A Novel Transgenic Model to Study Thyroid Axis Activity in Early Life Stage Medaka

Area of Science:

• Environmental Toxicology
• Endocrinology
• Aquatic Toxicology

Background:

• European regulations mandate identifying endocrine-disrupting chemicals (EDCs).
• Existing Organization for Economic Co-operation and Development (OECD) fish assays for Thyroid axis Active Chemicals (TACs) are lacking.
• Thyroid axis disruption is a key concern for aquatic ecosystems and human health.

Purpose of the Study:

• To develop a novel fish-based assay for detecting Thyroid axis Active Chemicals (TACs).
• To establish a screening tool for endocrine disruptors impacting the Hypothalamic-Pituitary-Thyroid axis.
• To provide a method for deciphering the mechanisms of TACs in medaka.

Main Methods:

• Development of a novel transgenic medaka (Oryzias latipes) line expressing green fluorescent protein (GFP) in thyrocytes.
• Utilizing the thyroglobulin gene promoter to link GFP expression to thyroid activity.
• Exposure of eleuthero-embryonic stage medaka to known thyroid axis activators (T₃, T₄) and inhibitors (PTU, TBBPA) for 72 hours.

Main Results:

• The transgenic line demonstrated inverse fluorescence proportional to thyroid axis activity.
• Thyroid-stimulating hormone (TSH) feedback loop regulation was reflected in thyrocyte size and fluorescence.
• Lowest observable effect concentrations (LOECs) were determined for T₃ (5 μg/L), T₄ (1 μg/L), PTU (8 mg/L), and TBBPA (5 mg/L).

Conclusions:

• The developed medaka assay is a promising tool for rapid screening of Thyroid axis Active Chemicals (TACs).
• This assay can aid in identifying endocrine disruptors and understanding their mechanisms of action.
• The study addresses a critical gap in OECD guidelines for fish-based TAC detection.