Endocrine Abstracts

The mineralocorticoid receptor (MR) is primarily involved in osmoregulation in mammals, with additional roles of brain-behaviour implicated. However, the understanding of this role is limited, partly due to the mortality of MR-knockout mice due to impaired Na⁺ reabsorption. Many steroidogenesis pathways and hormone receptors are highly conserved in zebrafish, providing a great potential to become a high-throughput model for translational endocrine research. My project is to characterise the role of the MR in zebrafish and evaluate whether zebrafish are an appropriate non-mammalian model of mineralocorticoid-resistance. I have created a viable zebrafish mutant line carrying a constitutive loss-of-function mutation in mr using CRISPR-Cas9 technology. Behavourial assays show an abnormal behavioural phenotype, with a significant increase in locomotion activity in the dark periods of standard dark/light interval assays; a potential output for high-throughput in vivo drug screening. Wholemount in situ hybridisation on 5 day-old zebrafish larvae showed a reduced expression of a transcriptional regulator of neurogenesis, neurod1, in mr homozygous mutants compared to wildtype sibling controls. In wildtype zebrafish, we showed differential expression of mr, the glucocorticoid receptor (gr) and 11hsdb2 during zebrafish development and between adult organs using qRT-PCR. The wildtype zebrafish brain exhibited a higher mr expression than osmoregulatory organs such as the gills and kidney. In the adult zebrafish brain, mr expression was localised at the periventricular gray zone of optic tectum, area with high proliferative cells that contribute to neuronal and glial lineages. Whilst in mammals the MR is primarily involved in the RAAS pathway to regulate electrolyte balance and blood volume, in zebrafish it appears to have an important role in the brain, affecting both behaviour and neuronal development. This zebrafish model of mineralocorticoid-resistance may provide further insights into the MR’s role in the brain and behaviour, the stress response and osmoregulation.