Cholinesterases in Tripartite Neuromuscular Synapse

Abstract

The neuromuscular junction (NMJ) is a tripartite synapse in which not only presynaptic and post-synaptic cells participate in synaptic transmission, but also terminal Schwann cells (TSC). Acetylcholine (ACh) is the neurotransmitter that mediates the signal between the motor neuron and the muscle but also between the motor neuron and TSC. ACh action is terminated by acetylcholinesterase (AChE), anchored by collagen Q (ColQ) in the basal lamina of NMJs. AChE is also anchored by a proline-rich membrane anchor (PRiMA) to the surface of the nerve terminal. Butyrylcholinesterase (BChE), a second cholinesterase, is abundant on TSC and anchored by PRiMA to its plasma membrane. Genetic studies in mice have revealed different regulations of synaptic transmission that depend on ACh spillover. One of the strongest is a depression of ACh release that depends on the activation of α7 nicotinic acetylcholine receptors (nAChR). Partial AChE deficiency has been described in many pathologies or during treatment with cholinesterase inhibitors. In addition to changing the activation of muscle nAChR, AChE deficiency results in an ACh spillover that changes TSC signaling. In this mini-review, we will first briefly outline the organization of the NMJ. This will be followed by a look at the role of TSC in synaptic transmission. Finally, we will review the pathological conditions where there is evidence of decreased AChE activity.

Keywords: acetylcholinesterase; butyrylcholinesterase; congenital myasthenic syndromes; neuromuscular junction; terminal Schwann cells.

FIGURE 1

Distribution of acetylcholinesterase (AChE) and Butyrylcholinesterase (BChE) at mouse neuromuscular junction (NMJ): (A) Schematic representation of nerve terminal, muscle fiber, and terminal Schwann cell (TSC). Acetylcholine (ACh) is released from the nerve terminal. AChE is organized in complexes (collagen-tailed forms A12) see Figure 2. AChE/collagen Q (ColQ) are clustered in the basal lamina. AChE is also tethered by proline-rich membrane anchor (PRiMA) on the membrane of the nerve terminal. ColQ/AChE controls the activation of muscle-type nicotinic acetylcholine receptors [nAChR; (α1)2β1γε]. BChE is anchored at the TSC by PRiMA. (B) Calcium waves are triggered in TSC by muscarinic agonist or ATP. TSCs regulate ACh release by using ATP as a gliotransmitter. ATP released by the nerve terminal triggers calcium waves in TSCs via activations of P2Y receptors. These calcium waves cause the release of ATP from TSCs. ATP, degraded to adenosine, activates A1 and A2a receptors. If the A1 receptor response dominates (3 × 600 stimuli protocol), it results in a depression of ACh release, if the A2a response dominates (1,800 stimuli protocol) it results in the facilitation of ACh release. (C) ACh controls its own release by different mechanisms that are revealed using cholinergic agonists and cholinesterase inhibitors. The addition of muscarine (muscarinic AChRs agonist) results in a decrease in the probability of release by activation of m2 muscarinic acetylcholine receptors (mAChR). On the contrary, the inhibition of AChE results in an increase in the probability of ACh release by activation of m1/m3 receptors. Inhibition of BChE results in a very limited decrease of the ACh release via activation of α7 nAChRs. When AChE and BChE are inhibited, in 50 Hz 10 s protocol, ACh activates α7 nAChRs, triggers calcium wave in TSCs that release adenosine (Ad) as a gliotransmitter. ACh release is greatly diminished.

FIGURE 2

ColQ functions at NMJ. (A) ColQ is required to cluster AChE in the basal lamina and interacts with multiple partners. ColQ is a multidomain protein. A signal peptide translocates the single-stranded collagen subunit into the endoplasmic reticulum where the collagen trimer is assembled from the C-terminal domain. AChE tetramers are organized by a proline-rich sequence (PRAD) of each of the ColQ subunits. The mature complex thus assembles 12 catalytic AChE subunits. Four heparan sulfate chains [perlecan (HSPG2) and other proteoglycans] interact with two clusters of basic residues (heparin-binding site HB1 and HB2) in the trimer of collagen. The C terminal of ColQ interacts with MuSK and thus participates in NMJ signaling. (B) Mutations in the ColQ gene in humans are responsible for a congenital myasthenic syndrome with AChE deficiency. The mutations in the domains lead to different properties, which are illustrated in the diagram. For example, a point mutation (176C-A) changes P59 to Q (Ohno et al., 2000), kept the full-length ColQ intact but ColQ cannot interact with AChE, whereas 107del215 create a deletion of exon 2 encoding PRAD and the absence of ColQ (Ohno et al., 1998). Similar mechanisms occur in the heparin-binding domain where the mutation G237D (Mihaylova et al., 2008) should affect the interaction with heparan sulfate but keep intact ColQ, whereas R236X produces a shorter single-stranded collagen. Similarly, ColQ with mutations in the C-terminal domain organizes AChE into tetramer but AChE is not clustered in the basal lamina.