Development of the corticosteroid stress axis and receptor expression in zebrafish

Abstract

Using zebrafish embryos and larvae, we examined the temporal patterns of cortisol and expression of genes involved in corticosteroid synthesis and signaling. Embryonic cortisol levels decreased approximately 70% from 1.5 h postfertilization (hpf) to hatch (approximately 42 hpf) and then increased 27-fold by 146 hpf. The mRNA abundances of steroidogenic acute regulatory protein, 11beta-hydroxylase and 11beta-hydroxysteroid dehydrogenase type 2, increased severalfold after hatch and preceded the rise of cortisol levels. In contrast to other teleosts that possess two glucocorticoid receptors (GRs) and one mineralocorticoid receptor (MR), only one GR and MR were identified in zebrafish, which were cloned and sequenced. GR mRNA abundance decreased from 1.5 to 25 hpf, rebounded, and then was stable from 49 to 146 hpf. MR transcripts increased continuously from 1.5 hpf and were 52-fold higher by 97 hpf. An acute cortisol response to a stressor was not detected until 97 hpf, whereas melanocortin type 2 receptor mRNA increased between 25 and 49 hpf. Collectively, the patterns of cortisol and the expression of cortisol biosynthetic genes and melanocortin type 2 receptor suggest that the corticoid stress axis in zebrafish is fully developed only after hatch. The temporal differences in GR, MR, and 11beta-hydroxysteroid dehydrogenase type 2 gene expression lead us to propose a key role for MR signaling by maternal cortisol during embryogenesis, whereas cortisol secretion after hatch may be regulating GR expression and signaling in zebrafish.

The knockdown of maternal glucocorticoid receptor mRNA alters embryo development in zebrafish

“…Maternal gr mRNA was available for translation until 8 hpf and was no longer detectable by WMISH at 9 hpf. During this period, embryo cortisol content is still high (Alsop and Vijayan, 2008) and presumably bindable by Gr. On the other hand, zygotic gr transcription is barely detectable at 6 hpf, but steadily progresses from 8 hpf onwards, reaching gr mRNA pre-cleavage level at 48 hpf (Pikulkaew et al, 2010).…”

“…On the other hand, zygotic gr transcription is barely detectable at 6 hpf, but steadily progresses from 8 hpf onwards, reaching gr mRNA pre-cleavage level at 48 hpf (Pikulkaew et al, 2010). Maternal cortisol is already halved around 15 hpf reaching a permanent minimum at 25 hpf, while interrenal steroidogenic enzymes and StAR protein are expressed only after hatching (48 hpf) to significantly raise larval cortisol at 75 hpf (Alsop and Vijayan, 2008). This may partly explain why the phenotype of gr splic MO larvae was essentially normal at 72 hpf.…”

“…The content at 1.5 h postfertilization (hpf) is around 10 pg/egg [recalculated from Alsop and Vijayan (2008) on the basis of measured 40 mg protein/egg] under standard culture conditions, but may be higher if parent females are in a stressful state. This initial deposit is rather labile, declining by 70% at 25 hpf and remaining at this low level until hatching at 48 hpf, when it is replaced by larval cortisol onwards (Alsop and Vijayan, 2008).…”

“…Owing to this instability, it was proposed that maternal Gr signaling is not critical during zebrafish embryogenesis and that rather maternal cortisol action might be mediated by the mineralocorticoid receptor (Mr), which, at lower levels, shows a progressive expression pattern of its mRNA and has equivalent affinity for cortisol (Alsop and Vijayan, 2008). The latter transcript, however, is minimal until 12 hpf and does not seem to be a good alternative to maternal gr mRNA (Pikulkaew et al, 2010).…”

Morphological alterations and acetylcholinesterase and monoamine oxidase inhibition in liver of zebrafish exposed to Aphanizomenon flos-aquae DC-1 aphantoxins

“…Zebrafish synthesize most typical vertebrate neurotransmitters and its neuroendocrine system demonstrates vigorous physiological responses to stress. It also has the classic advantages of small size, low cost, easy reproduction, a short breeding cycle, high egg production, external fertilization and development, transparent embryos and a well-documented genetic background (Panula et al, 2006;Alsop and Vijayan, 2008;Dai et al, 2014). Previous investigations showed PSPs may enter fish through the consumption of PSP-contaminated foodstuffs (Jiang et al, 2007).…”