Abstract
Despite the availability of synthetic thyroid hormone (TH) for therapeutic use, many patients with hypothyroidism do not feel well on replacement doses of TH. Indeed, hypothyroid children may develop various behavioral issues if under- or overtreated for hypothyroidism, suggesting the need of better individualized therapy. The development of stem cell-based models of thyroid replacement would greatly facilitate the treatment of post-surgical and congenital hypothyroidism and our understanding of the causes of congenital hypothyroidism. To generate thyroid follicular cells from human induced pluripotent stem cells (hiPSCs), we utilize a directed differentiation approach by activating endogenous signaling pathways we have previously identified in the mouse. We are employing a hiPSC-line with a GFP reporter targeted to the NKX2-1 locus allowing us to identify and purify cells positive for the transcription factor NKX2-1. Using these cells, we confirmed that BMP4 and FGF2 led to the induction of thyroid progenitors from foregut endoderm, expressing the four transcription factors in thyroid development NKX2-1, PAX8, HHEX and FOXE1. Subsequent differentiation of the purified, NKX2-1-positive progenitors led to follicular cell maturation, expressing genes related to TH synthesis, such as TG, TSHR, NIS (SLC5A5) and TPO. To assess the in vivo function of these hiPSC-derived thyroid follicular cells, we grafted the cells under the kidney capsule of radioablated, immunocompromised mice. First, hypothyroidism was confirmed 4 weeks post-ablation by increased TSH and low T4 levels. Next, mice received matured hiPSC-derived follicular cells, undifferentiated hiPSCs or sham surgery and were monitored for 8 weeks. In this preliminary test, no difference in TH levels was detected between the three groups. However, the grafted follicular cells formed a distinct cell population with thyroid-like histology. In order to optimize our directed differentiation protocol further and obtain a purer thyroid progenitor population, we generated a hiPSC line carrying a tdTomato reporter targeted to the PAX8 locus in addition to the NKX2-1-GFP-reporter. After directed differentiation cells sorted for NKX2-1-GFP and PAX8-tdTomato showed high induction of NKX2-1, PAX8, HHEX and FOXE1 expression, as well as expression of TG and TPO. Utilizing this double-reporter hiPSC line will enhance our ability to generate mature follicular cells for transplantation.