Ferredoxin 1B (FD×1B) is the essential mitochondrial redox partner for cortisol biosynthesis in zebrafish

Abstract

Mitochondrial cytochrome P450 (CYP) enzymes rely on electron transfer from the redox partner ferredoxin 1 (FDX1) for catalytic activity. Key steps in steroidogenesis require mitochondrial CYP enzymes and FDX1. Over 30 ferredoxin mutations have been explored in vitro; however, no spontaneously occurring mutations have been identified in humans leaving the impact of FDX1 on steroidogenesis in the whole organism largely unknown. Zebrafish are an important model to study human steroidogenesis, because they have similar steroid products and endocrine tissues. This study aimed to characterize the influence of ferredoxin on steroidogenic capacity in vivo by using zebrafish. Zebrafish have duplicate ferredoxin paralogs: fd×1 and fd×1b. Although fd×1 was observed throughout development andinmost tissues, fd×1bwas expressed after development of the zebrafish interrenal gland (counterpart to the mammalian adrenal gland). Additionally,fd×1b was restricted to adult steroidogenic tissues, such as the interrenal, gonads, and brain, suggesting that fd×1b was interacting with steroidogenic CYP enzymes. By using transcription activator-like effector nucleases, we generated fd×1b mutant zebrafish lines. Larvae with genetic disruption of fd×1b were morphologically inconspicuous. However, steroid hormone analysis by liquid chromatography tandem mass spectrometry revealed fd×1b mutants failed to synthesize glucocorticoids. Additionally, these mutants had an up-regulation of the hypothalamus-pituitary-interrenal axis and showed altered dark-light adaptation, suggesting impaired cortisol signaling. Antisense morpholino knockdown confirmed Fdx1b is required for de novo cortisol biosynthesis. In summary, by using zebrafish, we generated a ferredoxin knockout model system, which demonstrates for the first time the impact of mitochondrial redox regulation on glucocorticoid biosynthesis in vivo.