The C-terminal Tail of CRTH2 Is a Key Molecular Determinant That Constrains Gαi and Downstream Signaling Cascade Activation

Abstract

Prostaglandin D 2 activation of the seven-transmembrane receptor CRTH2 regulates numerous cell functions that are important in inflammatory diseases, such as asthma. Despite its disease implication, no studies to date aimed at identifying receptor domains governing signaling and surface expression of human CRTH2. We tested the hypothesis that CRTH2 may take advantage of its C-tail to silence its own signaling and that this mechanism may explain the poor functional responses observed with CRTH2 in heterologous expression systems. Although the C terminus is a critical determinant for retention of CRTH2 at the plasma membrane, the presence of this domain confers a signaling-compromised conformation onto the receptor. Indeed, a mutant receptor lacking the major portion of its C-terminal tail displays paradoxically enhanced G␣ i and ERK1/2 activation despite enhanced constitutive and agonist-mediated internalization. Enhanced activation of G␣ i proteins and downstream signaling cascades is probably due to the inability of the tail-truncated receptor to recruit ␤-arrestin2 and undergo homologous desensitization. Unexpectedly, CRTH2 is not phosphorylated upon agonist-stimulation, a primary mechanism by which GPCR activity is regulated. Dynamic mass redistribution assays, which allow label-free monitoring of all major G protein pathways in real time, confirm that the C terminus inhibits G␣ i signaling of CRTH2 but does not encode G protein specificity determinants. We propose that intrinsic CRTH2 inhibition by its C terminus may represent a rather unappreciated strategy employed by a GPCR to specify the extent of G protein activation and that this mechanism may compensate for the absence of the classical phosphorylation-dependent signal attenuation.2 is a lipid mediator that has been considered essential in the development of inflammatory diseases such as asthma and atopic dermatitis (1-3). It is the major cyclooxygenase metabolite synthesized in allergen-activated mast cells and is released upon their immunological activation (4). The biological effects of PGD 2 are mediated by two G protein-coupled receptors, DP1 and DP2/CRTH2 (chemoattractant receptor homologous molecule expressed on T helper type 2 cells), respectively (5, 6). DP1 activation leads to G␣ s -mediated elevation of intracellular cyclic AMP, whereas activation of CRTH2 results in an increase in intracellular Ca 2ϩ levels via the G␣ i pathway and a decrease in cAMP, but also G protein-independent, arrestin-mediated cellular responses have been observed (5-7).CRTH2 in particular is expressed on eosinophils, basophils, and T helper type 2 lymphocytes. Activation by PGD 2 or its active metabolites transduces the chemokinetic activity on these immune cells and, by doing so, mediates their recruitment to sites of inflammation (2, 3, 6, 8 -13). In mouse models of allergic asthma or atopic dermatitis, CRTH2 activation promotes eosinophilia and exacerbates pathology (14 -17). In humans, the proinflammatory role of CRTH2 is underscored by the finding that sequence...

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