A microfluorometric assay for cholinesterases, suitable for multiple kinetic determinations of picomoles of released thiocholine

A highly sensitive microfluorometric assay for cholinesterases has been developed. Enzymatic activity is measured by monitoring the thiocholine produced by specific hydrolysis of acetylthiocholine. This is carried out by reacting the thiocholine formed with the fluorogenic compound N-(4(7 diethylamino-4-methylcoumarin-3-yl)phenyl)maleimide to yield an intensely fluorescent product. The assay is linear over a range extending from a few picomoles to nanomoles of thiocholine. The specificity and accuracy of this microfluorometric assay were examined using microgram quantities of rat brain tissue as a source for cholinesterases. The specific activities and the Km values determined by this new method for both cholinesterase activities present in the brain (acetylcholine hydrolase, EC 3.1.1.7, and "nonspecific" cholinesterase-acylcholine acylhydrolase, EC 3.1.1.8) were identical to those reported earlier using the less sensitive spectrophotometric and radiometric methods. The background emission caused by nonenzymatic hydrolysis of the substrate is relatively low, and does not exceed background values encountered in other methods. The assay may be used for monitoring the kinetics of enzymatic activities in microscale reaction mixtures, providing a linear determination of the thiocholine produced over a period of at least 30 h at room temperature. The method can also be adapted for use in other enzymatic assays where reagents containing thiol groups can be produced or consumed.

Multiple molecular forms of acetylcholinesterase in the nematode Caenorhabditis elegans

Extracts of the nematode Caenorhabditis elegans contain five molecular forms of acetylcholinesterase (AChE) activity that can be separated by a combination of selective solubilization, velocity sedimentation, and ion-exchange chromatography. These are called form IA (5.2s), form IB (4.9s), form II (6.7s), form III (11.3s), and form IV (13.0s). All except form III are present in significant amounts in rapidly prepared extracts and are probably native; form III is probably derived autolytically from form IV. Most of forms IA and IB can be solubilized by repeated extractions without detergent, whereas forms II, III, and IV require detergent for effective solubilization and may therefore be membrane-bound. High salt concentrations are not required for, and do not aid in, the solubilization of these forms. For all forms, molecular weights and frictional ratios have been estimated by a combination of gel permeation chromatography and velocity sedimentations in both H2O and D2O. The molecular weight estimates range from 83,000 to 357,000 and only form II shows extensive asymmetry. The separated forms have been characterized with respect to substrate affinity, substrate specificity, inhibitor sensitivity, thermal inactivation, and detergent sensitivity. Judging by these properties, C. elegans is like other invertebrates in that none of its cholinesterase forms resembles either the "true" or the "pseudo" cholinesterase of vertebrates. However, internal comparison of the C. elegans forms clearly distinguishes forms IA, III, and IV as a group from forms IB and II; the former are therefore designated "class A" forms, the latter "class B" forms. Genetic evidence indicates that separate genes control class A and class B forms, and that these two classes overlap functionally. Several factors, including kinetic properties, molecular asymmetry, molecular size, and solubility, all suggest that a molecular model of the multiple cholinesterase forms observed in vertebrate electric organs probably does not apply in C. elegans. Potential functional roles and subunit structures of the multiple AChE forms within each C. elegans class are discussed.

Fibre types in chicken skeletal muscles and their changes in muscular dystrophy

1. Five major fibre types in chicken skeletal muscles are recognized, based upon their histochemical and morphological characteristics. A classification of these which is readily related to a commonly used classification of mammalian muscle fibre types is given.2. Seven muscles of the chicken were analysed in recognizing this range of fibre types. The proportions of the different types in each of these were determined. In some cases a gradient of fibre type composition exists across a single muscle.3. Measurements of muscle contraction were used in defining tonic muscles, which contain two fibre types. It was shown that in addition to the anterior latissimus dorsi (a.l.d.), previously well known to be a tonic muscle, two other muscles, the plantaris and the adductor profundus, are of the same class, but differ subtly from the a.l.d. in certain features. Gross red colouration is not a useful diagnostic feature of slow muscles, since the tonic adductor profundus, for example, is white.4. Fibres similar histochemically to mammalian type I (slow-twitch) occur in some of the avian twitch muscles investigated. These are oxidative in character, and despite the fact that they are multiply innervated we suggest that these are avian slow-twitch fibres.5. The patterns of cholinesterases found in a skeletal muscle correspond to its fibre type composition, with regard to both the concentrations and the proportions of the multiple forms of enzyme present. The distinctive patterns of those forms of acetylcholinesterase in the different fibre types are described.6. The fibre type composition is changed by inherited muscular dystrophy in a characteristic manner. This change has so far been found (at the earlier stages of the disease) only in the muscles with a predominance of type II B fibres in the normal chicken. Pathological changes within the fibres occur selectively in the type II B fibres, but there are exceptions to this and the effect can be greatly modified by the type of neighbouring fibres.

Comparison of the molecular forms of the cholinesterases in tissues of normal and dystrophic chickens

The levels and molecular forms of acetylcholinesterase (AChE, EC 3.1.1.7) and pseudocholinesterase (psiChE, EC 3.1.1.8) were examined in various skeletal muscles, cardiac muscles, and neural tissues from normal and dystrophic chickens. The relative amount of the heavy (Hc) form of AChE in mixed-fibre-type twitch muscles varies in proportion to the percentage of glycolytic fast-twitch fibres. Conversely, muscles with higher levels of oxidative fibres (i.e., slow-tonic oxidative-glycolytic fast-twitch, or oxidative slow-twitch) have higher proportions of the light (L) form of AChE. The effects of dystrophy on AChE and psiChE are more severe in muscles richer in glycolytic fast-twitch fibres (e.g., pectoral or posterior latissimus dorsi, PLD); there is no alteration of AChE or psiChE in a slow-tonic muscle. In the pectoral of PLD muscles from older dystrophic chickens, however, the AChE forms revert to a normal distribution while the pesChE pattern remains abnormal. Muscle psiChE is sensitive to collagenase in a similar way as is AChE, thus apparently having a similar tailed structure. Unlike skeletal muscle, cardiac muscle has very high levels of psiChE, present mainly as the L form; AChE is present mainly as the medium (M) form, with smaller amounts of L and Hc. The latter pattern of AChE forms resembles that seen in several neural tissues examined. No alterations in AChE or psiChE were found in cardiac or neural tissues from dystrophic chickens.

Biosynthesis and secretion of catalytically active acetylcholinesterase in Xenopus oocytes microinjected with mRNA from rat brain and from Torpedo electric organ

A novel technique was developed for monitoring the level of the mRNA species that direct the synthesis of acetylcholinesterase (AcChoEase; acetylcholine acetylhydrolase, EC 3.1.1.7), using microinjected Xenopus oocytes as a translation system. When injected with poly(A)-containing RNA from whole rat brain or rat cerebellum and from electric organ of Torpedo ocellata, Xenopus oocytes synthesize and secrete catalytically active cholinesterase. The newly synthesized enzyme, which is mostly secreted into the oocytes incubation medium, appears to be primarily AcChoEase because it is inhibited by the specific inhibitor BW 284C51. The new enzymatic activity can be detected after injection of as little as 12.5 ng of poly(A)-containing RNA per oocyte, and there is a linear dependence of the oocytes' ability to form AcChoEase on the amount of injected RNA. The AcChoEase mRNA displays a tau 1/2 of about 10 +/- 3 hr in injected oocytes. The abundance of AcChoEase mRNA in the total nonfractionated mRNA injected was calculated to be ca. 1 x 10(-5), a value similar to the level of AcChoEase protein determined in rat brain. The combination of the high turnover number of AcChoEase, the efficiency of the oocyte system, and the sensitivity of the assay used thus permit the accurate monitoring of the scarce mRNA species that direct the synthesis of this enzyme.