Expression and activity profiling of the steroidogenic enzymes of glucocorticoid biosynthesis and the <italic>fdx1</italic> co‐factors in zebrafish.

The spatial and temporal expression of steroidogenic genes in zebrafish has not been fully characterised. Because zebrafish are increasingly employed in endocrine and stress research, a better characterisation of steroidogenic pathways is required to target specific steps in the biosynthetic pathways. In the present study, we have systematically defined the temporal and spatial expression of steroidogenic enzymes involved in glucocorticoid biosynthesis (<italic>cyp21a2</italic>,<italic> cyp11c1</italic>,<italic> cyp11a1</italic>,<italic> cyp11a2</italic>,<italic> cyp17a1</italic>,<italic> cyp17a2</italic>,<italic> hsd3b1</italic>,<italic> hsd3b2</italic>), as well as the mitochondrial electron‐providing ferredoxin co‐factors (<italic>fdx1</italic>,<italic> fdx1b</italic>), during zebrafish development. Our studies showed an early expression of all these genes during embryogenesis. In larvae, expression of <italic>cyp11a2</italic>,<italic> cyp11c1</italic>,<italic> cyp17a2</italic>,<italic> cyp21a2</italic>,<italic> hsd3b1</italic> and <italic>fdx1b</italic> can be detected in the interrenal gland, which is the zebrafish counterpart of the mammalian adrenal gland, whereas the <italic>fdx1</italic> transcript is mainly found in the digestive system. Gene expression studies using quantitative reverse transcriptase‐PCR and whole‐mount <italic>in situ</italic> hybridisation in the adult zebrafish brain revealed a wide expression of these genes throughout the encephalon, including neurogenic regions. Using ultra‐high‐performance liquid chromatography tandem mass spectrometry, we were able to demonstrate the presence of the glucocorticoid cortisol in the adult zebrafish brain. Moreover, we demonstrate <italic>de novo</italic> biosynthesis of cortisol and the neurosteroid tetrahydrodeoxycorticosterone in the adult zebrafish brain from radiolabelled pregnenolone. Taken together, the present study comprises a comprehensive characterisation of the steroidogenic genes and the <italic>fdx</italic> co‐factors facilitating glucocorticoid biosynthesis in zebrafish. Furthermore, we provide additional evidence of <italic>de novo</italic> neurosteroid biosynthesising in the brain of adult zebrafish facilitated by enzymes involved in glucocorticoid biosynthesis. Our study provides a valuable source for establishing the zebrafish as a translational model with respect to understanding the roles of the genes for glucocorticoid biosynthesis and <italic>fdx</italic> co‐factors during embryonic development and stress, as well as in brain homeostasis and function. [ABSTRACT FROM AUTHOR]