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Gq pathway desensitizes chemotactic receptor-induced calcium signaling via inositol trisphosphate receptor
1Department of Internal Medicine and Physical Therapy, Faculty of Medicine, University of Tokyo, Japan.
Abstract:
Desensitization of a chemotactic receptor is an adaptive process that terminates inflammation. Although homologous desensitization can be well explained by the action of specific receptor kinases, the mechanisms of heterologous desensitization remain elusive. As an approach to evaluate the roles of Gq pathway in desensitization of calcium signaling, we expressed a constitutively active Gq alpha mutant (Gq alpha Q-L) together with platelet-activating factor (PAF) receptor in Xenopus laevis oocytes. Gq alpha Q-L expression completely attenuated the calcium-sensitive chloride current and the 45Ca release elicited by PAF. The Gq-mediated desensitization could not be ascribed to G protein/receptor uncoupling via receptor phosphorylation, because (i) PAF-induced inositol 1,4,5-trisphosphate (IP3) synthesis was only partially suppressed and (ii) a mutated PAF receptor devoid of all Ser and Thr in the third cytoplasmic loop and in the C-terminal tail was also completely desensitized by Gq alpha Q-L. In Gq alpha Q-L expressing oocytes, microinjection of IP3 failed to elicit the calcium response, and the IP3 receptor, detected by a specific antibody, disappeared. Thus, the Gq-mediated desensitization can be most likely explained by IP3 receptor down-regulation. These novel mechanisms may explain in part heterologous desensitization in chemotactic factor-stimulated inflammatory cells.
Insights
The Gq pathway desensitizes chemotactic receptors by down-regulating IP3 receptors, a novel mechanism explaining inflammation termination. This study reveals how Gq signaling impacts calcium signaling and receptor function.
Area of Science:
- Cellular Biology
- Immunology
- Molecular Pharmacology
Background:
- Chemotactic receptor desensitization is crucial for terminating inflammation.
- Homologous desensitization is understood, but heterologous desensitization mechanisms remain unclear.
- The role of the Gq pathway in calcium signaling desensitization requires investigation.
Purpose of the Study:
- To investigate the role of the Gq pathway in the desensitization of calcium signaling.
- To elucidate the mechanisms underlying heterologous desensitization of chemotactic receptors.
Main Methods:
- Expression of a constitutively active Gq alpha mutant (Gq alpha Q-L) and platelet-activating factor (PAF) receptor in Xenopus laevis oocytes.
- Measurement of calcium-sensitive chloride current and 45Ca release.
- Assessment of inositol 1,4,5-trisphosphate (IP3) synthesis and IP3 receptor levels.
Main Results:
- Gq alpha Q-L expression completely abolished PAF-induced calcium responses.
- Desensitization was independent of receptor phosphorylation, as a mutated PAF receptor was also desensitized.
- Microinjection of IP3 failed to elicit a response, and IP3 receptors disappeared in Gq alpha Q-L expressing oocytes.
Conclusions:
- Gq-mediated desensitization is primarily caused by IP3 receptor down-regulation.
- This novel mechanism contributes to understanding heterologous desensitization in inflammatory cells.
- The findings offer insights into adaptive processes terminating inflammation.