Review
Structural and functional organization of synaptic acetylcholinesterase

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Abstract

The expression of the synaptic asymmetric form of the enzyme acetylcholinesterase (AChE) depends of two different genes: the gene that encodes for the catalytic subunit and the gene that encodes for the collagenic tail, ColQ. Asymmetric AChE is specifically localized to the basal lamina at the neuromuscular junction (NMJ). This highly organized distribution pattern suggests the existence of one or more specific binding sites in ColQ required for its anchorage to the synaptic basal lamina. Recent evidence support this notion: first, the presence of two heparin-binding domains in ColQ that interact with heparan sulfate proteoglycans (HSPGs) at the synaptic basal lamina; and second, a knockout mouse for perlecan, a HSPG concentrated in nerve–muscle contact, in which absence of asymmetric AChE at the NMJ is observed. The physiological importance of collagen-tailed AChE form in skeletal muscle has been illustrated by the identification of several mutations in the ColQ gene. These mutations determine end-plate acetylcholinesterase deficiency and induce one type of synaptic functional disorders observed in Congenital Myasthenic Syndromes (CMSs).

Introduction

The physiological role of acetylcholinesterase (AChE, E.C. 3.1.1.7) at nicotinic cholinergic synapses is believed to be the termination of impulse transmission by rapid hydrolysis of the neurotransmitter acetylcholine (ACh). Because the whole process of synaptic transmission takes place within 1 ms, it demands a very precise temporo-spatial integration of the structural and functional components involved [29], [54]. The study of AChE extends back to the early 20th century when Dale [22] described a substance capable of cleaving choline esters. This preceded Loewi's [55] identification of acetylcholine (ACh) as the neurotransmitter of the cholinergic system, as well as findings of Marnay and Nachmanson [57] showing high concentration of AChE at the sites of nerve–muscle contacts. In 1942, the fundamental role of AChE in cholinergic transmission was firmly established by Eccles et al. [29]. Since then, extensive information has accumulated regarding the basic chemistry and physiology of AChE. The molecular structure of AChE has been determined by the study of the enzyme purified from tissues that are homologous to skeletal muscle, the electric organs of eel and the marine ray [59]. In 1979, Bon et al. [7] introduced a nomenclature that permits the classification of the multiple molecular forms of AChE. Two classes were defined: the globular and asymmetric forms. Globular forms are monomers (G1), dimers (G2) and tetramers (G4) of the catalytic subunit [7], [43], [59]; and asymmetric forms comprised to one (A4), two (A8) or three (A12) catalytic tetramers attached to a noncatalytic subunit [38], [42], [72]. The asymmetric forms are of particular interest because they have an unusual structure in which the catalytic subunits are attached to a long, triple-helical collagenous tail [59]. Asymmetric forms of AChE with properties similar to those from the electric organ exist in mammalian muscle [7]; in some species, these forms are found only in extracts obtained from regions of muscle containing endplates [18], [38].

This polymorphism, together with the distinct physicochemical characteristics and subcellular localization of the AChE forms, has made it difficult to precisely define their regulation. Indeed, several neuromuscular influences including neurotrophic substances, nerve impulse patterns and muscle mechanical activity have been implicated in the maintenance of AChE expression [35], [37], [56], [76], [77].

Section snippets

AChE at the vertebrate neuromuscular junction (NMJ)

The formation of NMJ comprises a high degree of specialization at the nerve–muscle contact where the synaptic basal lamina is present. The high concentration of AChE at the synaptic ECM is an important marker for the specialization of the junctional ECM, which constitutes <0.1% of the total muscle basal lamina area. During development, myotubes express asymmetric AChE over their entire surface, but then quickly accumulate it at the newly formed synapses [20], [84]. The G4 AChE is probably

Collagenic tail gene structure and role in NMJ

The molecular structure of ColQ is basically composed of three principal domains: the N-terminal domain a proline-rich domain (or PRAD) responsible for the recruitment of the catalytic subunits and their stabilization through disulfide bonds [6], a collagenic central domain composed of GXY triplets which form a triple helix and contains two belts of HBDs [23], [25] and the C-terminal region, which is a globular domain enriched in charged residues and cysteines involved in the trimeric ColQ

Skeletal muscle heparan sulfate proteoglycans (HSPGs)

Important progress has been recently achieved in the identification of the HSPG receptors for asymmetric AChE at the synaptic basal lamina. HSPGs are diverse multifunctional proteins from the cell surface or the ECM that can interact with several other molecules either through their protein cores or through their covalently attached GAG chains [5]. Different HSPG species can be isolated from mammalian skeletal muscle ECM [11]. Most of our knowledge of muscle HSPGs comes from in vitro studies

Perlecan anchors ColQ to the NMJ basal lamina

Perlecan and muscle agrin [54] are the main HSPGs present at the synaptic basal lamina. Both proteoglycans contain globular G-domain motifs in their core proteins that permit their binding to α-dystroglycan [15], [79] in the dystrophin-associated protein complex at the NMJ [9], [14]. It has been shown that the dystroglycan–perlecan complex serves as a cell surface acceptor for asymmetric AChE in Xenopus myotubes, and that it can concentrate AChE at the synapse by lateral migration at the plane

ColQ mutations determine a synaptic AChE deficiency, which results in a Congenital Myasthenic Syndromes (CMSs)

Congenital myasthenic syndromes (CMSs) are heterogeneous disorders caused by presynaptic, synaptic or postsynaptic defects [32], [33], [65]. The synaptic type of CMS is caused by the absence of the asymmetric AChE from the NMJ. It is a rare autosomal recessive disease, which was first described in 1977 by Engel et al. [30]. In the absence of the enzyme, the synaptic currents are prolonged and evoke repetitive compound muscle action potentials. The synaptic CMS with endplate AChE deficiency is

Acknowledgements

This work was supported by grants from FONDAP-Biomedicine No. 13980001. Partial support was obtained from the Millennium Institute of Fundamental and Applied Biology (MIFAB). N.C.I. and E.B. were recipients of a Presidential Chair in Science from the Chilean Government. E.B. is a Howard Hughes Medical Institute International Scholar.

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