Constitutive Activity among Orphan Class-A G Protein Coupled Receptors

PLoS One. 2015 Sep 18;10(9):e0138463. doi: 10.1371/journal.pone.0138463. eCollection 2015.


The purpose of this study was to evaluate the extent of constitutive activity among orphan class-A G protein coupled receptors within the cAMP signaling pathway. Constitutive signaling was revealed by changes in gene expression under control of the cAMP response element. Gene expression was measured in Chinese hamster ovary cells transiently co-transfected with plasmids containing a luciferase reporter and orphan receptor. Criteria adopted for defining constitutive activation were: 1) 200% elevation over baseline reporter gene expression; 2) 40% inhibition of baseline expression; and 3) 40% inhibition of expression stimulated by 3 μM forskolin. Five patterns of activity were noted: 1) inhibition under both baseline and forskolin stimulated expression (GPR15, GPR17, GPR18, GPR20, GPR25, GPR27, GPR31, GPR32, GPR45, GPR57, GPR68, GPR83, GPR84, GPR132, GPR150, GPR176); 2) no effect on baseline expression, but inhibition of forskolin stimulated expression (GPR4, GPR26, GPR61, GPR62, GPR78, GPR101, GPR119); 3) elevation of baseline signaling coupled with inhibition of forskolin stimulated expression (GPR6, GPR12); 4) elevation of baseline signaling without inhibition of forskolin stimulated expression (GPR3, GPR21, GPR52, GPR65); and 5) no effect on expression (GPR1, GPR19, GPR22, GPR34, GPR35, GPR39, GPR63, GPR82, GPR85, GPR87). Constitutive activity was observed in 75% of the orphan class-A receptors examined (30 of 40). This constitutive signaling cannot be explained by simple overexpression of the receptor. Inhibition of cAMP mediated expression was far more common (65%) than stimulation of expression (15%). Orphan receptors that were closely related based on amino acid homology tended to have similar effects on gene expression. These results suggest that identification of inverse agonists may be a fruitful approach for categorizing these orphan receptors and targeting them for pharmacological intervention.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • CHO Cells
  • Cell Line
  • Colforsin / pharmacology
  • Cricetulus
  • Cyclic AMP / metabolism
  • Gene Expression / drug effects
  • Gene Expression / genetics
  • Receptors, G-Protein-Coupled / genetics*
  • Receptors, G-Protein-Coupled / metabolism*
  • Signal Transduction / drug effects
  • Signal Transduction / genetics
  • Signal Transduction / physiology


  • Receptors, G-Protein-Coupled
  • Colforsin
  • Cyclic AMP

Grant support

Funding was provided by the cDNA Resource Center and Department of Biological Sciences at MS&T. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.