New insights into SREB orphan receptor roles in the ovary, using comparative transcriptomics across 3 fishes

Superconserved receptors expressed in brain (SREB) are a family of orphan G protein-coupled receptors with 3 members in most vertebrates (GPR27 or SREB1, GPR85 or SREB2, and GPR173 or SREB3). They are associated with diverse physiological processes, ranging from glucose homeostasis to ovarian development. Despite a lack of confirmed ligands, our understanding of these receptors has increased based on interactions with putative SREB3 ligands such as phoenixin (PNX) and the development of synthetic SREB1 agonists (A8535 and PT-91). However, to date no studies have used these agonists in any complex vertebrate system. The objective of this study was to compare the in vitro transcriptomic responses to PNX-20, A8535, and PT-91 in the ovaries of 3 fishes with different SREB systems: (1) zebrafish (Danio rerio) that exhibit similar receptors to mammals, (2) mummichog (Fundulus heteroclitus) that lost a receptor but gained a fish-specific member, and (3) pufferfish (Dichotomyctere nigroviridis) that exhibit all 4 receptors. Agonist interactions with fish SREB1s were confirmed using in silico characterizations. Zebrafish exhibited the strongest transcriptome response, including agonist-induced changes in oxidative phosphorylation and cell adhesion pathways. Mummichog exhibited a less robust response, possibly related to receptor evolutionary divergence, while pufferfish exhibited an intermediate transcriptome response. Short-term agonist exposure did not alter ovarian steroid levels ([17]estradiol, 17-hydroxyprogesterone, cortisol, testosterone, or 11-ketotestosterone). Collectively, the data reveal previously unrecognized developmental, metabolic, neural, and vascular gene networks associated with SREB activation and suggest both receptor-specific signaling and functional overlap within this family.