A dimeric form of acetylcholinesterase anchored through a glycolipid in mouse skeletal muscle

Expression of dimeric and tetrameric acetylcholinesterase isoforms on the surface of cultured bovine adrenal chromaffin cells

Acetylcholinesterase is a highly polymorphic enzyme, which can be anchored to the cell surface through several different mechanisms. Dimeric (G2) acetylcholinesterase isoforms are attached by a glycosyl-phosphatidyl-inositol (GPI) linkage, whereas tetrameric (G4) forms are linked through a 20 kilodalton hydrophobic subunit. Although cells of haemopoietic origin contain large amounts of G2 GPI-linked acetylcholinesterase, most tissues express only trace amounts of this isoform. We examined the expression of acetylcholinesterase isoforms in cultured bovine adrenal medullary chromaffin cells. Two major isoforms (G2 and G4) were identified on the cell surface. The G2 isoform, which accounted for approximately half the cell-surface enzyme activity, was linked to the membrane through a GPI anchor. After treatment with diisopropylfluorophosphate to completely inhibit cellular acetylcholinesterase, the G4 isoform was found to be resynthesised and transported to the cell surface more rapidly than the G2 isoform. As the addition of GPI anchors is known to be a very rapid step, this finding suggested that the G2 and G4 isoforms might be transported to the cell surface by two different mechanisms. This conclusion was supported by results from subcellular fractionation experiments. The ratio of G4/G2 membrane-bound acetylcholinesterase varied between different subcellular fractions. The membrane-bound G2 isoform was greatly enriched in a high-speed "microsomal" fraction. G4 acetylcholinesterase is known to be actively secreted by chromaffin cells in culture. Although the G4 isoform was present on the cell surface, most of the secreted enzyme was derived from an intracellular pool. Thus, it is unlikely that the cell-surface G4 isoform contributes significantly to the pool of secreted enzyme. Instead, the expression of two different membrane-bound isoforms may provide a means by which chromaffin cells can target the enzyme to different locations on the cell surface.

Calcium influxes and calmodulin modulate the expression and physicochemical properties of acetylcholinesterase molecular forms during development in vivo

1. Acetylcholinesterase (AcChoE; EC 3.1.1.7) exists in several molecular forms that may be anchored to cell membranes or associated with extracellular matrix. AcChoE bound to lipidic membranes is detergent extractable (DE AcChoE), whereas the enzyme associated with extracellular matrix is high salt soluble (HSS AcChoE). The latter variant is accumulated in synaptic regions by an unknown mechanism. 2. We have suggested previously that depolarization-induced Ca2+ influx is a major factor that modulates AcChoE synthesis in vivo, as well as the conversion of some DE AcChoE to HSS variant. In the present study, we have examined (i) the effects of depolarization-induced skeletal muscle inactivity and ionophore-induced Ca2+ influxes on the expression of AcChoE molecular forms and (ii) the hypothesis that Ca(2+)-dependent calmodulin may be involved in the conversion of at least some forms of DE AcChoE to HSS variant in vivo. 3. Chick embryos were treated in ovo during the early period of nerve-muscle interactions with d-tubocurarine (dTC; a competitive neuromuscular blocking agent) or with decamethonium (dMET; a depolarizing agent). Both dTC and dMET equally and significantly reduced embryonic neuromuscular activity (motility). However, dTC significantly decreased AcChoE overall activity, whereas dMET had virtually no effect on AcChoE expression, compared to controls. 4. Treatment of embryos with the Ca2+ ionophore A23187 significantly increased the total AcChoE activity as well as the DE/HSS ratio of each AcChoE molecular form. However, treatment with N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide (also termed W-7), a calmodulin antagonist, did not alter the total AcChoE activity, but significantly increased the DE/HSS ratio of AcChoE forms. 5. These results support the idea that (i) depolarization and/or Ca2+ influxes, but not muscle contraction, may regulate AcChoE expression in skeletal muscle and (ii) Ca(2+)-dependent calmodulin activation may be involved in the conversion of some DE AcChoE to their HSS variant in vivo.

Developmental modulation of physicochemical variants of the tailed asymmetric (16S) acetylcholinesterase by neuromuscular activity and innervation in the mouse embryo

We have studied the physicochemical properties of acetylcholinesterase (AChE) during embryonic development of normal and functionally impaired mouse skeletal muscle, focusing on the tailed asymmetric (16S) form of the enzyme. The muscle-specific 16S AChE exists in two different variants. One is associated with extracellular matrix and is high-salt soluble (HSS, also termed hydrophilic AChE), whereas the other form is anchored to cell membranes and is detergent extractable (DE, or hydrophobic AChE). Before innervation during normal embryonic development, both hydrophilic and hydrophobic 16S AChE exist in equal amounts. After muscle innervation, there was an increase (amounting three-fold on E18) in the levels of hydrophilic vs. hydrophobic 16S AChE. This alteration of the relative proportions of the two variants of 16S AChE did not occur in chronically inactive muscles either from the mouse mutant, muscular dysgenesis, or from tetrodotoxin-treated mouse embryos. Taken together with previous reports, the present results suggest that postsynaptic membrane depolarization-induced Ca2+ fluxes are important in modulating not only the synthesis of 16S AChE, but also the relative proportions of both physicochemical variants of this molecular form of AChE.

Rapid analysis of glycolipid anchors in amphiphilic dimers of acetylcholinesterases

1. We describe two simple procedures for the rapid identification of certain structural features of glycolipid anchors in acetylcholinesterases (AChEs). 2. Treatment with alkaline hydroxylamine (that cleaves ester-linked acyl chains but not ether-linked alkyl chains) converts molecules possessing a diacylglycerol, but not those with an alkylacylglycerol, into hydrophilic derivatives. AChEs in human and bovine erythrocytes possess an alkylacylglycerol (Roberts et al., J. Biol. Chem. 263:18766-18775, 1988; Biochem. Biophys. Res. Commun. 150:271-277, 1988) and are not converted to hydrophilic dimers by alkaline hydroxylamine. Amphiphilic dimers of AChE from Drosophila, from mouse erythrocytes, and from the human erythroleukaemia cell line K562 also resist the treatment with hydroxylamine and likely possess a terminal alkylacylglycerol. This indicates that the cellular pool of free glycolipids used as precursors of protein anchors is distinct from the pool of membrane phosphatidylinositols (which contain diacylglycerols). 3. Pretreatment with alkaline hydroxylamine is required to render the amphiphilic AChE from human erythrocytes susceptible to digestion by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (PI-PLC) (Toutant et al., Eur. J. Biochem. 180:503-508, 1989). We show here that this is also the case for the AChE from mouse erythrocytes, which therefore likely possesses an additional acyl chain in the anchor that prevents the action of PI-PLC. 4. In two sublines of K562 cells (48 and 243), we observed that AChE either was directly susceptible to PI-PLC (243) or required a prior deacylation by alkaline hydroxylamine (48). This suggests that glycolipid anchors in AChE of K562-48 cells, but not those in AChE of K562-243 cells, contain the additional acylation demonstrated in AChE from human erythrocytes. These observations illustrate the cell specificity (and the lack of species-specificity) of the structure of glycolipid anchors.

Phosphatidylinositol is involved in the attachment of tailed asymmetric acetylcholinesterase to neuronal membranes

1. We analyzed the mode of attachment of 16 S tailed acetylcholinesterase (AChE; EC 3.1.1.7) to rat superior cervical ganglion (SCG) neuronal membranes. Using extractions by high-salt (HS) and nonionic detergent (Triton X-100), we found two pools of 16 S AChE. 2. The detergent-extracted (DE) 16 S AChE was tightly bound to membranes through detergent-sensitive, high-salt insensitive interactions and was distinct from high-salt-soluble 16 S AChE. The detergent-extracted (DE) 16 S AChE constituted a significant proportion of about one-third of the total 16 S AChE. 3. Treatment of the neuronal membranes by a phosphatidylinositol-specific phospholipase C (PIPLC) resulted in the release of some, but not all DE 16 S AChE, indicating that a significant amount of the neuronal DE 16 S AChE, about one-third, is anchored to membranes through a phosphatidylinositol containing residue. Thus, a covalent association of a glycolipid and catalytic or structural AChE polypeptidic chains occurs not only for dimeric AChE but also for the asymmetric species of AChE. 4. The complex polymorphism of AChE is due not only to different globular or asymmetric associations of catalytic and structural subunits but also to the alternative existence of a transmembrane domain or a glycolipid membrane anchor.

Molecular cloning of mouse acetylcholinesterase: tissue distribution of alternatively spliced mRNA species

We have isolated cDNA clones encoding acetylcholinesterase from mouse muscle and brain. The polymerase chain reaction was used to amplify cDNA clones from C2 myotubes encoding the entire open reading frame and large segments of the 5' and 3' untranslated regions. The muscle cDNA clones were used to isolate clones from a brain library encoding the same mRNA species. The mouse clones encode a catalytic subunit containing a C-terminal sequence similar to that of the hydrophilic species of Torpedo. The mouse acetylcholinesterase sequence shares approximately 88% and 61% amino acid identity with bovine and Torpedo acetylcholinesterases, respectively, but only 52% identity with mouse butyrylcholinesterase, the sequence of which we have also deduced by molecular cloning. Northern blot and RNAase protection analyses indicate that the cDNA clones were derived from the acetylcholinesterase transcript that predominates in most expressing tissues. In contrast, erythroid cells are enriched in an mRNA species whose sequence diverges from that of the cDNA in the region encoding the C-terminus of the enzyme.

Amphiphilic and hydrophilic molecular forms of acetylcholinesterase in membranes derived from sarcoplasmic reticulum of skeletal muscle

Native molecular forms of acetylcholinesterase (AChE) present in a microsomal fraction enriched in SR of rabbit skeletal muscle were characterized by sedimentation analysis in sucrose gradients and by digestion with phospholipases and proteinases. The hydrophobic properties of AChE forms were studied by phase-partition of Triton X-114 and Triton X-100-solubilized enzyme and by comparing their migration in sucrose gradient containing either Triton X-100 or Brij 96. We found that in the microsomal preparation two hydrophilic 13.5 S and 10.5 S forms and an amphiphilic 4.5 S form exist. The 13.5 S is an asymmetric molecule which by incubation with collagenase and trypsin is converted into a 'lytic' 10.5 S form. The hydrophobic 4.5 S form is the predominant one in extracts prepared with Triton X-100. Proteolytic digestion of the membranes with trypsin brought into solution a significant portion of the total activity. Incubation of the membranes with phospholipase C failed to solubilize the enzyme. The sedimentation coefficient of the amphiphilic 4.5 S form remained unchanged after partial reduction, thus confirming its monomeric structure. Conversion of the monomeric amphiphilic form into a monomeric hydrophilic molecule was performed by incubating the 4.5 S AChE with trypsin. This conversion was not produced by phospholipase treatment.