Bioorganic Research Institute, Suntory Foundation for Life Sciences, Osaka 618-8503, Japan.
Endocrinology. 2012 Apr;153(4):1841-9. doi: 10.1210/en.2011-1668. Epub 2012 Jan 31.
The endocrine and neuroendocrine systems for reproductive functions have diversified as a result of the generation of species-specific paralogs of peptide hormones and their receptors including GnRH and their receptors (GnRHR), which belong to the class A G protein-coupled receptor family. A protochordate, Ciona intestinalis, has been found to possess seven GnRH (tGnRH-3 to -8 and Ci-GnRH-X) and four GnRHR (Ci-GnRHR1 to -4). Moreover, Ci-GnRHR4 (R4) does not bind to any Ciona GnRH and activate any signaling pathways. Here we show novel functional diversification of GnRH signaling pathways via G protein-coupled receptor heterodimerization among Ciona GnRHR. R4 was shown to heterodimerize with R2 specifically in test cells of vitellogenic oocytes by coimmunoprecipitation. The R2-R4 heterodimerization in human embryonic kidney 293 cells cotransfected with R2 and R4 was also observed by coimmunoprecipitation and fluorescent energy transfer analyses. Of particular interest is that the R2-R4 heterodimer decreases the cAMP production in a nonligand-selective manner via shift of activation of Gs protein to Gi protein by R2, compared with R2 monomer/homodimer. Considering that the R1-R4 heterodimer elicits 10-fold more potent Ca²⁺ mobilization than R1 monomer/homodimer in a ligand-selective manner but does not affect cAMP production, these results indicate that R4 regulates differential GnRH signaling cascades via heterodimerization with R1 and R2 as an endogenous allosteric modulator. Collectively, the present study suggests that the heterodimerization among GnRHR paralogs, including the species-specific orphan receptor subtype, is involved in rigorous and diversified GnRHergic signaling of the protochordate, which lacks a hypothalamus-pituitary gonad axis.
生殖功能的内分泌和神经内分泌系统因肽激素及其受体（包括 GnRH 和其受体 GnRHR）的物种特异性同源物的产生而多样化，这些受体属于 A 类 G 蛋白偶联受体家族。现已发现，文昌鱼（一种原索动物）拥有七种 GnRH（tGnRH-3 至 -8 和 Ci-GnRH-X）和四种 GnRHR（Ci-GnRHR1 至 -4）。此外，Ci-GnRHR4（R4）不与任何文昌鱼 GnRH 结合，也不激活任何信号通路。在这里，我们通过 Ci-GnRHR 之间的 G 蛋白偶联受体异源二聚化展示了 GnRH 信号通路的新的功能多样化。通过共免疫沉淀，我们发现 R4 仅在卵黄生成卵母细胞的生殖细胞中与 R2 特异性异源二聚化。在共转染 R2 和 R4 的人胚肾 293 细胞中，也通过共免疫沉淀和荧光能量转移分析观察到 R2-R4 异源二聚体。特别有趣的是，与 R2 单体/同源二聚体相比，R2-R4 异源二聚体通过 R2 将 Gs 蛋白的激活转移到 Gi 蛋白，以非配体选择性的方式降低 cAMP 的产生。考虑到 R1-R4 异源二聚体以配体选择性的方式比 R1 单体/同源二聚体引起 10 倍更强的 Ca²⁺动员，但不影响 cAMP 的产生，这些结果表明 R4 通过与 R1 和 R2 异源二聚化作为内源性变构调节剂来调节不同的 GnRH 信号级联。总的来说，本研究表明，包括物种特异性孤儿受体亚型在内的 GnRHR 同源物的异源二聚化参与了原索动物严格和多样化的 GnRH 信号转导，而原索动物缺乏下丘脑-垂体-性腺轴。
