Inactivation of G-protein-coupled Receptor 48 (Gpr48/Lgr4) Impairs Definitive Erythropoiesis at Midgestation through Down-regulation of the ATF4 Signaling Pathway

G-protein-coupled receptors (GPCRs), one of the most versatile groups of cell surface receptors, can recognize specific ligands from neural, hormonal, and paracrine organs and regulate cell growth, proliferation, and differentiation. Gpr48/LGR4 is a recently identified orphan GPCR with unknown functions. To reveal the functions of Gpr48 in vivo, we generated Gpr48(-/-) mice and found that Gpr48(-/-) fetuses displayed transient anemia during midgestation and abnormal definitive erythropoiesis. The dramatic decrease of definitive erythroid precursors (Ter119(pos) population) in Gpr48(-/-) fetal liver at E13.5 was confirmed by histological analysis and blood smear assays. Real-time PCR analyses showed that in Gpr48(-/-) mice both adult hemoglobin alpha and beta chains were decreased while embryonic hemoglobin chains (zeta, beta H1, and epsilon y) were increased, providing another evidence for the impairment of definitive erythropoiesis. Furthermore, proliferation was suppressed in Gpr48(-/-) fetal liver with decreased c-Myc and cyclin D1 expression, whereas apoptosis was unaffected. ATF4, a key transcription factor in erythropoiesis, was down-regulated in Gpr48(-/-) fetal livers during midgestation stage through the cAMP-PKA-CREB pathway, suggesting that Gpr48 regulated definitive erythropoiesis