In vivo Characterization of Endocrine Disrupting Chemical Effects via Thyroid Hormone Action Indicator Mouse

Summary

The Thyroid Hormone Action Indicator mouse model was developed to enable tissue-specific quantification of local thyroid hormone action using its endogenous regulatory machinery. Recently, it has been shown that the model is suitable for characterizing endocrine-disrupting chemicals interacting with thyroid hormone economy, both by ex vivo and in vivo methodologies.

Abstract

Thyroid hormones (TH) play a critical role in cell metabolism and tissue function. TH economy is susceptible to endocrine disrupting chemicals (EDCs) that can disturb hormone production or action. Many environmental pollutants are EDCs, representing an emerging threat to both human health and agricultural production. This has led to an increased demand for proper test systems to examine the effects of potential EDCs. However, current methodologies face challenges. Most test systems use endogenous markers regulated by multiple, often complex regulatory processes, making it difficult to distinguish direct and indirect effects. Moreover, in vitro test systems lack the physiological complexity of EDC metabolism and pharmacokinetics in mammals. Additionally, exposure to environmental EDCs usually involves a mixture of multiple compounds, including in vivo generated metabolites, so the possibility of interactions cannot be ignored. This complexity makes EDC characterization difficult. The Thyroid Hormone Action Indicator (THAI) mouse is a transgenic model that carries a TH-responsive luciferase reporter system, enabling the assessment of tissue-specific TH action. One can evaluate the tissue-specific effects of chemicals on local TH action by quantifying luciferase reporter expression in tissue samples. Furthermore, with in vivo imaging, the THAI mouse model allows for longitudinal studies on the effects of potential EDCs in live animals. This approach provides a powerful tool for testing long-term exposure, complex treatment structures, or withdrawal, as it enables the assessment of changes in local TH action over time in the same animal. This report describes the process of in vivo imaging measurements on THAI mice. The protocol discussed here focuses on developing and imaging hyper- and hypothyroid mice, which can serve as controls. Researchers can adapt or expand the treatments presented to meet their specific needs, offering a foundational approach for further investigation.

Introduction

Thyroid hormone (TH) signaling is a fundamental regulator of cellular metabolism, essential for normal development and optimal tissue function in adulthood¹. Within tissues, TH action is finely controlled by a complex molecular machinery, allowing for tissue-specific maintenance of local TH levels. This autonomy of different tissues from circulating TH levels is of great importance^2,3,4.

Numerous chemicals have the potential to disrupt endocrine functions and are found in the environment as pollutants. It is a growing concern that these molecules can enter the food chain through wastewater and agricultural production, thereby impacting the health of livestock and humans^5,6,7.

One of the significant challenges in addressing this issue is the sheer number of compounds involved, including both authorized and already banned, but still persistently present, molecules. In recent years, substantial efforts have been made to develop test systems for screening and identifying the disruptive potential of various chemicals^8,9,10,11. While these methods excel in high-throughput screening of thousands of compounds and identifying potential threats, a detailed analysis of specific in vivo effects of these molecules is essential to establish the hazards of human exposure. Thus, a multifaceted approach is necessary when studying and characterizing endocrine-disrupting chemicals (EDCs).

In the context of TH regulation, understanding the tissue-specific consequences of EDC exposure requires quantifying local TH action. Although several in vivo models have been developed for this purpose, most rely on endogenous markers as their output measure. Despite being physiological, these markers are subject to numerous regulatory mechanisms, both direct and indirect, making their interpretation more challenging. Therefore, characterizing EDC effects on TH regulation at the tissue level remains a significant challenge^12,13.

To address the challenges of measuring tissue-specific TH signaling, the Thyroid Hormone Action Indicator (THAI) mouse model was recently developed. This model allows for specific quantification of changes in local TH action under endogenous conditions. A luciferase transgene was introduced into the mouse genome, which is highly sensitive to regulation by TH action¹⁴. This model has demonstrated effectiveness in answering various research questions that require quantifying changes in local tissue TH signaling^{14,15,16,17,18}.

Recognition of one potential use of the THAI model is characterizing tissue-specific effects of EDCs on TH signaling. The model has recently been employed successfully to investigate the tissue-specific effects of tetrabromobisphenol A and diclazuril on TH signaling¹⁵. Here, baseline protocols are presented for utilizing in vivo imaging techniques on the THAI model as a test system for characterizing EDCs that disrupt TH function. This method leverages the bioluminescent nature of the luciferin-luciferase reaction. Essentially, the transgenically expressed luciferase enzyme catalyzes the oxidation of administered luciferin, generating luminescent light proportional to the amount of luciferase in the tissue (Figure 1). Consequently, the biological response measured is luciferase activity, which has been validated as a suitable measure of local TH action¹⁴. While the THAI model is applicable for quantifying TH action in virtually all tissues, in vivo imaging primarily focuses on TH action in the small intestine (ventral imaging) and the interscapular brown adipose tissue (BAT, dorsal imaging)¹⁴.

A significant advantage of the in vivo imaging technique is that it eliminates the need to sacrifice animals for measurements. This allows investigators to design longitudinal and follow-up experiments as self-controlled studies, reducing between-subjects bias and the number of animals used. This aspect is particularly crucial in EDC characterization, and the method's strength and versatility for this purpose have been previously demonstrated^14,15.

Protocol

The present protocol was reviewed and approved by the Animal Welfare Committee at the Institute of Experimental Medicine (PE/EA/1490-7/2017, PE/EA/106-2/2021). The presented data is from FVB/Ant background14, 3-month-old male THAI mice (n = 3-6/group). FVB/Ant background THAI animals tend to have highly pigmented spots on their skin that may distort measurements. Hence, search for pigmented spots on the skin of the imaged area after fur removal. Animals do not require special housing conditions un…

Representative Results

Generally, the measured radiance ranges from magnitudes of 105 to 1010 p/s/cm2/sr. However, exact values can vary among animals within the same image and across different images. Therefore, comparing raw data might be misleading. It's crucial to establish control and background signals in all experiments, making self-controlled designs highly recommended. Figure 2</strong…

Discussion

The threats posed by Endocrine-Disrupting Chemicals (EDCs) to human health are well recognized; however, research on EDCs faces formidable challenges. These challenges are partially a consequence of the complexity of the endocrine system. Many EDCs have been identified to simultaneously disrupt multiple endocrine systems²². Additionally, in the context of Thyroid Hormone (TH) economy, there exists an additional layer of complexity due to tissue-specific differences in regulating TH action. This co…

Materials

3,5,3'-triiodothyronine (T3)	Merck	T2877
Animals, mice			THAI mouse
Eye protection gel	Oculotect		1000 IU/g
Falcon tube	Thermo Fisher Scientific		50 mL volume
Iodine-free chow diet	Research Diets	custom
IVIS Lumina II in vivo imaging system	Perkin Elmer	–
Ketamine	Vetcentre	E1857
Living Image software 4.5	Perkin Elmer	–	provided with the instrument
Measuring cylinder			250 mL
methimazole	Merck	M8506
Microfuge tubes	Eppendorf		For diluting treatment materials
NaClO4	Merck	71852
Na-luciferin, substrate	Goldbio	103404-75-7
NaOH	Merck	101052833
Phoshphate buffer saline	Chem Cruz	sc-362302
Pipette	Gilson		For diluting treatment materials
Pipette tips	Axygen		For diluting treatment materials
Shaving cream/epilator/shaver			Personal preference
Syringe	B Braun		1 mL volume
Syringe needle	B Braun		0.3 x 12 mm
Xylazine	Vetcentre	E1852