Article

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

ABSTRACT

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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Available from: Eugene P Brandon, Jan 06, 2016
    • "NMJ development and maintenance is actively balanced by signals on both the presynaptic (neuronal) and postsynaptic (muscle) side of the junction that act to stabilize or disperse the postsynaptic signaling apparatus in response to innervation. Cdk5 acts at several levels of NMJ development to regulate both positive (Cdk5 promotes the neuregulin-induced acetylcholine receptor[AChR]transcription) and negative signals (acetylcholine[ACh]-mediated Cdk5 activity disperses AChR clusters that are not stabilized by agrin) that act on AChR clustering (Fu et al., 2001Fu et al., , 2005Lin et al., 2005). As a response to ACh agonists, which initiate a signaling cascade that destabilizes the NMJs, C2C12 myotubes upregulate both membrane-associated and total Cdk5 activity, induce p35 membrane recruitment and ultimately trigger nestin Thr316 phosphorylation 518 JuliaLindqvist et al.Author's personal copy (Yang et al., 2011). "
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    ABSTRACT: Current research utilizes the specific expression pattern of intermediate filaments (IF) for identifying cellular state and origin, as well as for the purpose of disease diagnosis. Nestin is commonly utilized as a specific marker and driver for CNS progenitor cell types, but in addition, nestin can be found in several mesenchymal progenitor cells, and it is constitutively expressed in a few restricted locations, such as muscle neuromuscular junctions and kidney podocytes. Alike most other members of the IF protein family, nestin filaments are dynamic, constantly being remodeled through posttranslational modifications, which alter the solubility, protein levels, and signaling capacity of the nestin filaments. Through its interactions with kinases and other signaling executors, resulting in a complex and bidirectional regulation of cell signaling events, nestin has the potential to determine whether cells divide, differentiate, migrate, or stay in place. In this review, the broad and similar roles of IFs as dynamic signaling scaffolds, is exemplified by observations of nestin functions and its interaction with the cyclin- dependent kinase 5, the atypical kinase in the family of cyclin-dependent kinases.
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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.
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    • "Signals released by axons also regulate circuit assembly. For example, agrin, secreted by motor axons, regulates development of the postsynaptic membrane at the vertebrate neuromuscular junction (Burgess et al., 1999; Gautam et al., 1996; Kummer et al., 2006; Lin et al., 2005; Misgeld et al., 2005; Nitkin et al., 1987), and brain-derived neurotrophic factor (BDNF), produced by sympathetic axons, induces pruning of less active neighboring sympathetic axons (Singh et al., 2008). In addition, within the Drosophila visual system, anterograde signals regulating cell proliferation and differentiation control the numerical matching of photoreceptors with their synaptic targets (see below). "
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