The A12 acetylcholinesterase and polypeptide composition of electric organ basal lamina of Electrophorus and some Torpedinae fishes

At least two receptors of asymmetric acetylcholinesterase are present at the synaptic basal lamina of Torpedo electric organ

Asymmetric acetylcholinesterase (AChE) is anchored to the basal lamina (BL) of cholinergic synapses via its collagenic tail, yet the complement of matrix receptors involved in its attachment remains unknown. The development of a novel overlay technique has allowed us to identify two Torpedo BL components that bind asymmetric AChE: a polypeptide of approximately 140 kDa and a doublet of 195-215 kDa. These were found to stain metachromatically with Coomassie blue R-250, were solubilized by acetic acid, and were sensitive to collagenase treatment. Upon sequence analysis, the 140 kDa polypeptide yielded a characteristic collagenous motif. Another AChE-binding BL constituent, identified by overlay, corresponded to a heparan sulfate proteoglycan. Lastly, we established that this proteoglycan, but not the collagenous proteins, interacted with at least one heparin binding domain of the collagenic tail of AChE. Our results indicate that at least two BL receptors are likely to exist for asymmetric AChE in Torpedo electric organ.

A major portion of synaptic basal lamina acetylcholinesterase is detached by high salt- and heparin-containing buffers from rat diaphragm muscle and Torpedo electric organ

Collagen-tailed asymmetric acetylcholinesterase (AChE) forms are believed to be anchored to the synaptic basal lamina via electrostatic interactions involving proteoglycans. However, it was recently found that in avian and rat muscles, high ionic strength or polyanionic buffers could not detach AChE from cell-surface clusters and that these buffers solubilized intracellular non-junctional asymmetric AChE rather than synaptic forms of the enzyme. In the present study, asymmetric AChE forms were specifically solubilized by ionic buffers from synaptic basal lamina-enriched fractions, largely devoid of intracellular material, obtained from the electric organ of Torpedo californica and the end plate regions of rat diaphragm muscle. Furthermore, foci of AChE activity were seen to diminish in size, number, and staining intensity when the rat synaptic basal lamina-enriched preparations were treated with the extraction buffers. In the case of Torpedo, almost all the AChE activity was removed from the pure basal lamina sheets. We therefore conclude that a major portion of extracellular collagen-tailed AChE is extractable from rat and Torpedo synaptic basal lamina by high ionic strength and heparin buffers, although some non-extractable AChE activity remains associated with the junctional regions.

Characterization of acetylcholinesterase from human heart auricles: evidence for the presence of a G-form sensitive to phosphatidylinositol-specific phospholipase c

1. Acetylcholinesterase (AChE) is an important enzyme of the cholinergic system in mammals. 2. We report here the subcellular association of the AChE molecular forms in the normal human heart auricle. 3. Both globular (G) and asymmetric (A) forms were identified using velocity sedimentation and sequential extraction procedures. 4. G forms corresponds to 84% and A forms account for 16% of the total AChE activity. 5. Of G forms 64% of AChE activity correspond to the G1 monomer and of the A forms the class I-A account for 80% of AChE activity. 6. In addition, treatment of the cardiac membranes with the enzyme phosphatidylinositol-specific phospholipase c (PIPLC) results in the solubilization of AChE activity. 7. This means that a G2 AChE dimer with a glycolipid anchoring domain is present in the human heart.

Dermatan sulfate and de-sulfated heparin solubilized collagen-tailed acetylcholinesterase from the rat neuromuscular junction

We are interested in the study of the interactions involved in the attachment of collagen-tailed acetylcholinesterase (AChE) to the synaptic basal lamina. The fact that AChE occupies less than 0.1% of the muscle basal lamina, suggests that there is a very high specificity in the interaction that defines its distribution. We have previously found that asymmetric AChE is bound to the neuromuscular junction via heparan sulfate proteoglycans. Sulfated glycosaminoglycans as heparan sulfate and heparin extracted the asymmetric AChE from the synaptic basal lamina. Here we show that dermatan sulfate as well as de-sulfated heparin, are also able to extract collagen-tailed AChE. Taking into account that the solubilization of the asymmetric AChE is concomitant with the liberation of a dermatan sulfate proteoglycan from the rat neuromuscular junction, the present results open the possibility that the collagen-tailed AChE is also anchored to dermatan sulfate proteoglycans at the synaptic basal lamina.

Isolation of the heparan sulfate proteoglycans from the extracellular matrix of rat skeletal muscle

We have previously shown that asymmetric collagen-tailed acetylcholinesterase (AChE) is anchored to the extracellular matrix (ECM) by heparan sulfate proteoglycans (HSPGs). Here we present our studies on the characterization of such PGs from the ECM of rat skeletal muscles. After radiolabeling with 35SO4 for 24h, PGs were extracted from the muscle ECM with 4.0 M guanidine-HCl containing protease inhibitors. PGs were subsequently isolated using sequential DEAE-Sephacel chromatography, digestion with chondroitinase ABC, and Sepharose CL-4B. Two different hydrodynamic size species of HSPGs were found. One type had a Mr of 4-6 X 10(5) (Kav = 0.25) as estimated by gel chromatography in the presence of 1% SDS and accounted for 75% of the total HSPGs. The other HSPG had a Mr 1.5-2.5 X 10(5) (Kav = 0.41). The glycosaminoglycan (GAG) side chains (Mr 20,000 and 12,000) were found composed only of heparan sulfate as determined by nitrous acid oxidation and heparitinase treatment. The large-sized HSPG, which is concentrated in synaptic regions, contains only GAG chains of Mr 20,000, suggesting that each HSPG contains only one kind of heparan sulfate chain in its structure. Our results definitively establish by biochemical criteria that the basement membrane of mammalian skeletal muscle contains HSPGs, the likely matrix receptor for the immobilization of the asymmetric collagen-tailed AChE at the neuromuscular junction.

Glycosaminoglycan composition of electric organ basement membranes

The basement membranes of the innervated surface of the electric organ of Discopyge tschudii present a high concentration of mucopolysaccharides as revealed by intense ruthenium red-positive reaction. Glycosaminoglycans (GAGs) were isolated and characterized from these pure basement membranes by using a combination of agarose gel electrophoresis and enzymatic degradation with specific enzymes. The isolated basement membrane showed a high concentration of GAGs (130 mg/g of dry tissue); of this amount 49% was hyaluronic acid, 24% was chondroitin-6-sulfate, 12% was heparan sulfate, and 14% was dermatan sulfate. Controlled digestion with heparinase and heparitinases I and II was used to study the structural features of the heparan sulfate. Four unsaturated disaccharide units were found in the heparan sulfate: disulfated, N-sulfated, N-acetylated, and N-acetylated O-sulfated disaccharides. The disaccharide units of the cholinergic heparan sulfate present a high amount of disulfated disaccharides and a low amount of N-acetylated O-sulfated disaccharides. The N-sulfated disaccharides, in contrast to the N-acetylated ones, were found through all the structure of the cholinergic heparan sulfate. Finally our work shows for the first time the presence of dermatan sulfate in the basal lamina of the electric organ.

The synaptic form of acetylcholinesterase binds to cell-surface heparan sulfate proteoglycans

We have previously communicated that heparan sulfate and heparin released 16S acetylcholinesterase (AChE) from cholinergic synapses. These experiments suggest that heparan-like molecules are involved in the anchorage of AChE to the neuromuscular junction. In order to prove the in vivo interaction between the 16S AChE and heparan sulfate residues, the binding of exogenously added 16S enzyme to intact cells rich in cell-surface heparan sulfate proteoglycans was examined; 16S AChE form was shown to bind to intact endothelial cells in a specific, time-dependent, saturable fashion. A single class of binding sites was involved and at saturation around 2.52 X 10(11) molecules of 16S AChE/cm2 were bound. Fifty percent of the binding of the 16S AChE was blocked by heparan sulfate, heparin, or previous treatment of the cell with heparitinase. The binding was reversed by exogenous heparin, but not by chondroitin sulfate or hyaluronic acid. Our results demonstrate that the synaptic form of AChE binds to heparan sulfate proteoglycans on the surface of the cell.

Anchorage of collagen-tailed acetylcholinesterase to the extracellular matrix is mediated by heparan sulfate proteoglycans

Heparan sulfate and heparin, two sulfated glycosaminoglycans (GAGs), extracted collagen-tailed acetylcholinesterase (AChE) from the extracellular matrix (ECM) of the electric organ of Discopyge tschudii. The effect of heparan sulfate and heparin was abolished by protamine; other GAGs could not extract the esterase. The solubilization of the asymmetric AChE apparently occurs through the formation of a soluble AChE-GAG complex of 30S. Heparitinase treatment but not chondroitinase ABC treatment of the ECM released asymmetric AChE forms. This provides direct evidence for the vivo interaction between asymmetric AChE and heparan sulfate residues of the ECM. Biochemical analysis of the electric organ ECM showed that sulfated GAGs bound to proteoglycans account for 5% of the total basal lamina. Approximately 20% of the total GAGs were susceptible to heparitinase or nitrous acid oxidation which degrades specifically heparan sulfates, and approximately 80% were susceptible to digestion with chondroitinase ABC, which degrades chondroitin-4 and -6 sulfates and dermatan sulfate. Our experiments provide evidence that asymmetric AChE and carbohydrate components of proteoglycans are associated in the ECM; they also indicate that a heparan sulfate proteoglycan is involved in the anchorage of the collagen-tailed AChE to the synaptic basal lamina.

Age-related responses of skeletal muscle after ectopic innervation, with particular reference to 16S acetylcholinesterase, in adult rats

The formation of ectopic junctions between the foreign fibular nerve and the soleus muscle of young (35-day-old) and mature (200-day-old) adult rats was induced by severing the normal nerve 4 weeks after implanting the foreign nerve. The various molecular forms of acetylcholinesterase (AChE) were studied both at the implanted region and at the original denervated endplates. The velocity of contraction was also studied. In young rats the 16S form was first detected in the ectopic junctions around day 5 after reinnervation; this form rapidly increased during the following weeks, reaching a plateau by day 20. By contrast, in mature rats the appearance of the 16S AChE was dramatically delayed; in fact, it could not be observed before day 80 after reinnervation. (The 16S AChE form appeared at day 20 after reinnervation in the original denervated endplates of young rats; however, at the same time, no effect was observed in mature animals.) The original, slow muscle fibers of the soleus became faster upon reinnervation; this change occurred also much earlier in younger than in mature rats. Our results indicate a loss of plasticity in the skeletal muscle of mature rats. We suggest caution in the use of the ectopic innervation model to study development in mature adult rats.

Binding of the asymmetric forms of acetylcholinesterase to heparin

The interaction between acetylcholinesterase (EC 3.1.1.7) and heparin, a sulphated glycosaminoglycan, was studied by affinity chromatography. A specific binding of the asymmetric acetylcholinesterase to an agarose gel containing covalently bound heparin was demonstrated. This interaction required an intact collagenous tail, shown by the fact that the binding is abolished by pretreatment with collagenase. The globular forms did not bind to the column. Both total and intracellular asymmetric acetylcholinesterase forms isolated from the endplate region of the rat diaphragm muscle showed higher affinity for the heparin than did the enzyme from the non-endplate region. The binding to the resin was destabilized with 0.55 M-NaCl, and, among the various glycosaminoglycans tested, only heparin was able to displace the acetylcholinesterase bound to the column. Our results added further support to the concept that the asymmetric acetylcholinesterase forms are immobilized on the synaptic basal lamina via interactions with heparin-like molecules, probably related to heparan sulphate proteoglycans.

The electric organ of Discopyge tschudii: its innervated face and the biology of acetylcholinesterase

An ultrastructural, histochemical, and biochemical study of the electric organ of the South American Torpedinid ray, Discopyge tschudii, was carried out. Fine structural cytochemical localization of acetylcholinesterase (AChE) indicated that most of the esterase was associated with the basal lamina. Electron microscopy indicated no marked differences in the electrocyte ultrastructure between Discopyge and Torpedo californica. Discopyge electric organ possessed three molecular forms, two asymmetric forms (16 S and 13 S) and one globular hydrophobic form (6.5 S). The asymmetric 16 S AChE form was solubilized by heparin, a sulfated glycosaminoglycan, suggesting that heparin-like macromolecules are involved in the binding of the enzyme to the basal lamina. Our results show that cell-free translated AChE peptides, synthesized using Discopyge electric organ poly(A+) RNA, correspond to a main band of 62,000 daltons which probably represents the catalytic subunit of the asymmetric AChE.

16S acetylcholinesterase of the extracellular matrix is assembled within mouse muscle cells in culture

The present paper examines where the extracellular-matrix (ECM) 16S acetylcholinesterase (AChE, EC 3.1.1.7) is assembled in muscle cells in culture. The existence of an internal pool of 16S AChE was detected by using AChE inhibitors of differing membrane permeability. After irreversible inhibition of all cellular esterase, the newly synthesized 16S form appears in an intracellular compartment and is only later detected on the cell surface. Results show that the ECM 16S AChE is assembled within muscle cells.