Distinct Muscarinic Acetylcholine Receptor Subtypes Contribute to Stability and Growth, But Not Compensatory Plasticity, of Neuromuscular Synapses

Abstract

Muscarinic acetylcholine receptors (mAChRs) modulate synaptic function, but whether they influence synaptic structure remains unknown. At neuromuscular junctions (NMJs), mAChRs have been implicated in compensatory sprouting of axon terminals in paralyzed or denervated muscles. Here we used pharmacological and genetic inhibition and localization studies of mAChR subtypes at mouse NMJs to demonstrate their roles in synaptic stability and growth but not in compensatory sprouting. M 2 mAChRs were present solely in motor neurons, whereas M 1 , M 3 , and M 5 mAChRs were associated with Schwann cells and/or muscle fibers. Blockade of all five mAChR subtypes with atropine evoked pronounced effects, including terminal sprouting, terminal withdrawal, and muscle fiber atrophy. In contrast, methoctramine, an M 2/4 -preferring antagonist, induced terminal sprouting and terminal withdrawal, but no muscle fiber atrophy. Consistent with this observation, M 2Ϫ/Ϫ but no other mAChR mutant mice exhibited spontaneous sprouting accompanied by extensive loss of parental terminal arbors. Terminal sprouting, however, seemed not to be the causative defect because partial loss of terminal branches was common even in the M 2 Ϫ/Ϫ NMJs without sprouting. Moreover, compensatory sprouting after paralysis or partial denervation was normal in mice deficient in M 2 or other mAChR subtypes. We also found that many NMJs of M 5 Ϫ/Ϫ mice were exceptionally small and reduced in proportion to the size of parental muscle fibers. These findings show that axon terminals are unstable without M 2 and that muscle fiber growth is defective without M 5 . Subtype-specific muscarinic signaling provides a novel means for coordinating activity-dependent development and maintenance of the tripartite synapse.

Synergistic Action of Presynaptic Muscarinic Acetylcholine Receptors and Adenosine Receptors in Developmental Axonal Competition at the Neuromuscular Junction

“…Although it is not clear whether all mAChR subtypes are present in the NMJ [30,31] , some of these receptors play an important regulatory role in ACh release during development [14,16,18,32] and in the adult [16,33,34] . In the adult NMJ, M 1 and M 2 mAChRs modulate evoked transmitter release by positive and negative feedback, respectively [14,31,34] .…”

“…In the adult NMJ, M 1 and M 2 mAChRs modulate evoked transmitter release by positive and negative feedback, respectively [14,31,34] . However, during NMJ synaptogenesis, the functional significance of the subtypes is different.…”

Acetylcholine negatively regulates development of the neuromuscular junction through distinct cellular mechanisms

“…ACh may also mediate these effects by activating nonpostsynaptic muscarinic AChRs in embryonic MNs or Schwann cells, because distinct muscarinic AChR subtypes are expressed by adult MNs, Schwann cells, and muscle (13, 21, 22, 52) and play a role in the stability of adult NMJs (52). A more detailed analysis of both nicotinic and muscarinic AChRs in embryonic NMJ cell types will help clarify the molecular mechanism by which ACh regulates presynaptic differentiation in nAChRα1-deficient mice.…”

Perisynaptic Schwann Cells at the Neuromuscular Synapse: Adaptable, Multitasking Glial Cells

“…3A) (Robitaille 1995;Robitaille et al 1997;Castonguay and Robitaille 2001;Rochon et al 2001). At adult NMJs, detection of synaptic transmission by PSCs is mediated by muscarinic receptors (M1, M3, or M5) (Wright et al 2009) and by purinergic receptors, in particular adenosine A1 receptors (Rochon et al 2001). Although the characterization of the muscarinic receptor system follows a clear nomenclature, the properties of the purinergic receptor systems still elude a clear classification (Robitaille et al 1997;Rochon et al 2001;Rousse et al 2010).…”