Adenosine receptors (ADORs) are G-protein coupled purinoceptors that have several functions including regulation of chloride secretion via CFTR in human airway and kidney. We cloned an ADOR from Squalus acanthias (shark) that likely regulates CFTR in the rectal gland. Phylogenic- and expression- analyses indicate that elasmobranch ADORs are non-olfactory, and appear to represent extant predecessors of mammalian ADORs. We therefore designate the shark ADOR as the A0 receptor. We co-expressed A0 with CFTR in Xenopus laevis oocytes and characterized the coupling of A0 to the chloride channel. Two electrode voltage clamping was performed and current-voltage (I-V) responses were recorded to monitor CFTR status. Only in A0- and CFTR- co-injected oocytes did adenosine analogs produce a significant concentration-dependent activation of CFTR consistent with its electrophysiological signature. A pharmacological profile for A0 was obtained for ADOR agonists and antagonists that differed markedly from all mammalian ADOR subtypes (agonists: R-PIA > S-PIA > CGS21680 > CPA > 2ClADO > CV1808 = DPMA > NECA) and (antagonists: DPCPX > PD115199 > 8PT > CGC > CGS15943). Structures of human ADORs permitted a high-confidence homology model of the shark A0 core which revealed unique structural features of ancestral receptors. We conclude: (1) A0 is a novel and unique adenosine receptor ancestor by functional and structural criteria; (2) A0 likely activates CFTR in vivo and this receptor activates CFTR in oocytes indicating an evolutionary coupling between ADORs and chloride secretion; and (3) A0 appears to be a non-olfactory evolutionary ancestor of all four mammalian ADOR subtypes.

中文翻译：

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非嗅鲨鲨腺苷受体以独特的药理和结构特征激活CFTR

腺苷受体（ADOR）是G蛋白偶联的嘌呤受体，具有多种功能，包括通过人呼吸道和肾脏中的CFTR调节氯化物的分泌。我们从角鲨（Squalus acanthias，鲨鱼）克隆了一个ADOR，它可能调节直肠中的CFTR。系统发育和表达分析表明，弹性分支ADOR是非嗅觉的，并且似乎代表了现有的哺乳动物ADOR的前身。因此，我们将鲨鱼ADOR指定为A0受体。我们在非洲爪蟾卵母细胞中与CFTR共表达A0，并表征A0与氯通道的偶联。进行了两个电极电压钳位，并记录了电流-电压（IV）响应以监视CFTR状态。仅在A0和CFTR共注射的卵母细胞中，腺苷类似物才产生与CFTR的电生理特征一致的显着浓度依赖性的CFTR激活。对于与所有哺乳动物ADOR亚型明显不同的ADOR激动剂和拮抗剂，获得了A0的药理学特征（激动剂：R-PIA> S-PIA> CGS21680> CPA> 2ClADO> CV1808 = DPMA> NECA）和（拮抗剂：DPCPX> PD115199 > 8PT> CGC> CGS15943）。人类ADORs的结构允许鲨鱼A0核心具有高可信度的同源性模型，该模型揭示了祖先受体的独特结构特征。我们得出以下结论：（1）从功能和结构上看，A0是一种新颖独特的腺苷受体祖先；（2）A0可能在体内激活CFTR，而该受体在卵母细胞中激活CFTR，表明ADOR和氯化物分泌之间存在进化耦合。（3）A0似乎是所有四种哺乳动物ADOR亚型的非嗅觉进化祖先。