Background and Purpose : Gpr19 encodes an evolutionarily conserved orphan G-protein-coupled receptor (GPCR) with no established physiological function in vivo. The purpose of this study was to determine the role of Gpr19 in the circadian clock system. Experimental Approach : We examined whether and how the master circadian clock neurons in the suprachiasmatic nucleus (SCN) express Gpr19 . By analysing Gpr19 -deficient ( Gpr19 −/− ) mice, we asked whether Gpr19 has a role in modulating free-running period and light resetting capacity of the circadian clock. Key Results : Compared with the known common core clock genes, Gpr19 was identified to show several distinct yet limited features related to the circadian clock. Gpr19 mRNA was mainly expressed in the middle-to-dorsal region of the SCN. A conserved cAMP-responsive element within the Gpr19 promoter drove the circadian expression of Gpr19 . Gpr19 −/− mice exhibited a prolonged circadian period and a delayed initiation of daily locomotor activity in a 12-h light/12-h dark cycle. Gpr19 deficiency caused the downregulation of several genes that normally peak during the night, including Bmal1 and Gpr176 . Gpr19 −/− mice had a reduced capacity for phase shift to early subjective night light. The defect was only observed for phase-delay, but not phase-advance, and accompanied by reduced response of c-Fos expression in the dorsal region of the SCN, while apparently normal in the ventral part of the SCN, in Gpr19 −/− mice. Conclusion and Implications : Gpr19 is an SCN-enriched orphan GPCR with a distinct role in circadian regulation and thus may be a potential target for alleviating circadian clock disorders.
The suprachiasmatic nucleus (SCN) is the master circadian clock in mammals and is properly entrained by environmental light cycle. However, the molecular mechanism(s) determining the magnitude of phase shift by light is still not fully understood. The orphan G-protein-coupled receptor Gpr176 is enriched in the SCN, controls the pace (period) of the circadian rhythm in behavior but is not apparently involved in the light entrainment; Gpr176−/− animals display a shortened circadian period in constant darkness but their phase-resetting responses to light are normal. Here, we performed microarray analysis and identified enhanced mRNA expression of neuromedin U (Nmu) and neuromedin S (Nms) in the SCN of Gpr176−/− mice. By generating C57BL/6J-backcrossed Nmu/Nms/Gpr176 triple knockout mice, we noted that the mutant mice had a greater magnitude of phase shift in response to early subjective night light than wildtype mice, while Nmu/Nms double knockout mice as well as Gpr176 knockout mice are normal in the phase shifts induced by light. At the molecular level, Nmu−/−Nms−/−Gpr176−/− mice had a reduced induction of Per1 and cFos mRNA expression in the SCN by light and mildly upregulated circadian expression of Per2, Prok2, Rgs16, and Rasl11b. These expressional changes may underlie the phenotype of the Nmu/Nms/Gpr176 knockout mice. Our data argue that there is a mechanism requiring Nmu, Nms, and Gpr176 for the proper modulation of light-induced phase shift in mice. Simultaneous modulation of Nmu/Nms/Gpr176 may provide a potential target option for modulating the circadian clock.
Abstract Gpr19 encodes an evolutionarily conserved orphan G-protein-coupled receptor (GPCR) with currently no established physiological role in vivo. We characterized Gpr19 expression in the suprachiasmatic nucleus (SCN), the locus of the master circadian clock in the brain, and determined its role in the context of the circadian rhythm regulation. We found that Gpr19 is mainly expressed in the dorsal part of the SCN, with its expression fluctuating in a circadian fashion. A conserved cAMP-responsive element in the Gpr19 promoter was able to produce circadian transcription in the SCN. Gpr19 −/− mice exhibited a prolonged circadian period and a delayed initiation of daily locomotor activity. Gpr19 deficiency caused the downregulation of several genes that normally peak during the night, including Bmal1 and Gpr176 . In response to light exposure at night, Gpr19 −/− mice had a reduced capacity for light-induced phase-delays, but not for phase-advances. This defect was accompanied by reduced response of c-Fos expression in the dorsal region of the SCN, while apparently normal in the ventral area of the SCN, in Gpr19 −/− mice. Thus, our data demonstrate that Gpr19 is an SCN-enriched orphan GPCR with a distinct role in circadian regulation and may provide a potential target option for modulating the circadian clock.
