A nonolfactory shark adenosine receptor activates CFTR with unique pharmacology and structural features

Am J Physiol Cell Physiol. 2021 May 1;320(5):C892-C901. doi: 10.1152/ajpcell.00481.2020. Epub 2021 Mar 10.


Adenosine receptors (ADORs) are G protein-coupled purinoceptors that have several functions including regulation of chloride secretion via cystic fibrosis transmembrane conductance regulator (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 nonolfactory and appear to represent extant predecessors of mammalian ADORs. We therefore designate the shark ADOR as the A0 receptor. We coexpressed 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-coinjected 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-phenyl-isopropyl adenosine (R-PIA) > S-phenyl-isopropyl adenosine (S-PIA) > CGS21680 > N6-cyclopentyladenosine (CPA) > 2-chloroadenosine (2ClAdo) > CV1808 = N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA) > N-ethyl-carboxyl adenosine (NECA); and antagonists: 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) > PD115199 > 1,3-dimethyl-8-phenylxanthine (8PT) > CGS15943]. Structures of human ADORs permitted a high-confidence homology model of the shark A0 core that revealed unique structural features of ancestral receptors. We conclude that 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 nonolfactory evolutionary ancestor of all four mammalian ADOR subtypes.

Keywords: CFTR; adenosine receptor; chloride secretion; molecular evolution; shark; structural biology.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Chlorides / metabolism*
  • Cloning, Molecular
  • Cystic Fibrosis Transmembrane Conductance Regulator / genetics
  • Cystic Fibrosis Transmembrane Conductance Regulator / metabolism*
  • Evolution, Molecular
  • Female
  • Fish Proteins / genetics
  • Fish Proteins / metabolism*
  • Humans
  • Male
  • Membrane Potentials
  • Phylogeny
  • Protein Conformation
  • Purinergic P1 Receptor Agonists / pharmacology
  • Purinergic P1 Receptor Antagonists / pharmacology
  • Receptors, Purinergic P1 / drug effects
  • Receptors, Purinergic P1 / genetics
  • Receptors, Purinergic P1 / metabolism*
  • Salt Gland / metabolism*
  • Squalus acanthias / genetics
  • Squalus acanthias / metabolism*
  • Structure-Activity Relationship
  • Xenopus laevis


  • Chlorides
  • Fish Proteins
  • Purinergic P1 Receptor Agonists
  • Purinergic P1 Receptor Antagonists
  • Receptors, Purinergic P1
  • Cystic Fibrosis Transmembrane Conductance Regulator