Subacute microcystin-LR exposure alters the metabolism of thyroid hormones in juvenile zebrafish (Danio Rerio).

Zidong Liu,Dapeng Li,Rong Tang,Ying Wang,Qing Hu

doi:10.3390/toxins7020337

Abstract

Microcystin-LR (MC-LR) has been detected extensively in the aquatic environment and has the potential to disturb the thyroid endocrine system. However, limited information is available on the effects of subacute MC-LR exposure on fish thyroid hormone (TH) metabolism. In the present study, juvenile zebrafish (Danio rerio) were exposed to MC-LR at environmentally relevant concentrations (0, 1, 5, and 25 μg/L) for 28 days. Whole-body TH content and thyroid follicle histology were used as direct endpoints to assess thyroid disruption. The activities of iodothyronine deiodinases (IDs) and the transcription of selected genes associated with TH synthesis were also investigated to study the underlying mechanisms of endocrine disruption. Exposure of zebrafish to MC-LR significantly increased whole-body thyroxine (T4) content but decreased whole-body triiodothyronine (T3) content. We also observed hypertrophy and hyperplasia of the thyroid follicle epithelial cells, as well as up-regulation of corticotropin-releasing hormone (CRH), thyroid-stimulating hormone (TSH), thyroid peroxidase (TPO), and transthyretin (TTR) genes. The decreases in ID1 and ID2 activities coupled with an increase in ID3 activity were observed in MC-LR treatment groups. These results demonstrate that exposure to MC-LR at environmental concentrations results in the disturbance of TH homeostasis by disrupting the synthesis and conversion of THs.

Highlights

Cyanobacterial blooms occur constantly worldwide and have been regarded as a serious environmental issue [1]
Whole-body T4 concentration was significantly increased in the 25 μg/L MC-LR exposure group as compared to the control group after 14 days of exposure
There were no significant differences in the FT4 levels among MC-LR treatment groups (Figure 1B)

Summary

Introduction

Cyanobacterial blooms occur constantly worldwide and have been regarded as a serious environmental issue [1]. The microcystins (MCs) are the most commonly identified toxins in freshwater blooms [2]. Despite the identification of nearly 80 structurally different MCs, microcystin-LR (MC-LR). Is recognized as being the most toxic and is distributed worldwide in freshwater environments [3,4]. A few studies have reported that maximal dissolved MCs concentration reached to 78 μg/L [5], the environmental concentration of dissolved MCs during most cyanobacterial blooms in lakes usually range from 0.1 to 10 μg/L [6]. Fish are exposed to MCs through ingestion of cyanobacterial cells or through contact with the surrounding water passively [7]

Methods

Results

Discussion

Conclusion