GPR19

GPR19 is a class A orphan-derived G protein-coupled receptor that links the secreted peptide adropin to MAPK/ERK1/2 signaling and to the regulation of organismal physiology spanning circadian timing, ingestive behavior, and whole-body metabolism (PMID:28476646, PMID:34789778). Adropin activation of GPR19 stimulates ERK1/2 phosphorylation and upregulates E-cadherin, driving a mesenchymal-to-epithelial transition and reducing invasion in breast cancer cells (PMID:28476646); in the rat hypothalamus GPR19 is required for adropin's suppression of water-deprivation-induced drinking (PMID:26739651). In the brain, GPR19 is enriched in the dorsal suprachiasmatic nucleus where its expression oscillates under control of a cAMP-responsive element, and its loss lengthens circadian period, blunts light-induced phase delays and c-Fos induction, and downregulates night-peaking clock genes including Bmal1 and Gpr176 (PMID:34789778). Systemic Gpr19 deletion raises energy expenditure in both sexes but produces male-specific glucose intolerance and diet-induced hepatomegaly with reduced hepatic fatty acid oxidation gene expression, defining a sex-dependent metabolic role (PMID:37061564).

1. 2016 Medium

   Identified GPR19 as the candidate central receptor through which adropin acts, addressing how this peptide exerts effects on ingestive behavior.

   Evidence siRNA/antisense knockdown of GPR19 in rat medial basal hypothalamus with water-drinking behavioral assay

   PMID:26739651

   Open questions at the time

   • No direct adropin-GPR19 binding or in vitro signaling demonstrated in this study
   • Mechanism downstream of receptor in hypothalamic neurons not defined
2. 2017 Medium

   Established adropin as an endogenous ligand for GPR19 and connected the receptor to ERK1/2 signaling and an epithelial gene program, defining its molecular signaling axis.

   Evidence GPR19 overexpression plus adropin stimulation with ERK1/2 phosphorylation, E-cadherin expression, and invasion/migration assays in breast cancer cells

   PMID:28476646

   Open questions at the time

   • G protein coupling specificity not resolved
   • Direct receptor-ligand affinity not quantified
   • Single-lab overexpression context
3. 2021 High

   Demonstrated a circadian function for GPR19 in the dorsal SCN, showing it shapes clock period and light-induced phase resetting rather than acting only peripherally.

   Evidence Gpr19-knockout mice with locomotor activity monitoring, promoter-CRE reporter assay, c-Fos immunostaining, and SCN clock-gene expression analysis

   PMID:34789778

   Open questions at the time

   • Whether adropin is the relevant ligand in the SCN was not tested
   • Signaling pathway linking GPR19 to clock-gene regulation not defined
4. 2023 Medium

   Revealed a sex-dependent role for GPR19 in systemic metabolism, addressing whether the receptor influences whole-body energy and glucose handling.

   Evidence Gpr19 knockout mice with metabolic cage measurements, glucose tolerance tests, and hepatic gene expression under high-fat diet

   PMID:37061564

   Open questions at the time

   • Mechanistic basis of male-specific hepatic phenotype unresolved
   • Tissue-specific contributions not separated by conditional deletion
5. 2023 Low

   Proposed that GPR19 expression level dictates which stress, DNA-damage, or senescence signaling paradigm is engaged, exploring dose-dependent receptor outputs.

   Evidence Proteomic, molecular, and bioinformatic analysis of HEK293 cells with titrated GPR19 expression

   PMID:37239845

   Open questions at the time

   • Limited direct functional validation beyond proteomic/informatic correlation
   • No ligand-dependent confirmation of the proposed paradigms
   • Single cell line and single lab

• The G protein coupling, structural basis of adropin recognition, and how a single receptor integrates circadian, metabolic, and behavioral outputs across tissues remain unresolved.
• No defined G protein/transducer coupling
• No structural model of the adropin-GPR19 interaction
• Tissue-specific signaling mechanisms not mechanistically linked