Glucocorticoid-Induced Attenuation of the Inflammatory Response in Zebrafish

Functional analysis reveals no transcriptional role for the glucocorticoid receptor β-isoform in zebrafish

In humans, two splice variants of the glucocorticoid receptor (GR) exist: the canonical α-isoform, and the β-isoform, which has been shown to have a dominant-negative effect on hGRα. Previously, we have established the occurrence of a GR β-isoform in zebrafish, and in the present study we have investigated the functional role of the zebrafish GRβ (zGRβ). Reporter assays in COS-1 cells demonstrated a dominant-negative effect of zGRβ but no such effect was observed in zebrafish PAC2 cells using induction of the fk506 binding protein 5 (fkbp5) gene as readout. Subsequently, we generated a transgenic fish line with inducible expression of zGRβ. Transcriptome analysis suggested transcriptional regulation of genes by zGRβ in this line, but further validation failed to confirm this role. Based on these results, its low expression level and its poor evolutionary conservation, we suggest that the zebrafish GR β-isoform does not have a functional role in transcriptional regulation.

Common deregulated gene expression profiles and morphological changes in developing zebrafish larvae exposed to environmental-relevant high to low concentrations of glucocorticoids

Synthetic glucocorticoids have been detected in environmental waters and their biological potency have raised concerns of their impact on aquatic vertebrates especially fish. In this study, developing zebrafish larvae exposed to representative glucocorticoids (dexamethasone, prednisolone and triamcinolone) at 50 pM to 50 nM from 3 h post-fertilisation to 5 days post-fertilisation were investigated. Microarray analysis identified 1255, 1531, and 2380 gene probes, which correspondingly mapped to 660, 882 and 1238 human/rodent homologs, as deregulated by dexamethasone, prednisolone and triamcinolone, respectively. A total of 248 gene probes which mapped to 159 human/rodent homologs were commonly deregulated by the three glucocorticoids. These homologs were associated with over 20 molecular functions from cell cycle to cellular metabolisms, and were involved in the development and function of connective tissue, nervous, haematological, and digestive systems. Glucocorticoid receptor signalling, NRF2-mediated oxidative stress response and RAR signalling were among the top perturbed canonical pathways. Morphological analyses using four transgenic zebrafish lines revealed that the hepatic and endothelial-vascular systems were affected by all three glucocorticoids while nervous, pancreatic and myeloid cell systems were affected by one of them. Quantitative real-time PCR detected significant change in the expression of seven genes at 50 pM of all three glucocorticoids, a concentration comparable to total glucocorticoids reported in environmental waters. Three genes (cry2b, fbxo32, and klhl38b) responded robustly to all glucocorticoid concentrations tested. The common deregulated genes with the associated biological processes and morphological changes can be used for biological inference of glucocorticoid exposure in fish for future studies.

The inflammatory chemokine Cxcl18b exerts neutrophil-specific chemotaxis via the promiscuous chemokine receptor Cxcr2 in zebrafish

Cxcl18b is a chemokine found in zebrafish and in other piscine and amphibian species. Cxcl18b is a reliable inflammatory marker; however, its function is yet to be elucidated. Here, we found that Cxcl18b is chemotactic towards neutrophils, similarly to Cxcl8a/Interleukin-8, the best characterised neutrophil chemoattractant in humans and teleosts. Like Cxcl8a, Cxcl18b-dependent recruitment required the chemokine receptor Cxcr2, while it was unaffected by depletion of the other two neutrophil receptors cxcr1 and cxcr4b. To visualise cxcl18b induction, we generated a Tg(cxcl18b:eGFP) reporter line. The transgene is induced locally upon bacterial infection with the fish pathogen Mycobacterium marinum, but strikingly is not directly expressed by infected cells. Instead, cxcl18b is induced by non-phagocytic uninfected cells that compose the stroma of the granulomas, typical inflammatory lesions formed upon mycobacterial infections. Together, these results suggest that Cxcl18b might be an important contributor to neutrophil chemotaxis in the inflammatory microenvironment and indicate that the zebrafish model could be explored to further investigate in vivo the biological relevance of different Cxcl8-like chemokine lineages.