Cholinesterases in Tripartite Neuromuscular Synapse

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.

USH2A is a skin end-organ protein necessary for vibration sensing in mice and humans

Fingertip mechanoreceptors comprise sensory neuron endings together with specialized skin cells that form the end-organ. Exquisitely sensitive vibration-sensing neurons are associated with Meissner’s corpuscles and Pacinian corpuscles1. Such end-organ structures have been recognized for more than 160 years, but their exact functions have remained a matter of speculation. Here we examined the role of USH2A in touch sensation in humans and mice. The USH2A gene encodes a transmembrane protein with a very large extracellular domain. Pathogenic USH2A mutations cause Usher syndrome associated with hearing loss and visual impairment2. We show that patients with biallelic pathogenic USH2A mutations also have profound impairments in vibrotactile touch perception. Similarly, mice lacking the USH2A protein showed severe deficits in a forepaw vibrotactile discrimination task. Forepaw rapidly-adapting mechanoreceptors (RAMs) recorded from Ush2a−/− mice innervating Meissner’s corpuscles showed profound reductions in their vibration sensitivity. However, the USH2A protein was not expressed in sensory neurons, but was found in specialized terminal Schwann cells in Meissner’s corpuscles. Loss of this large extracellular tether-like protein in corpuscular end-organs innervated by RAMs was sufficient to reduce the vibration sensitivity of mechanoreceptors. Thus, USH2A expressed in corpuscular end-organs associated with vibration sensing is required to properly perceive vibration. We propose that cells within the corpuscle present a tether-like protein that may link to mechanosensitive channels in sensory endings to facilitate small amplitude vibration detection essential for the perception of fine textured surfaces.

Premature satellite cell activation before injury accelerates myogenesis and disrupts neuromuscular junction maturation in regenerating muscle

Satellite cell (SC) activation, mediated by nitric oxide (NO), is essential to myogenic repair, whereas myotube function requires innervation. Semaphorin (Sema) 3A, a neuro-chemorepellent, is thought to regulate axon guidance to neuromuscular junctions (NMJs) during myotube differentiation. We tested whether “premature” SC activation (SC activation before injury) by a NO donor (isosorbide dinitrate) would disrupt early myogenesis and/or NMJs. Adult muscle was examined during regeneration in two models of injury: myotoxic cardiotoxin (CTX) and traumatic crush (CR) ( n = 4–5/group). Premature SC activation was confirmed by increased DNA synthesis by SCs immediately in pretreated mice after CTX injury. Myotubes grew faster after CTX than after CR; growth was accelerated by pretreatment. NMJ maturation, classified by silver histochemistry (neurites) and acetylcholinesterase (AchE), and α-bungarotoxin staining (Ach receptors, AchRs) were delayed by pretreatment, consistent with a day 6 rise in the denervation marker γ-AchR. With pretreatment, S100B from terminal Schwann cells (TSCs) increased 10- to 20-fold at days 0 and 10 after CTX and doubled 6 days after CR. Premature SC activation disrupted motoneuritogenesis 8–10 days post-CTX, as pretreatment reduced colocalization of pre- and postsynaptic NMJ features and increased Sema3A-65. Premature SC activation before injury both accelerated myogenic repair and disrupted NMJ remodeling and maturation, possibly by reducing Sema3A neuro-repulsion and altering S100B. This interpretation extends the model of Sema3A-mediated motoneuritogenesis during muscle regeneration. Manipulating the timing and type of Sema3A by brief NO effects on SCs suggests an important role for TSCs and Sema3A-65 processing in axon guidance and NMJ restoration during muscle repair.

Terminal Schwann cell and vacant site mediated synapse elimination at developing neuromuscular junctions

Synapses undergo transition from polyinnervation by multiple axons to single innervation a few weeks after birth. Synaptic activity of axons and interaxonal competition are thought to drive this developmental synapse elimination and tested as key parameters in quantitative models for further understanding. Recent studies of muscle synapses (endplates) show that there are also terminal Schwann cells (tSCs), glial cells associated with motor neurons and their functions, and vacant sites (or vacancies) devoid of tSCs and axons proposing tSCs as key effectors of synapse elimination. However, there is no quantitative model that considers roles of tSCs including vacancies. Here we develop a stochastic model of tSC and vacancy mediated synapse elimination. It employs their areas on individual endplates quantified by electron microscopy-based analyses assuming that vacancies form randomly and are taken over by adjacent axons or tSCs. The model reliably reproduced synapse elimination whereas equal or random probability models, similar to classical interaxonal competition models, did not. Furthermore, the model showed that synapse elimination is accelerated by enhanced synaptic activity of one axon and also by increased areas of vacancies and tSCs suggesting that the areas are important structural correlates of the rate of synapse elimination.