Discovery of a functional glucocorticoid receptor beta-isoform in zebrafish

In humans, two glucocorticoid receptor (GR) splice variants exist: GRalpha and GRbeta, which are identical between amino acids 1-727 and then diverge. Whereas GRalpha (the canonical GR) acts as a ligand-activated transcription factor, GRbeta does not bind traditional glucocorticoid agonists, lacks GRalpha's transactivational activity, and acts as a dominant-negative inhibitor of GRalpha. It has been suggested that this receptor isoform is involved in the induction of glucocorticoid resistance in asthma patients. Unfortunately, a GR beta-isoform has been detected in only humans, and therefore, an animal model for studies on this isoform is lacking. In the present study, we demonstrate that in zebrafish a GR isoform exists that diverges from the canonical zebrafish GR at the same position as human GRbeta from human GRalpha. The zebrafish GR beta-isoform acts as a dominant-negative inhibitor in reporter assays, and the extent of inhibition and the effective GRalpha/GRbeta ratio is similar to studies performed with the human GR isoforms. In addition, the subcellular localization of zebrafish GRbeta is similar to its human equivalent. Finally, expression levels of GRalpha and GRbeta were determined in adult zebrafish tissues and at several developmental stages. Both receptor isoforms were detected throughout the body, and GRbeta mRNA levels were relatively low compared with GRalpha mRNA levels, as in humans. Thus, for the first time, a GR beta-isoform has been identified in a nonhuman animal species, shedding new light on the relevance of this GR splice variant and providing a versatile animal model for studies on the GR system.