title: The bile acid receptor TGR5 activates the TRPA1 channel to induce itch in mice.
abstract: OBJECTIVE
  Patients with cholestatic disease have increased systemic concentrations of bile acids (BAs) and profound pruritus. The G-protein-coupled BA receptor 1 TGR5 (encoded by GPBAR1) is expressed by primary sensory neurons; its activation induces neuronal hyperexcitability and scratching by unknown mechanisms. We investigated whether the transient receptor potential ankyrin 1 (TRPA1) is involved in BA-evoked, TGR5-dependent pruritus in mice.
  
  
  METHODS
  Co-expression of TGR5 and TRPA1 in cutaneous afferent neurons isolated from mice was analyzed by immunofluorescence, in situ hybridization, and single-cell polymerase chain reaction. TGR5-induced activation of TRPA1 was studied in in HEK293 cells, Xenopus laevis oocytes, and primary sensory neurons by measuring Ca(2+) signals. The contribution of TRPA1 to TGR5-induced release of pruritogenic neuropeptides, activation of spinal neurons, and scratching behavior were studied using TRPA1 antagonists or Trpa1(-/-) mice.
  
  
  RESULTS
  TGR5 and TRPA1 protein and messenger RNA were expressed by cutaneous afferent neurons. In HEK cells, oocytes, and neurons co-expressing TGR5 and TRPA1, BAs caused TGR5-dependent activation and sensitization of TRPA1 by mechanisms that required Gβγ, protein kinase C, and Ca(2+). Antagonists or deletion of TRPA1 prevented BA-stimulated release of the pruritogenic neuropeptides gastrin-releasing peptide and atrial natriuretic peptide B in the spinal cord. Disruption of Trpa1 in mice blocked BA-induced expression of Fos in spinal neurons and prevented BA-stimulated scratching. Spontaneous scratching was exacerbated in transgenic mice that overexpressed TRG5. Administration of a TRPA1 antagonist or the BA sequestrant colestipol, which lowered circulating levels of BAs, prevented exacerbated spontaneous scratching in TGR5 overexpressing mice.
  
  
  CONCLUSIONS
  BAs induce pruritus in mice by co-activation of TGR5 and TRPA1. Antagonists of TGR5 and TRPA1, or inhibitors of the signaling mechanism by which TGR5 activates TRPA1, might be developed for treatment of cholestatic pruritus.
authors.affiliation: Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia.
authors.forename: TinaMarie
authors.initials: T
authors.lastname: Lieu
authors.affiliation: Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia.
authors.forename: Gihan
authors.initials: G
authors.lastname: Jayaweera
authors.affiliation: Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia.
authors.forename: Peishen
authors.initials: P
authors.lastname: Zhao
authors.affiliation: Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia; Department of Anatomy and Neuroscience, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Poole
authors.affiliation: Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia.
authors.forename: Dane
authors.initials: D
authors.lastname: Jensen
authors.affiliation: Health Innovations Research Institute and School of Medical Sciences, RMIT University, Bundoora, Victoria, Australia.
authors.forename: Megan
authors.initials: M
authors.lastname: Grace
authors.affiliation: Health Innovations Research Institute and School of Medical Sciences, RMIT University, Bundoora, Victoria, Australia.
authors.forename: Peter
authors.initials: P
authors.lastname: McIntyre
authors.affiliation: Department of Anatomy and Neuroscience, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Romke
authors.initials: R
authors.lastname: Bron
authors.affiliation: Department of Anatomy and Neuroscience, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Yvette M
authors.initials: YM
authors.lastname: Wilson
authors.affiliation: Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Matteus
authors.initials: M
authors.lastname: Krappitz
authors.affiliation: Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Silke
authors.initials: S
authors.lastname: Haerteis
authors.affiliation: Institut für Zelluläre und Molekulare Physiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Korbmacher
authors.affiliation: Charles Institute for Translational Dermatology, University College Dublin, Dublin, Ireland.
authors.forename: Martin S
authors.initials: MS
authors.lastname: Steinhoff
authors.affiliation: Department of Health Sciences, Clinical Pharmacology Unit, University of Florence, Florence, Italy.
authors.forename: Romina
authors.initials: R
authors.lastname: Nassini
authors.affiliation: Department of Health Sciences, Clinical Pharmacology Unit, University of Florence, Florence, Italy.
authors.forename: Serena
authors.initials: S
authors.lastname: Materazzi
authors.affiliation: Department of Health Sciences, Clinical Pharmacology Unit, University of Florence, Florence, Italy.
authors.forename: Pierangelo
authors.initials: P
authors.lastname: Geppetti
authors.affiliation: Department of Surgery, University of California, San Francisco, San Francisco, California.
authors.forename: Carlos U
authors.initials: CU
authors.lastname: Corvera
authors.affiliation: Monash Institute of Pharmaceutical Sciences, Parkville, Victoria, Australia; Department of Pharmacology, University of Melbourne, Parkville, Victoria, Australia. Electronic address: Nigel.Bunnett@Monash.edu.
authors.forename: Nigel W
authors.initials: NW
authors.lastname: Bunnett
mesh_terms: D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D002779:Cholestasis / Q000150:complications / Q000378:metabolism*; D004195:Disease Models, Animal; D005727:Ganglia, Spinal / Q000166:cytology / Q000378:metabolism; D019886:Gastrin-Releasing Peptide / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D018345:Mice, Knockout; D045265:Natriuretic Peptides / Q000378:metabolism; D009475:Neurons, Afferent / Q000166:cytology / Q000378:metabolism; D009619:Nociceptors / Q000378:metabolism; D009865:Oocytes / Q000166:cytology / Q000378:metabolism; D061251:Primary Cell Culture; D011537:Pruritus / Q000209:etiology / Q000378:metabolism*; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism*; D000074025:TRPA1 Cation Channel; D050051:Transient Receptor Potential Channels / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D001647:Bile Acids and Salts; C513561:Gpbar1 protein, mouse; D045265:Natriuretic Peptides; D043562:Receptors, G-Protein-Coupled; D000074025:TRPA1 Cation Channel; D050051:Transient Receptor Potential Channels; C475685:Trpa1 protein, mouse; D019886:Gastrin-Releasing Peptide
keywords: Itching; Liver; Mouse Model; Signal Transduction