Neurotransmitter Acetylcholine Negatively Regulates Neuromuscular Synapse Formation by a Cdk5-Dependent Mechanism

The Salk Institute, La Jolla, California 92037, USA.
Neuron (Impact Factor: 15.05). 06/2005; 46(4):569-79. DOI: 10.1016/j.neuron.2005.04.002
Source: PubMed


Synapse formation requires interactions between pre- and postsynaptic cells to establish the connection of a presynaptic nerve terminal with the neurotransmitter receptor-rich postsynaptic apparatus. At developing vertebrate neuromuscular junctions, acetylcholine receptor (AChR) clusters of nascent postsynaptic apparatus are not apposed by presynaptic nerve terminals. Two opposing activities subsequently promote the formation of synapses: positive signals stabilize the innervated AChR clusters, whereas negative signals disperse those that are not innervated. Although the nerve-derived protein agrin has been suggested to be a positive signal, the negative signals remain elusive. Here, we show that cyclin-dependent kinase 5 (Cdk5) is activated by ACh agonists and is required for the ACh agonist-induced dispersion of the AChR clusters that have not been stabilized by agrin. Genetic elimination of Cdk5 or blocking ACh production prevents the dispersion of AChR clusters in agrin mutants. Therefore, we propose that ACh negatively regulates neuromuscular synapse formation through a Cdk5-dependent mechanism.

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    • "In this regard, neural-and muscle-derived molecules have been described to play such inhibitory roles at the vertebrate NMJ. One the one hand, the neurotransmitter ACh displays an AChRdisaggregating activity at the most abundant non-innervated ( " extrasynaptic " ) domains of the muscle membrane at nascent NMJs (Lin et al., 2005; Misgeld et al., 2005). ACh acts through the cyclin-dependent kinase 5 (Cdk5) which phosphorylates the intermediate filament protein nestin, that becomes dissociated from the cytoskeletal network and is subsequently degraded (Fu et al., 2005; Yang et al., 2011). "
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    ABSTRACT: Cumulative evidence indicates that Wnt pathways play crucial and diverse roles to assemble the neuromuscular junction (NMJ), a peripheral synapse characterized by the clustering of acetylcholine receptors (AChR) on postsynaptic densities. The molecular determinants of Wnt effects at the NMJ are still to be fully elucidated. We report here that the Wnt receptor Frizzled-9 (Fzd9) is expressed in developing skeletal muscles during NMJ synaptogenesis. In cultured myotubes, gain-and loss-of-function experiments revealed that Fzd9-mediated signaling impairs the AChR-clustering activity of agrin, an organizer of postsynaptic differentiation. Overexpression of Fzd9 induced the cytosolic accumulation of β-catenin, a key regulator of Wnt signaling. Consistently, Fzd9 and β-catenin localize in the postsynaptic domain of embryonic NMJs in vivo. Our findings represent the first evidence pointing to a crucial role of a Fzd-mediated, β-catenin-dependent signaling on the assembly of the vertebrate NMJ.
    Frontiers in Cellular Neuroscience 04/2014; 8(1). DOI:10.3389/fncel.2014.00110 · 4.29 Impact Factor
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    • "There is a careful balance between signals promoting and inhibiting postsynaptic stabilisation. A compromised presynaptic terminal may no longer provide enough positive signals, such as AGRN (McMahan, 1990; Misgeld et al. 2005), that stabilise the clustering of AChR at the postsynaptic side, to counteract a prolonged exposure to ACh, which is also a signal for AChR cluster dissipation (Lin et al. 2005; Misgeld et al. 2005). "
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    ABSTRACT: MuSK myasthenia gravis is a rare, severe autoimmune disease of the neuromuscular junction, only identified in 2001, with unclear pathogenic mechanisms. In this review we describe the clinical aspects that distinguish MuSK MG from AChR MG, review what is known about the role of MuSK in the development and function of the neuromuscular junction, and discuss the data that address how the antibodies to MuSK lead to neuromuscular transmission failure.
    Journal of Anatomy 03/2013; 224(1). DOI:10.1111/joa.12034 · 2.10 Impact Factor
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    • "Muscle depolarization induced by released ACh is a negative signal for AChRs. It inhibits AChR localization, stability [62], and transcription along the muscle fiber by stimulation of cyclin-dependent kinase 5 [63, 64], protein kinase C (PKC), and Ca2+/calmodulin-dependent kinase II (CaMKII) [65]. Indeed, mutant mice lacking acetyltransferase, a biosynthetic enzyme for ACh, develop faster and larger AChR clusters; they also exhibit hyperinnervation due to a broader distribution of AChR clusters along the muscle fiber [66, 67]. "
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    ABSTRACT: Skeletal muscle innervation is a multi-step process leading to the neuromuscular junction (NMJ) apparatus formation. The transmission of the signal from nerve to muscle occurs at the NMJ level. The molecular mechanism that orchestrates the organization and functioning of synapses is highly complex, and it has not been completely elucidated so far. Neuromuscular junctions are assembled on the muscle fibers at very precise locations called end plates (EP). Acetylcholine receptor (AChR) clusterization at the end plates is required for an accurate synaptic transmission. This review will focus on some mechanisms responsible for accomplishing the correct distribution of AChRs at the synapses. Recent evidences support the concept that a dual transcriptional control of AChR genes in subsynaptic and extrasynaptic nuclei is crucial for AChR clusterization. Moreover, new players have been discovered in the agrin-MuSK pathway, the master organizer of postsynaptical differentiation. Mutations in this pathway cause neuromuscular congenital disorders. Alterations of the postynaptic apparatus are also present in physiological conditions characterized by skeletal muscle wasting. Indeed, recent evidences demonstrate how NMJ misfunctioning has a crucial role at the onset of age-associated sarcopenia.
    03/2012; 3(1):13-23. DOI:10.1007/s13539-011-0041-7
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