The Bile Acid Receptor TGR5 Activates the TRPA1 Channel to Induce Itch in Mice

Abstract

BACKGROUND & AIMS 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 Ca2+ 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 Ca2+. 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.

Optogenetic activation of mechanically insensitive afferents in mouse colorectum reveals chemosensitivity

“…The present study revealed that AHS-sensitive and IS-sensitive MIAs represent discrete afferent groups with distinct transformation characteristics. The effects of BS on colorectal MIAs were briefly reported previously and also studied here; BS enhances permeability of the colorectal mucosa (37) and may act on afferents via TRPA1 channels (23). However, MIAs did not appear to directly encode BS stimulation because only a small proportion of MIAs gave direct responses.…”

Bile acid activated receptors are targets for regulation of integrity of gastrointestinal mucosa

“…The mucosal application of physiological concentrations of DCA and LCA (less than 100 lM) to a flat sheet preparation of mouse colon evokes an ascending contraction and descending relaxation of circular muscle, consistent with stimulation of peristalsis, and concomitantly stimulates release of the peristaltic transmitters 5-HT and CGRP. GP-BAR1 deletion abolishes these effects, and antagonism of 5-HT 4 receptors and CGRP receptors blocks GP-BAR1-evoked peristalsis, consistent with the localization of GP-BAR1 in enterochromaffin cells and intrinsic primary afferent neurons [14][15][16]. Considered together, these results suggest that GP-BAR1 mediates the prokinetic actions of bile acids in the colon and that the receptor is required for normal defecation.…”

“…Bile acids and GP-BAR1-selective agonists induce hyperexcitability of dorsal root ganglia neurons and stimulate the release of itch transmitters, including CGRP and leucine enkephalin. Further, intradermal injection of bile acids and a GP-BAR1-selective agonist triggers a scratching behavior caused by CGRP--and opioid-dependent mechanisms in mice [16]. Thus, bile acids activate GP-BAR1 on sensory nerves, stimulating the release of neuropeptides in the spinal cord that transmit itch.…”

“…In mice, peptidergic neurons of dorsal root ganglia and spinal cord that transmit itch and pain express GP-BAR1, which is also expressed by dermal macrophages that contain opioid-like immune reactivity [16]. Bile acids and GP-BAR1-selective agonists induce hyperexcitability of dorsal root ganglia neurons and stimulate the release of itch transmitters, including CGRP and leucine enkephalin.…”

Pathogenesis and Management of Pruritus in PBC and PSC

“…The same group further showed that TGR5 coexpressed with TRPA1 and unconjugated bile salts caused scratching behaviour by the coactivation of TRPA1. DCA-induced itching was reduced by antagonists of TRPA1 or in TRPA1 -/-mice [97] . Of note, unconjugated DCA is present only in trace amounts in humans [98] .…”