In vivo Characterization of Endocrine Disrupting Chemical Effects via Thyroid Hormone Action Indicator Mouse
October 6th, 2023
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.
Endocrine disruptors are increasingly present pollutants and they can greatly interfere with thyroid hormone economy. This is a major concern since thyroid hormones are important regulators of biological function. One of our major aims is to measure whether specific molecules and their metabolites are able to disrupt thyroid hormone economy in vivo in mammals.
While there are screening methods for such compounds, little is known on how they act in vivo on tissues. This is due to the lack of proper test systems aiming to characterize their tissue-specific effects. Additionally, this would require the quantitative measurement of local thyroid hormone action that is also challenging.
We developed the thyroid hormone action indicator, or THAI, mouse model in order to characterize thyroid hormone action in the tissues. We realized this would be adequate to measure the thyroid-related effects of endocrine disruptors in a tissue-specific manner. Notably, the THAI model would allow us to do so in a living mammalian organism.
The THAI model enables the in vivo detection of endogenous changes in local thyroid hormone action through bioluminescent imaging. This enables the use of crossover designs, follow-up studies, self-control studies, and continuous disruptor exposure. Notably, the THAI model enables us to lower the required number of animals used in the study.
To begin, launch the software compatible with the in vivo imaging system. Log in and wait for the imaging wizard panel to fully load. Click on the Initialize button on the imaging wizard panel to initiate the cooling of the camera.
Then set the temperature of the heating pad between 30 to 37 degrees Celsius to keep the animals warm. Next, gently pinch the foot pads of the anesthetized animals to assess their pedal reflex. Intraperitoneally inject 10 microliters per gram body weight of sodium Luciferin into the shaved animals.
Now align the animal against the camera's center as denoted by the symbol in the imaging instrument. Set the imaging time in the image wizard panel to three minutes for luminescence imaging and select the check boxes for both Photo and Luminescence. Click on Acquire to take the first measurement.
After all measurements have been recorded, convert the image scale to radiance. Now determine the optimal binning and color scale settings for the images and keep it uniform for all images. Then click on ROI tool on the Tool Palette panel and use the Place ROI option to select the areas of interest on the image.
Lastly, click on Measure ROIs and copy the resulting data for exporting into the organizing or statistical software of choice. In vivo imaging of the THAI mice showed that areas that lacked fur, such as foot pads, tail and nose, had higher basal signals. Luciferase signal was influenced by the thyroid hormone status.
Robust signals were observed in the testicular region. The thyroid hormone action was significantly heightened in the brown adipose tissue of cold-stressed THAI mice. The treatment did not impact the luciferase signals from the foot pads and tail.
Imaging studies also captured the accumulation and clearance of diclazuril, an endocrine-disrupting compound, over a three-week period.
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Overview
The Thyroid Hormone Action Indicator (THAI) mouse model was developed to enable tissue-specific quantification of local thyroid hormone action. This model is particularly useful for characterizing the effects of endocrine-disrupting chemicals on thyroid hormone economy through both ex vivo and in vivo methodologies.
Key Study Components
Area of Science
- Endocrinology
- Toxicology
- In vivo Imaging
Background
- Endocrine disruptors can interfere with thyroid hormone economy in mammals.
- Thyroid hormones are crucial regulators of biological functions.
- Existing screening methods for endocrine disruptors have limitations in assessing their tissue-specific effects in vivo.
- A need exists for test systems that can quantitatively measure local thyroid hormone action.
Purpose of Study
- Characterize thyroid hormone action in tissues using the THAI mouse model.
- Measure the tissue-specific effects of endocrine disruptors in vivo.
- Assess the impact of specific molecules and their metabolites on thyroid hormone economy.
Methods Used
- The THAI mouse model is used for in vivo imaging of thyroid hormone action.
- Animals are subjected to bioluminescent imaging to detect changes in local thyroid hormone activity.
- Luciferin is administered intraperitoneally for imaging assessment.
- Monitoring includes imaging time settings and analysis of specific regions of interest (ROIs).
Main Results
- In vivo imaging revealed higher basal luciferase signals in non-furred areas such as foot pads and tail.
- Significant thyroid hormone action was noted in the brown adipose tissue during cold stress.
- It was found that the treatment did not impact luciferase signals from certain areas.
- Imaging studies tracked the accumulation and clearance of the endocrine disruptor diclazuril over three weeks.
Conclusions
- The THAI model enables an in vivo understanding of thyroid hormone dynamics and its disruption by chemicals.
- This model facilitates the characterization of endocrine disruptors’ effects in a living organism, potentially reducing the number of animals needed for studies.
- Implications extend to a better understanding of thyroid hormone regulation and environmental pollutant impacts on mammalian physiology.
