Scientists from the Universite´ Libre de Bruxelles, Belgium in collaboration with scientists from the Lillehei Heart Institute at the University of Minnesota, University of Chicago, and Ghent University, in Merelbeke, Belgium have differentiated engineered mouse embryonic stem cells into thyroid cells that make thyroid hormone, organize themselves into a thyroid, and even rescue thyroid deficient mice.
In a paper published in the international journal Nature, lead author Francesco Antonica and her colleagues used mouse embryonic stem cells for these experiments. Antonica and others engineered these cells to express two transcription factors; NKX2-1 and PAX8. They used a trick to engineer these cells so that they would only express these genes if they were treated with the drug doxycycline. A variety of experiments showed that the genetic manipulation of the cells did not affect their pluripotency.
After genetically engineering their mouse embryonic stem cells, they grew half of them without doxycycline and the other half in the presence of doxycycline. Three days after growing cells on doxycycline, the cells expressed high levels of Pax8 and NKX2-1, and also showed high levels of expression of thyroid-specific genes such as thyroid-stimulating hormone (TSH) receptor (Tshr), the sodium/iodide symporter NIS (Slc5a5) and thyroglobulin (Tg), as well as Foxe1, which is yet another key transcription factor for thyroid development. In contrast, the cultures without doxycycline showed no such changes in gene expression.
These cells, however, did not stop there. 22 days after being grown on doxycycline, the cells rounded up and formed clusters that exactly resemble those found in a living thyroid gland. The resemblance to thyroid glands, however, was not superficial. Thyroid-specific proteins were detected in these clusters. Those cells formed a circle that surrounded a space and it also showed proper localization of thyroid-specific proteins. There is a protein found on the bottom of the thyroid celll called NIS, which stands for sodium/iodide symporter. This proteins transports two sodium cations (Na+) for each iodide anion (I–) into thyroid cells. The uptake of iodide into follicular cells of the thyroid gland is the first step in the synthesis of thyroid hormone. Another protein found at the bottom of thyroid cells called E-cadherin helps the cells stick to each other.
These ESC-derived thyroid cells also express E-cadherin at the bottom of the cell. Also, at the other end of the cell (the apical end) thyroid cells express a protein called zona occludens 1 (ZO-1). These ESC-derived thyroid cells also express ZO-1 at the top of the cell. Finally, thyroglobulin, which is the precursor version of thyroid hormone was also expressed in these cells and was also found in the space at the center of the cell clusters – just like in a thyroid gland.
So, it looks like a thyroid, it makes the same genes as a thyroid, but is it a thyroid functionally speaking? To answer this question, Antonica and colleagues took normal mice and feed them radioactive iodine. This destroys the thyroid and they were able to confirm that these mice were devoid of thyroid activity and showed the symptoms of hypothyroidism. Then they grafted their ESC-derived thryoid cells into the kidney capsule of the hypothyroid mice. The grafts took and tissue examination showed that the grafts looked like thyroid tissue and also expressed thyroid-specific proteins. Therefore, transplantation of the ESC-derived thyroid tissue does not change its characteristics.
Amazingly, 8 or 9 hypothyroid mice that had received the grafts recovered full thyroid function. Those hypothyroid mice transplanted with ESCs that were not grown in the presence of doxycycline showed no signs of recovery from hypothyroidism.
These experiments show that ESCs can be differentiated into thyroid follicles that can serve as an excellent model for thyroid physiology and development. Such a model system can also be used to test thyroid drugs and model thyroid diseases. Additionally, such cells can also potentially be used to treat thyroid diseases. If this technology can be recapitulated with human pluripotent stem cells – particularly with induced pluripotent stem cells and might be patient specific, then a ready-made treatment for patients who have lost their thyroids as a result of surgery is potentially at hand.