Transcriptional and metabolic effects of glucocorticoid receptor α and β signaling in zebrafish

In humans and zebrafish, two glucocorticoid receptor splice variants exist: the canonical GR α-isoform (GRα), and the GR β-isoform (GRβ). In the present study we have used the zebrafish model system in order to reveal genes affected by each of these two receptor isoforms. By injecting zebrafish embryos with different splice-blocking morpholinos, we could knock down both GR isoforms or could target the alternative splicing of the GR pre-mRNA in favor of the GR β-isoform. In addition, specific GRβ overexpression was achieved by injecting mRNA. Embryos were treated with the synthetic glucocorticoid dexamethasone and transcriptome analysis was performed. Two distinct gene clusters were found that are regulated by GRα. One that was regulated by GRα under basal conditions (presence of endogenous cortisol only), and one that is regulated upon increased activation of GRα (using a pharmacological dose of dexamathasone). GRβ may act as a dominant-negative inhibitor of GRα when GRβ is overexpressed and the GRα expression level is knocked down simultaneously, but without GRα knockdown no evidence for this activity was found. In addition, the data indicate regulation of gene transcription through other mechanisms of action by GRβ. We also investigated the concentrations of several metabolites using NMR spectroscopy. We found that dexamethasone treatment and knockdown of GRα together with overexpression of GRβ had opposite effects on glucose, amino acid and fatty acid levels. Thus, we have shed new light on the molecular mechanisms of glucocorticoid-induced effects on metabolism, which are known to increase the risk of obesity, hyperglycemia and diabetes.