ORF48-07 - Formation of Mineralocorticoid and Glucocorticoid Receptor Heterodimers Results in Dissociative Corticosteroid Signalling

The steroid hormone cortisol plays a crucial role in our response to stress, influencing various physiological systems. These effects are mediated by two receptors: the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). Both receptors function as ligand-activated transcription factors, binding as homodimers to response elements in target genes. Due to their structural similarity, MR and GR can also heterodimerize; however, the physiological relevance of such dimerization events remains largely unknown. In our recent work using GR and MR knockout zebrafish, we uncovered increasing evidence of MR/GR co-regulation of cortisol-mediated anti-inflammatory action. To investigate whether MR/GR heterodimers, specifically, mediate cortisol action during inflammation, we developed mutant receptors that could selectively form homo- or heterodimers and expressed these receptors in zebrafish larvae from an MR/GR double knockout line. Indeed, cortisol’s inhibition of macrophage migration during inflammation depended on heterodimer formation. These results are distinct from the regulation of metabolic endpoints by cortisol, where elevated glucose and muscle wasting are primarily mediated by GR homodimers. Additionally, cortisol-induced hyperactive behaviour was also regulated by heterodimers. To identify molecular targets of MR/GR heterodimers, we next performed RNA-sequencing. Our findings showed that the homodimers and heterodimers regulate distinct gene expression patterns. For example, in the context of behaviour, genes involved in glutamatergic signalling were regulated only by heterodimers. Subsequent mutagenesis of the glucocorticoid response element in the gene encoding the metabotropic glutamate receptor 3 resulted in an abolishment of the cortisol-induced hyperactivity, supporting heterodimer-induced regulation. In summary, our research not only establishes the physiological relevance of MR/GR heterodimerization in vivo but also highlights a potential mechanism of dissociative corticosteroid signalling.