title: The GPCR OGR1 (GPR68) mediates diverse signalling and contraction of airway smooth muscle in response to small reductions in extracellular pH.
abstract: OBJECTIVE
  Previous studies have linked a reduction in pH in airway, caused by either environmental factors, microaspiration of gastric acid or inflammation, with airway smooth muscle (ASM) contraction and increased airway resistance. Neural mechanisms have been shown to mediate airway contraction in response to reductions in airway pH to < 6.5; whether reduced extracellular pH (pHo) has direct effects on ASM is unknown.
  
  
  METHODS
  Intracellular signalling events stimulated by reduced pHo in human cultured ASM cells were examined by immunoblotting, phosphoinositide hydrolysis and calcium mobilization assays. ASM cell contractile state was examined using magnetic twisting cytometry. The expression of putative proton-sensing GPCRs in ASM was assessed by real-time PCR. The role of ovarian cancer G protein-coupled receptor 1 (OGR1 or GPR68) in acid-induced ASM signalling and contraction was assessed in cultures subjected to siRNA-mediated OGR1 knockdown.
  
  
  RESULTS
  ASM cells responded to incremental reductions in pHo (from pH 8.0 to pH 6.8) by activating multiple signalling pathways, involving p42/p44, PKB, PKA and calcium mobilization. Coincidently, ASM cells contracted in response to decreased pHo with similar 'dose'-dependence. Real-time PCR suggested OGR1 was the only proton-sensing GPCR expressed in ASM cells. Both acid-induced signalling (with the exception of PKB activation) and contraction were significantly attenuated by knockdown of OGR1.
  
  
  CONCLUSIONS
  These studies reveal OGR1 to be a physiologically relevant GPCR in ASM cells, capable of pleiotropic signalling and mediating contraction in response to small reductions in extracellular pH. Accordingly, ASM OGR1 may contribute to asthma pathology and represent a therapeutic target in obstructive lung diseases.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201-1075, USA.
authors.forename: H
authors.initials: H
authors.lastname: Saxena
authors.forename: D A
authors.initials: DA
authors.lastname: Deshpande
authors.forename: B C
authors.initials: BC
authors.lastname: Tiegs
authors.forename: H
authors.initials: H
authors.lastname: Yan
authors.forename: R J
authors.initials: RJ
authors.lastname: Battafarano
authors.forename: W M
authors.initials: WM
authors.lastname: Burrows
authors.forename: G
authors.initials: G
authors.lastname: Damera
authors.forename: R A
authors.initials: RA
authors.lastname: Panettieri
authors.forename: T D
authors.initials: TD
authors.lastname: Dubose
authors.forename: S S
authors.initials: SS
authors.lastname: An
authors.forename: R B
authors.initials: RB
authors.lastname: Penn
mesh_terms: D001980:Bronchi / Q000166:cytology / Q000187:drug effects; D018929:Cell Culture Techniques; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D045604:Extracellular Fluid / Q000737:chemistry*; D006801:Humans; D006851:Hydrochloric Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007213:Indomethacin / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D060888:Real-Time Polymerase Chain Reaction; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D014132:Trachea / Q000166:cytology / Q000187:drug effects
chemical_list: D016861:Cyclooxygenase Inhibitors; C486281:GPR68 protein, human; D043562:Receptors, G-Protein-Coupled; D000242:Cyclic AMP; D006851:Hydrochloric Acid; D007213:Indomethacin