Article

Β-Catenin stabilization in skeletal muscles, but not in motor neurons, leads to aberrant motor innervation of the muscle during neuromuscular development in mice

Department of Neuroscience, UT Southwestern Medical Center, Dallas, TX 75390, USA.
Developmental Biology (Impact Factor: 3.55). 04/2012; 366(2):255-67. DOI: 10.1016/j.ydbio.2012.04.003
Source: PubMed

ABSTRACT

β-Catenin, a key component of the Wnt signaling pathway, has been implicated in the development of the neuromuscular junction (NMJ) in mice, but its precise role in this process remains unclear. Here we use a β-catenin gain-of-function mouse model to stabilize β-catenin selectively in either skeletal muscles or motor neurons. We found that β-catenin stabilization in skeletal muscles resulted in increased motor axon number and excessive intramuscular nerve defasciculation and branching. In contrast, β-catenin stabilization in motor neurons had no adverse effect on motor innervation pattern. Furthermore, stabilization of β-catenin, either in skeletal muscles or in motor neurons, had no adverse effect on the formation and function of the NMJ. Our findings demonstrate that β-catenin levels in developing muscles in mice are crucial for proper muscle innervation, rather than specifically affecting synapse formation at the NMJ, and that the regulation of muscle innervation by β-catenin is mediated by a non-cell autonomous mechanism.

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    • "Mice subjected to conditional depletion of β-catenin in the muscle precursor Pax7+ cell lineage (Ctnnb1F/F;Pax7-Cre mice) show reduced muscle mass and fewer slow myofibers (Hutcheson et al., 2009). Mice expressing a constitutively active, stabilized β-catenin in the Pax7+ lineage (CtnnbΔex3;Pax7-Cre mice) exhibit reduced myogenesis but more slow myofibers (Hutcheson et al., 2009;Liu et al., 2012). The dysregulation of Wnt/β-catenin signaling can thus lead to severe developmental defects and perturb muscle homeostasis. "
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    Preview · Article · Oct 2015 · Biology Open
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    • "In turn, specific stabilization of β-catenin in muscle, but not in neuronal cells, resulted in excessive nerve branching and defasciculation , while NMJ formation and function were unaffected (Liu et al., 2012). Interestingly, β-catenin deficient mice contain bigger AChR clusters than control animals (Li et al., 2008; Wang et al., 2008), further supporting the view that β-catenin signaling in muscle cells plays a negative role on NMJ formation and growth. "
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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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    • "The muscle derived BMP-4 subsequently promotes development of presynaptic motor neuron terminals. Indeed, β-catenin stabilization in skeletal muscles (not limited to the neuromuscular junction area) results in increased motor axon number and excessive intramuscular nerve defasciculation and branching [41]. Taken together, our experiments have identified a novel interaction between canonical Wnt and BMP signaling that plays a role in myofiber type specification. "
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    ABSTRACT: Background The Wnts are secreted proteins that play important roles in skeletal myogenesis, muscle fiber type diversification, neuromuscular junction formation and muscle stem cell function. How Wnt proteins orchestrate such diverse activities remains poorly understood. Canonical Wnt signaling stabilizes β-catenin, which subsequently translocate to the nucleus to activate the transcription of TCF/LEF family genes. Methods We employed TCF-reporter mice and performed analysis of embryos and of muscle groups. We further isolated fetal myoblasts and performed cell and molecular analyses. Results We found that canonical Wnt signaling is strongly activated during fetal myogenesis and weakly activated in adult muscles limited to the slow myofibers. Muscle-specific transgenic expression of a stabilized β-catenin protein led to increased oxidative myofibers and reduced muscle mass, suggesting that canonical Wnt signaling promotes slow fiber types and inhibits myogenesis. By TCF-luciferase reporter assay, we identified Wnt-1 and Wnt-3a as potent activators of canonical Wnt signaling in myogenic progenitors. Consistent with in vivo data, constitutive overexpression of Wnt-1 or Wnt-3a inhibited the proliferation of both C2C12 and primary myoblasts. Surprisingly, Wnt-1 and Wnt-3a overexpression up-regulated BMP-4, and inhibition of BMP-4 by shRNA or recombinant Noggin protein rescued the myogenic inhibitory effect of Wnt-1 and Wnt-3a. Importantly, Wnt-3a or BMP-4 recombinant proteins promoted slow myosin heavy chain expression during myogenic differentiation of fetal myoblasts. Conclusions These results demonstrate a novel interaction between canonical Wnt and BMP signaling that induces myogenic differentiation towards slow muscle phenotype.
    Full-text · Article · Mar 2013 · Skeletal Muscle
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