Department of Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Columbia University Medical Center, New York, NY 10032, USA.
Genetics. 2013 Aug;194(4):885-901. doi: 10.1534/genetics.113.152660. Epub 2013 May 20.
Cyclic guanosine monophosphate (cGMP) is a key secondary messenger used in signal transduction in various types of sensory neurons. The importance of cGMP in the ASE gustatory receptor neurons of the nematode Caenorhabditis elegans was deduced by the observation that multiple receptor-type guanylyl cyclases (rGCs), encoded by the gcy genes, and two presently known cyclic nucleotide-gated ion channel subunits, encoded by the tax-2 and tax-4 genes, are essential for ASE-mediated gustatory behavior. We describe here specific mechanistic features of cGMP-mediated signal transduction in the ASE neurons. First, we assess the specificity of the sensory functions of individual rGC proteins. We have previously shown that multiple rGC proteins are expressed in a left/right asymmetric manner in the functionally lateralized ASE neurons and are required to sense distinct salt cues. Through domain swap experiments among three different rGC proteins, we show here that the specificity of individual rGC proteins lies in their extracellular domains and not in their intracellular, signal-transducing domains. Furthermore, we find that rGC proteins are also sufficient to confer salt sensory responses to other neurons. Both findings support the hypothesis that rGC proteins are salt receptor proteins. Second, we identify a novel, likely downstream effector of the rGC proteins in gustatory signal transduction, a previously uncharacterized cyclic nucleotide-gated (CNG) ion channel, encoded by the che-6 locus. che-6 mutants show defects in gustatory sensory transduction that are similar to defects observed in animals lacking the tax-2 and tax-4 CNG channels. In contrast, thermosensory signal transduction, which also requires tax-2 and tax-4, does not require che-6, but requires another CNG, cng-3. We propose that CHE-6 may form together with two other CNG subunits, TAX-2 and TAX-4, a gustatory neuron-specific heteromeric CNG channel complex.
环鸟苷酸 (cGMP) 是一种在各种感觉神经元信号转导中使用的关键第二信使。在秀丽隐杆线虫的 ASE 味觉受体神经元中，cGMP 的重要性是通过观察到多个受体型鸟苷酸环化酶 (rGC)，由 gcy 基因编码，以及目前已知的两个环核苷酸门控离子通道亚基，由 tax-2 和 tax-4 基因编码，对 ASE 介导的味觉行为是必需的。我们在这里描述了 ASE 神经元中 cGMP 介导的信号转导的特定机制特征。首先，我们评估了单个 rGC 蛋白的感觉功能的特异性。我们之前已经表明，多个 rGC 蛋白在功能上偏侧化的 ASE 神经元中以左右不对称的方式表达，并需要感知不同的盐线索。通过三个不同的 rGC 蛋白之间的结构域交换实验，我们在这里表明，单个 rGC 蛋白的特异性在于它们的细胞外结构域，而不是它们的细胞内信号转导结构域。此外，我们发现 rGC 蛋白也足以赋予其他神经元盐感觉反应。这两个发现都支持 rGC 蛋白是盐受体蛋白的假说。其次，我们在味觉信号转导中鉴定出 rGC 蛋白的一种新的、可能的下游效应物，一种以前未被描述的环核苷酸门控 (CNG) 离子通道，由 che-6 基因座编码。che-6 突变体在味觉感觉转导中表现出缺陷，与缺乏 tax-2 和 tax-4 CNG 通道的动物观察到的缺陷相似。相比之下，需要 tax-2 和 tax-4 的热感觉转导不需要 che-6，但需要另一个 CNG，cng-3。我们提出 CHE-6 可能与另外两个 CNG 亚基 TAX-2 和 TAX-4 一起形成一个味觉神经元特异性异源 CNG 通道复合物。
