Eicosanoid-GPCR Signaling Enhances Hematopoiesis and Marrow Transplant

Small molecule treatment of hematopoietic stem cells ex vivo has the potential to expand these cells or increase their engraftability. Previously, we discovered that ex vivo treatment of marrow with 11,12-epoxyeicosatrienoic acid (EET) enhances the engraftment of hematopoietic stem and progenitor cells in both zebrafish and mammals. Additionally, EET treatment promotes specification of HSPC from the hemogenic endothelium, suggesting a broad pro-hematopoietic role of this molecule. Indeed, bioactive lipids play an important role as signaling molecules both during embryo development and adult tissue homeostasis. However, due to their small-molecule nature, identifying their receptors biochemically has been a long-standing challenge which impedes the understanding of the biological processes they regulate. The identity of the EET receptor remains unknown despite more than a decade of research. Here, we utilized a novel bioinformatic approach to identify candidate EET receptors and identified a candidate functional in cell culture, zebrafish and mouse assays.

EET signaling is known to be G-protein dependent, suggesting its receptor is a G-protein coupled receptor (GPCR). We performed RNAseq on U937 monocytes, EaHy endothelial cells, and PC3M-LN4 prostate cancer cells, three human cell lines with clear EET-responsive phenotypes. These three cell lines expressed 37 GPCR in common at a basal level of greater than or equal to 0.3 fragments per kilobase per million reads (FPKM). 27 of these GPCR were also expressed in a non-EET-responsive cell line, HEK293, leaving only 10 candidate EET receptors. We screened 7 of these candidates for EET-responsiveness using a cell-culture based β-arrestin recruitment assay. Of these, only GPR132 exhibited EET-dependent recruitment of β-arrestin to the cell membrane, indicating GPCR activation. GPR132 was previously identified as a receptor for a variety of small oxygenated fatty acids, and we confirmed that these related molecules induce GPR132-dependent β-arrestin recruitment. We additionally treated developing zebrafish embryos with these molecules. Like EET, these GPR132 ligands increased HSPC numbers in the zebrafish aorta-gonad-mesonephros (AGM) and caused ectopic expression of the HSPC marker runx1 in the zebrafish tail, a phenotype that was previously seen only with EET treatment. To test the requirement of GPR132 for EET signaling, we knocked down the zebrafish ortholog of GPR132 by morpholino injection, which prevented the EET-induced increase of runx1in both the AGM and tail. Finally, we performed competitive whole bone marrow transplant using wildtype and GPR132-/- mice as donors and found that while treatment with EET increases engraftment of WT donor cells, no such improvement is seen in GPR132-/- cells. GPR132 is thus required in both zebrafish and mice for EET phenotypes.

Combining bioinformatic, biochemical, and genetic approaches, we identified GPR132 as a receptor for EET involved in regulating hematopoiesis and marrow transplant. GPR132 thus represents a therapeutic target for the enhancement of hematopoietic stem cell transplant, and genetic manipulation of GPR132 could help illuminate the endogenous roles of its fatty acid ligands.