GPR17 is an Essential Component of the Negative Feedback Loop of the Sonic Hedgehog Signalling Pathway in Neural Tube Development

Abstract

Dorsal-ventral pattern formation of the neural tube is regulated by temporal and spatial activities of extracellular signalling molecules. Sonic hedgehog (Shh) assigns ventral neural subtypes via activation of the Gli transcription factors. Shh activity changes dynamically during neural differentiation, but the mechanisms responsible for regulating this dynamicity are not fully understood. Here we show that the P2Y-type G-protein coupled receptor GPR17 is involved in temporal regulation of the Shh signal. GPR17 was expressed in the ventral progenitor regions of the neural tube and acted as a negative regulator of the Shh signal in chick embryos. While the activation of the GPR17-related signal inhibited ventral identity, perturbation of GPR17 expression led to aberrant expansion of ventral neural domains. Notably, perturbation of GPR17 expression partially inhibited the negative feedback of Gli activity. Moreover, GPR17 increased cAMP activity, suggesting that it exerts its function by inhibiting the processing of Gli3 protein. GPR17 also negatively regulated Shh signalling in neural cells differentiated from mouse embryonic stem cells, suggesting that GPR17 function is conserved among different organisms. Our results demonstrate that GPR17 is a novel negative regulator of Shh signalling in a wide range of cellular contexts.\n\nAuthor SummaryDuring neural development, determination of cell fate and the progress of differentiation are regulated by extracellular signal molecules, including Sonic Hedgehog (Shh). Shh forms a gradient within the embryonic organ of the central nervous system, or the neural tube, and a variety of cells are produced corresponding to the concentration. While the signal concentration is critical for cell fate, recent studies have revealed that the intracellular signal intensity does not always correspond to the Shh concentration. Rather, the intracellular signal intensity changes over time. Importantly, the signal intensity peaks and gradually decreases thereafter, and the half-life of the Shh signal contributes to the cell fate determination. However, the mechanisms for this temporal change are not fully understood.\n\nBy using chick embryos and mouse embryonic stem cells as model systems, we demonstrate that the G-protein coupled receptor, GPR17, is an essential regulator for the negative feedback of the Shh signal during neural development. While GPR17 gene expression is induced by the Shh signal, GPR17 perturbs the Shh signalling pathway. This negative function of GPR17 on the Shh signal is conserved among different vertebrate species. The collective data demonstrate that GPR17 is a negative regulator for the Shh signalling pathway in a wide range of the cellular contexts.