Yilmaz Atilgan, Kattamuri Chandramohan, Ozdeslik Rana N, Schmiedel Carolyn, Mentzer Sarah, Schorl Christoph, Oancea Elena, Thompson Thomas B, Fallon Justin R
Department of Neuroscience, Brown University, Providence, RI 02912, USA. Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, RI 02912, USA.
Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati, Medical Sciences Building, Cincinnati, OH 45267, USA.
Sci Signal. 2016 Sep 6;9(444):ra87. doi: 10.1126/scisignal.aaf0890.
Bone morphogenetic proteins (BMPs) function in most tissues but have cell type-specific effects. Given the relatively small number of BMP receptors, this exquisite signaling specificity requires additional molecules to regulate this pathway's output. The receptor tyrosine kinase MuSK (muscle-specific kinase) is critical for neuromuscular junction formation and maintenance. Here, we show that MuSK also promotes BMP signaling in muscle cells. MuSK bound to BMP4 and related BMPs with low nanomolar affinity in vitro and to the type I BMP receptors ALK3 and ALK6 in a ligand-independent manner both in vitro and in cultured myotubes. High-affinity binding to BMPs required the third, alternatively spliced MuSK immunoglobulin-like domain. In myoblasts, endogenous MuSK promoted BMP4-dependent phosphorylation of SMADs and transcription of Id1, which encodes a transcription factor involved in muscle differentiation. Gene expression profiling showed that MuSK was required for the BMP4-induced expression of a subset of genes in myoblasts, including regulator of G protein signaling 4 (Rgs4). In myotubes, MuSK enhanced the BMP4-induced expression of a distinct set of genes, including transcripts characteristic of slow muscle. MuSK-mediated stimulation of BMP signaling required type I BMP receptor activity but was independent of MuSK tyrosine kinase activity. MuSK-dependent expression of Rgs4 resulted in the inhibition of Ca(2+) signaling induced by the muscarinic acetylcholine receptor in myoblasts. These findings establish that MuSK has dual roles in muscle cells, acting both as a tyrosine kinase-dependent synaptic organizing molecule and as a BMP co-receptor that shapes BMP transcriptional output and cholinergic signaling.
骨形态发生蛋白（BMPs）在大多数组织中发挥作用，但具有细胞类型特异性效应。鉴于BMP受体的数量相对较少，这种精确的信号特异性需要额外的分子来调节该信号通路的输出。受体酪氨酸激酶MuSK（肌肉特异性激酶）对神经肌肉接头的形成和维持至关重要。在此，我们表明MuSK在肌肉细胞中也促进BMP信号传导。MuSK在体外以低纳摩尔亲和力与BMP4及相关BMPs结合，并且在体外和培养的肌管中以配体非依赖性方式与I型BMP受体ALK3和ALK6结合。与BMPs的高亲和力结合需要第三个可变剪接的MuSK免疫球蛋白样结构域。在成肌细胞中，内源性MuSK促进BMP4依赖的SMAD磷酸化和Id1的转录，Id1编码一种参与肌肉分化的转录因子。基因表达谱分析表明，MuSK是成肌细胞中BMP4诱导的一组基因表达所必需的，包括G蛋白信号调节因子4（Rgs4）。在肌管中，MuSK增强了BMP4诱导的一组不同基因的表达，包括慢肌特征性转录本。MuSK介导的BMP信号刺激需要I型BMP受体活性，但独立于MuSK酪氨酸激酶活性。MuSK依赖的Rgs4表达导致成肌细胞中由毒蕈碱型乙酰胆碱受体诱导的Ca(2+)信号传导受到抑制。这些发现表明MuSK在肌肉细胞中具有双重作用，既作为酪氨酸激酶依赖性的突触组织分子，又作为塑造BMP转录输出和胆碱能信号传导的BMP共受体。
