Role of ERK1/2 in Neuromuscular Synapses and Myofiber Development in vivo

描述（申请人提供）：细胞外信号调节激酶1和2（ERK 1/2）是典型的细胞内有丝分裂原活化蛋白激酶（MAPK）。ERK 1/2在许多细胞类型中被多种生长因子和细胞因子激活。体外研究表明ERK 1/2参与细胞增殖、分化和凋亡过程中的许多细胞过程。ERK活性在培养骨骼肌肌管的分化中具有刺激和抑制作用，其随着这一漫长过程的阶段而变化。然而，迄今为止，还没有关于ERK 1/2在体内发育肌纤维中的作用的基因靶向研究的报道。 由运动神经元释放的聚集蛋白诱导和/或维持脊椎动物神经肌肉接头（NMJ）处的乙酰胆碱受体（AChR）聚集和突触后分化的其他方面，NMJ是脊髓中的运动神经元和骨骼肌纤维之间的突触。聚集蛋白通过结合和激活含有LDL受体相关蛋白4（Lrp4）和肌肉特异性激酶（MuSK）的受体复合物而起作用。我们最近报道，在培养的肌管聚集蛋白诱导ERK 1/2激活的Lrp4/MuSK依赖的方式，这负调控聚集蛋白诱导AChR集群的能力。此外，我们的初步数据表明，ERK2在肌纤维中的损失导致在年轻成年小鼠的NMJ维持缺陷。在这些动物中，NMJ显示出类似于在老年小鼠或mdx小鼠的NMJ中所见的片段化，mdx小鼠是杜氏肌营养不良症的模型。 R21应用的目的是研究ERK 1/2在体内对神经肌肉突触形成和维持以及骨骼肌纤维分化和成熟的意义。一种有条件的基因靶向策略，最近在周围神经的其他细胞中取得了成果。 系统将用于选择性地在骨骼肌纤维中表达Erk1/2。 上述实验的结果将阐明ERK 1/2在NMJ的聚集蛋白信号传导中的体内作用，特别是在骨骼肌纤维的发育中的作用。此外，它们可能为正常衰老、杜氏肌营养不良和肌肉中表达的ERK 1/2下游底物突变引起的疾病（如与Coffin-Lowry综合征相关的核糖体S6激酶2（Rsk 2））的分子机制提供新的见解。

DESCRIPTION (provided by applicant): Extracellular-signal regulated kinases 1 and 2 (ERK1/2) are the prototypical intracellular mitogen activated protein kinases (MAPK). ERK1/2 are activated by multiple growth factors and cytokines in many cell types. Studies in vitro implicate ERK1/2 in a myriad of cellular processes during proliferation, differentiation and apoptosis. ERK activity has both stimulatory and inhibitory roles in the differentiation of culture skeletal myotubes that vary with the stage of this protracted process. However, no gene targeting investigations on the role of ERK1/2 in developing muscle fibers in vivo have been reported to date. Agrin released by motoneurons induces and/or maintains acetylcholine receptor (AChR) clustering and other aspects of postsynaptic differentiation at the vertebrate neuromuscular junction (NMJ), the synapse between a motoneuron in the spinal cord and a skeletal muscle fiber. Agrin acts by binding and activating a receptor complex containing LDL receptor related protein 4 (Lrp4) and muscle specific kinase (MuSK). We reported recently that in cultured myotubes agrin induces ERK1/2 activation in an Lrp4/MuSK-dependent fashion and that this negatively regulates the ability of agrin to induce AChR clusters. Furthermore, our preliminary data show that loss of ERK2 in myofibers leads to defects in NMJ maintenance in young adult mice. In these animals, NMJs display fragmentation similar to than seen in NMJs of old mice or in mdx mice, which model Duchenne muscular dystrophy. The aim of this R21 application is to investigate the significance in vivo of ERK1/2 for neuromuscular synapse formation and maintenance and for skeletal myofiber differentiation and maturation. A strategy of conditional gene targeting that has recently bear fruit in other cells of the peripheral nervous system will be used to selectively inactivate Erk1/2 in skeletal muscle fibers. Results from the above experiments will clarify the in vivo role of ERK1/2 in agrin signaling at the NMJ, in particular, and in the development of skeletal muscle fibers, in general. In addition, they may provide new insights into the molecular mechanisms underlying normal aging, Duchenne muscular dystrophy and diseases caused by mutation of ERK1/2 downstream substrates expressed in muscle, such as ribosomal S6 kinase 2 (Rsk2), which is linked to Coffin-Lowry syndrome.