A novel receptor for bile salts – the G protein-coupled receptor MRGX4 is expressed on sensory neurons

Background&Aims:

Mas gene-related G protein-coupled receptors (Mrg) represent a class of pruriceptors expressed on sensory neurons. Pruritus is a frequent symptom in various hepatobiliary disorders. The recent beneficial effects of ileal bile acid transport inhibitors in patients suffering from cholestatic pruritus have raised interest in bile salts as pruritogens. We hypothesized that cholephilic substances in serum of cholestatic patients may activate MRG receptors thereby causing pruritus.

Methods:

Human and murine MRG receptors were cloned and stably expressed in HEK293 cells. Activation of MRG receptors in HEK293 or DRGs by cholephilic substances were measured by changes in cytosolic free calcium (Ca²⁺)_i using ratiometric fluorimetry. In mice scratch activity after intradermal pruritogen injection was quantified observer-independently measuring electric fields induced by implanted magnets. In 15 healthy volunteers itch intensity was quantified on a numeric rating scale after intradermal injection or focal application.

Results:

We investigated the potential of human cholestatic serum and bile for activation of human and murine MRG receptors. Fractions containing bile salts resulted in rise of (Ca²⁺)_i in hMRGX4 expressing HEK cells but neither of other human or murine MRG isoforms nor cultures of freshly dissociated murine DRGs. Analyzing human bile salts revealed that certain bile salt species (referred to as hMRGX4-activating bile salt species) were capable of activating hMRGX4 while others did not. The EC50 values ranged within the pathophysiological range of low micromolar concentrations. Intradermally injected bile salts did not cause significant scratching behavior in mice. In contrast, in humans focally applied or intradermally injected hMRGX4-activating bile salts caused significant itch intensity compared to non-hMRGX4 activating bile salts. Laser-doppler imaging indicated no widespread axonreflex erythema excluding relevant mast cells activation.

Conclusions:

These data unravel a novel signaling pathway for bile salt subspecies that may contribute to cholestatic pruritus and may explain the differences between murine and human cholestasis. The hMRGX4 receptor represents a promising drug target to alleviate cholestatic pruritus.