Studies on the Molecular Mechanism of Neuromuscular Junction Formation

我们以前已经表明，γ-亚基的ε-亚基的替代是负责肌肉发育过程中乙酰胆碱受体的功能改变。在试图建立一个在体外系统中的乙酰胆碱受体亚基的开关发生，肌细胞系（L 6和C2 C12）和原代培养的成肌细胞进行了测试的乙酰胆碱受体亚基在分化过程中的表达。在成肌细胞向肌管发育过程中，α、β、γ和δ亚基的mRNA显著增加，而ε亚基的mRNA仅在原代培养细胞发育的后期才能检测到，而在细胞系中未检测到。β-和δ-亚基特异性mRNA（称为α-AChR）的不寻常之处在于，其通道在不存在激动剂的情况下以高频率自发打开。因此，γ-和ε-亚基对于稳定未配体受体通道的闭合状态是重要的。此外，乙酰胆碱激活的alphabetadelta-AChR的平均通道开放时间比包含γ-或ε-亚基的完整受体的平均通道开放时间长得多。它组装的容易性提高了一种可能性，即α-乙酰胆碱受体有助于在发育过程中发生的受体特性的一些变化。

We have previously shown that the replacement of the gamma-subunit by the epsilon-subunit is responsible for the functional alteration of the acetylcholine receptor during muscle development. In an attempt to establish an in vitro system in which the switch of the acetylcholine receptor subunits takes place, myogenic cells lines (L6 and C2C12) and primary culture of myoblasts were tested for expression of the acetylcholine receptor subunits during differentiation. The alpha-, beta-, gamma-and delta-subunit mRNAs increased prominently during development from myoblasts to myotubes, whereas the epsilon-subunit was detected only in late stage of the development of the primary cultured cells, but not in the cell lines.The acetylcholine receptor expressed in Xenopus oocytes injected with the bovine alpha-, beta- and delta-subunit-specific mRNAs (referred to as alphabetadelta-AChR) is unusual in that its channel opens spontaneously at a high frequency in the absence of agonist. Thus, the gamma-and epsilon-subunits are important for stabilizing the closed state of the unliganded receptor channel. Furthermore, the mean channel open time of acetylcholine-activated alphabetadelta-AChR is considerably longer than that of the complete receptors containing either the gamma- or epsilon-subunit. The ease with which it is assembled raises the possibility that the alphabetadelta-AChR contributes to some of the variation in receptor properties that occur during development.