Abstract
Estrogens act by binding to estrogen receptors alpha and beta (ERα, ERβ), ligand-dependent transcription factors that play crucial roles in sex differentiation, tumor growth and cardiovascular physiology. Estrogens also activate the G protein-coupled estrogen receptor (GPER), however the function of GPER in vivo is less well understood. Here we find that GPER is required for normal heart rate in zebrafish embryos. Acute exposure to estrogens increased heart rate in wildtype and in ERα and ERβ mutant embryos but not in GPER mutants. GPER mutant embryos exhibited reduced basal heart rate, while heart rate was normal in ERα and ERβ mutants. We detected gper transcript in discrete regions of the brain and pituitary but not in the heart, suggesting that GPER acts centrally to regulate heart rate. In the pituitary, we observed gper expression in cells that regulate levels of thyroid hormone triiodothyronine (T3), a hormone known to increase heart rate. Compared to wild type, GPER mutants had reduced levels of T3 and estrogens, suggesting pituitary abnormalities. Exposure to exogenous T3, but not estradiol, rescued the reduced heart rate phenotype in gper mutant embryos, demonstrating that T3 acts downstream of GPER to regulate heart rate. Using genetic and mass spectrometry approaches, we find that GPER regulates maternal estrogen levels, which are required for normal embryonic heart rate. Our results demonstrate that estradiol plays a previously unappreciated role in the acute modulation of heart rate during zebrafish embryonic development and suggest that GPER regulates embryonic heart rate by altering maternal estrogen levels and embryonic T3 levels.
Highlights
Zebrafish are an established model for human cardiovascular development and function [1] with conserved estrogen signaling [2,3,4]
Using zebrafish with mutations in genes that respond to estrogens, we found that heart rate is regulated not by the typical molecules that respond to estrogens–the nuclear estrogen receptors–but
Our results are consistent with previous results in cultured cells demonstrating that progesterone binds G protein-coupled estrogen receptor (GPER) with less than 0.01% binding affinity compared to E2 [23]
Summary
Zebrafish are an established model for human cardiovascular development and function [1] with conserved estrogen signaling [2,3,4]. Studies using GPER-deficient mice implicate GPER in ventricular hypertrophy [16], regulation of blood pressure and vascular tone [17, 18] and atherosclerosis progression [19], but whether nuclear ER signaling is required for GPER function in these contexts is unknown. These studies examined GPER function in adult animals, while the role of GPER during embryonic development is not well understood. We use zebrafish embryos, an established model of human development, to reveal a new function for GPER during cardiovascular development
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