Gpr137b is an orphan G-protein-coupled receptor associated with M2 macrophage polarization

Macrophages are classified mainly into two subtypes, M1 and M2, which exhibit distinct phenotypes, based on their microenvironment. Although recent studies have suggested that G-protein-coupled receptors (GPCRs) are associated with M1/M2 macrophage polarization, available information on GPCR-mediated macrophage polarization is still limited. In the present study, we identified Gpr137b as an orphan GPCR abundantly expressed in RAW264, a mouse macrophage cell line, and illuminated its role in M2 macrophage polarization. We generated Gpr137b-knockout (Gpr137b-KO) clones of RAW264 cells using the CRISPR/Cas9 genome editing system. Two independent Gpr137b-KO clones were isolated, which were demonstrated to have frameshifting 188-nucleotide deletions at a region containing the ATG start codon of Gpr137b. Consistently, qRT-PCR analysis revealed that the deleted region is not transcribed. We then treated the Gpr137b-KO and wildtype RAW264 cells with interleukin-4 (IL-4) to induce M2 macrophage polarization. Microarray analysis revealed that the IL-4-induced gene expression of representative M2 macrophage markers was significantly reduced in the Gpr137b-KO cells, and this was validated by qRT-PCR analysis. By contrast, M1 macrophage marker gene expression induced by lipopolysaccharide was unaffected by Gpr137b-KO. Collectively, the current study shows that Gpr137b is a possible regulator of M2 macrophage polarization.