Genetic Analysis of Collagen Q: Roles in Acetylcholinesterase and Butyrylcholinesterase Assembly and in Synaptic Structure and Function

Abstract

Acetylcholinesterase (AChE) occurs in both asymmetric forms, covalently associated with a collagenous subunit called Q (ColQ), and globular forms that may be either soluble or membrane associated. At the skeletal neuromuscular junction, asymmetric AChE is anchored to the basal lamina of the synaptic cleft, where it hydrolyzes acetylcholine to terminate synaptic transmission. AChE has also been hypothesized to play developmental roles in the nervous system, and ColQ is also expressed in some AChE-poor tissues. To seek roles of ColQ and AChE at synapses and elsewhere, we generated ColQ-deficient mutant mice. ColQ −/− mice completely lacked asymmetric AChE in skeletal and cardiac muscles and brain; they also lacked asymmetric forms of the AChE homologue, butyrylcholinesterase. Thus, products of the ColQ gene are required for assembly of all detectable asymmetric AChE and butyrylcholinesterase. Surprisingly, globular AChE tetramers were also absent from neonatal ColQ −/− muscles, suggesting a role for the ColQ gene in assembly or stabilization of AChE forms that do not themselves contain a collagenous subunit. Histochemical, immunohistochemical, toxicological, and electrophysiological assays all indicated absence of AChE at ColQ −/− neuromuscular junctions. Nonetheless, neuromuscular function was initially robust, demonstrating that AChE and ColQ do not play obligatory roles in early phases of synaptogenesis. Moreover, because acute inhibition of synaptic AChE is fatal to normal animals, there must be compensatory mechanisms in the mutant that allow the synapse to function in the chronic absence of AChE. One structural mechanism appears to be a partial ensheathment of nerve terminals by Schwann cells. Compensation was incomplete, however, as animals lacking ColQ and synaptic AChE failed to thrive and most died before they reached maturity.

Abundant Tissue Butyrylcholinesterase and Its Possible Function in the Acetylcholinesterase Knockout Mouse

“…These forms have a characteristically high sedimentation velocity and a distinctive salt-dependent solubility that reflects the collagen tail. Equivalent asymmetric A12 and A8 collagen-tailed forms of BChE have been found in muscle tissue of the rat (Vigny et al, 1978;Berman et al, 1987) and mouse (Feng et al, 1999). Our sucrose density gradients detected small amounts of A12 BChE.…”

Transcriptional Regulation of Acetylcholinesterase-associated Collagen ColQ

“…Physiologically, the hydrolysis of acetylcholine at nmjs depends on AChE collagen-tailed forms (5). Several lines of evidence indicate that the unique expression pattern of these molecular forms in muscle depends primarily on the regulation of ColQ expression.…”

“…AChE T catalytic subunits produce amphiphilic monomers and dimers and nonamphiphilic homotetramers as well as heteromeric associations with anchoring proteins, ColQ and PRiMA, which allow their functional localization in cholinergic synapses (2). Collagen ColQ thus characterizes the collagen-tailed forms (A forms) of AChE and butyrylcholinesterase (3)(4)(5), which are localized in the basal lamina at neuromuscular junctions (nmjs) of vertebrates (6,7); in these molecules (A 4 , A 8 , A 12 ), one, two, or three tetramers of catalytic subunits are disulfide-linked to the strands of a triple helix of ColQ collagen. The cDNAs encoding ColQ have been cloned in Torpedo and mammals (4,8,9).…”

Two Different Heparin-binding Domains in the Triple-helical Domain of ColQ, the Collagen Tail Subunit of Synaptic Acetylcholinesterase

“…In the case of asymmetric AChE, the collagen ColQ is responsible for the localization of the enzyme at the vertebrate neuromuscular junction. Inactivation of the ColQ gene in mice or mutations in the human ColQ gene result in the absence of enzyme accumulation at the neuromuscular junction and are the cause of a congenital myasthenic syndrome (type 1c) (1,2).…”

“…At vertebrate cholinergic synapses, acetylcholinesterase (AChE) 1 rapidly hydrolyzes the neurotransmitter acetylcholine, thereby terminating synaptic transmission. This key function does not only require a high catalytic turnover number but also a strategic positioning of the enzyme.…”