Distribution of acetylcholinesterase molecular forms in brain, nerve and muscle tissue of Torpedo marmorata

The multiple biological roles of the cholinesterases

It is tacitly assumed that the biological role of acetylcholinesterase is termination of synaptic transmission at cholinergic synapses. However, together with its structural homolog, butyrylcholinesterase, it is widely distributed both within and outside the nervous system, and, in many cases, the role of both enzymes remains obscure. The transient appearance of the cholinesterases in embryonic tissues is especially enigmatic. The two enzymes' extra-synaptic roles, which are known as 'non-classical' roles, are the topic of this review. Strong evidence has been presented that AChE and BChE play morphogenetic roles in a variety of eukaryotic systems, and they do so either by acting as adhesion proteins, or as trophic factors. As trophic factors, one mode of action is to directly regulate morphogenesis, such as neurite outgrowth, by poorly understood mechanisms. The other mode is by regulating levels of acetylcholine, which acts as the direct trophic factor. Alternate substrates have been sought for the cholinesterases. Quite recently, it was shown that levels of the aggression hormone, ghrelin, which also controls appetite, are regulated by butyrylcholinesterase. The rapid hydrolysis of acetylcholine by acetylcholinesterase generates high local proton concentrations. The possible biophysical and biological consequences of this effect are discussed. The biological significance of the acetylcholinesterases secreted by parasitic nematodes is reviewed, and, finally, the involvement of acetylcholinesterase in apoptosis is considered.

G2-acetylcholinesterase is presynaptically localized in Torpedo electric organ

In Torpedo electric organ, much of the acetylcholinesterase (AChE) is a globular dimer (G2), anchored to the plasma membrane via covalently attached phosphatidylinositol and selectively solubilized by a bacterial phosphatidylinositol-specific phospholipase C. While the structure of this form of the enzyme is well-established, the ultrastructural localization of G2-AChE is still unclear. Selective solubilization with phosphatidylinositol-specific phospholipase C was, therefore, combined with immunocytochemistry at the electron microscope level, in order to localize G2-AChE in electric organ of Torpedo ocellata. Thin sections of electric organ were labelled with antibodies raised against Torpedo AChE, followed by gold-conjugated second antibodies, before or after exposure to the phospholipase. For comparison, the location of AChE was examined using histochemical methods. We show that (1) immunolabelling is concentrated in the synaptic clefts between nerve terminals and the innervated face of the electrocyte; (2) this labelling co-localizes with AChE histochemical reaction products; and (3) prior exposure to the phospholipase causes a decrease in AChE-associated labelling. Quantitative analysis of immunolabelling in the synaptic clefts shows that the phospholipase treatment had reduced primary labelling at or adjacent to the presynaptic membrane. Together with our earlier biochemical and immunofluorescent evidence, these results support our previous assignment of a neuronal and synaptic localization for G2-AChE in Torpedo electric organ.

Immunocytochemical localization of phosphatidylinositol-anchored acetylcholinesterase in excitable membranes of Torpedo ocellata

In Torpedo electric organ much of the acetylcholinesterase is a 'globular' dimer (G2), anchored to the plasma membrane via covalently attached phosphatidylinositol and solubilized by a bacterial phosphatidylinositol-specific phospholipase C. This suggested that selective solubilization with phosphatidylinositol-specific phospholipase C, coupled with immunocytochemistry, might be used to localize G2 acetylcholinesterase in excitable tissues of Torpedo. Cryostat sections of electric organ, electromotor nerve, electric lobe and back muscle from Torpedo ocellata were labelled, using three different antibody preparations to Torpedo acetylcholinesterase, followed by a fluorescent second antibody, before and after exposure to the phospholipase. Sites of innervation on electrocytes and myofibers were labelled selectively, as were motor and electromotor nerves. In all these cases labelling was substantially diminished by prior exposure to the phospholipase. The results support our previous assignment, based on biochemical evidence, for a neuronal and synaptic localization of the G2 acetylcholinesterase in Torpedo. Electric lobe acetylcholinesterase appears insensitive to the phospholipase treatment and lacks certain epitopes present in both electric organ and electromotor nerve enzyme. This suggests that substantial processing of the G2 form occurs concomitantly with its movement from the electric lobe into the electromotor nerve.

Torpedo electromotor system development: biochemical differentiation of Torpedo electrocytes in vitro

The accumulation of 2 postsynaptic proteins--the acetylcholine receptor and acetylcholinesterase, total protein and lactate dehydrogenase levels, and the evolution of the multiple molecular forms of acetylcholinesterase (exhibiting apparent sedimentation coefficients of 17, 13, 11 and 6S) have been examined in aneural cultures of embryonic Torpedo electric organ explanted before, during or after electrocyte differentiation and the onset of synaptogenesis. During electrocyte differentiation in vitro, with explants taken before the 38 mm stage, the relative proportions of the 17, 13 and 11S forms change in vitro as in vivo but the 6S form remains abnormally dominant. In tissue explants taken from 38 to 47 mm stage embryos, the 4 major molecular forms of acetylcholinesterase differentiate in a manner identical to that observed in vivo. In explants taken after the onset of synaptogenesis (55-80 mm stages), the proportions of the acetylcholinesterase forms change as in vivo only during the first week in vitro whilst accumulation is occurring at the normal in vivo rate. The switch to the high acetylcholine receptor and acetylcholinesterase accumulation rate that occurs when synaptogenesis begins in vivo is not observed after any time lag in vitro with tissue explanted before the stage (55 mm) at which synaptogenesis begins. The effects on acetylcholinesterase and acetylcholine receptor accumulation of supplementing the medium with a neural tissue extract are described. The experiments were designed to elucidate the factors and mechanisms that regulate the differentiation and formation of chemical synapses using the electric organ of Torpedo marmorata as a model system. The results demonstrate that the complex changes occurring in the multiple molecular forms of acetylcholinesterase during electrocyte differentiation are not under direct neural control but that the switch to an increased acetylcholinesterase and acetylcholine receptor accumulation rate may be triggered by an external, possible neural factor.

Solubilization of membrane-bound acetylcholinesterase by a phosphatidylinositol-specific phospholipase C

Phosphatidylinositol-specific phospholipase C (PIPLC) quantitatively solubilizes acetylcholinesterase (AChE) from purified synaptic plasma membranes and intact synaptosomes of Torpedo ocellata electric organ. The solubilized AChE migrates as a single peak of sedimentation coefficient 7.0S upon sucrose gradient centrifugation, corresponding to a subunit dimer. The catalytic subunit polypeptide of AChE is the only polypeptide detectably solubilized by PIPLC. This selective removal of AChE does not affect the amount of acetylcholine released from intact synaptosomes upon K+ depolarization. PIPLC also quantitatively solubilizes AChE from the surface of intact bovine and rat erythrocytes, but only partially solubilizes AChE from human and mouse erythrocytes. The AChE released from rat and human erythrocytes by PIPLC migrates as a approximately 7S species on sucrose gradients, corresponding to a catalytic subunit dimer. PIPLC does not solubilize particulate AChE from any of the brain regions examined of four mammalian species. Several other phospholipases tested, including a nonspecific phospholipase C from Clostridium welchii, fail to solubilize AChE from Torpedo synaptic plasma membranes, rat erythrocytes, or rat striatum.

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)

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 hydrophobic dimer of acetylcholinesterase from Torpedo californica electric organ is solubilized by phosphatidylinositol-specific phospholipase C

A dimeric form of acetylcholinesterase from the electric organ of Torpedo californica was solubilized by phosphatidylinositol-specific phospholipase C from Staphylococcus aureus. The solubilized enzyme had a sedimentation coefficient of 7.3S which was not modified by detergents. The high salt-soluble asymmetric forms of acetylcholinesterase were not solubilized by the phospholipase. Our data suggest that the hydrophobic dimer of acetylcholinesterase may be associated with the plasma membrane through a specific interaction involving phosphatidylinositol.

Comparative affinity chromatography of acetylcholinesterases from five vertebrate species

The efficacy of N-methylacridinium affinity chromatography in the purification of acetylcholinesterases from chicken, rat, calf and human brain and from the electric organ of the electric fish Torpedo marmorata has been investigated. Retention of the enzymes on the N-methylacridinium columns exceeded 90% in all instances except for the chicken enzyme, where 40-80% retention was observed depending on the acridinium concentration. Sucrose density gradient centrifugation profiles revealed no difference between the distribution of molecular forms in the crude extracts and in the partially purified fractions eluted from the columns by decamethonium iodide.

Changes in acetylcholinesterase molecular forms during the embryonic development of Torpedo marmorata

Multiple molecular forms of acetylcholinesterase from electric organ and electric lobe of Torpedo marmorata were examined at various developmental stages by sucrose density sedimentation. Four major forms were characterized by their apparent sedimentation coefficients of 6 S, 11 S, 13 S, and 17 S. Embryonic lobe possessed at early stages predominantly the 11 S form. With maturation the 17 S form became the most abundant. The early embryonic stages of the electric organ were characterized by predominating amounts of 6 S and 11 S forms. With differentiation of the postsynaptic membrane of the developing electrocytes, 13 S and 17 S forms replaced the slower-sedimenting forms. Concomitant with the formation of synaptic contacts, a transient increase in the 13 S form was followed by a dramatic accumulation of rapid-sedimenting 17 S form. The establishment of fully functional synapses was accompanied by an increase in the amount of the hydrophobic 6 S form. At birth, equal amounts of 6 S and 17 S form were found, with the other forms present in only trace amounts. The observed characteristic changes correlated with morphological and physiological events, indicating a close functional relationship between the accumulation of the 17 S form and synapse formation and the accumulation of the 6 S form and onset of function.