Thyrotropin regulates thyroglobulin mRNA splicing and differential processing

Directed differentiation of mouse embryonic stem cells into thyroid follicular cells

Elucidating the molecular mechanisms leading to the induction and specification of thyroid follicular cells is important for our understanding of thyroid development. To characterize the key events in this process, we previously established an experimental embryonic stem (ES) cell model system, which shows that wild-type mouse CCE ES cells can give rise to thyrocyte-like cells in vitro. We extend our analysis in this report by using a genetically manipulated ES cell line in which green fluorescent protein (GFP) cDNA is targeted to the TSH receptor (TSHR) gene, linking GFP expression to the transcription of the endogenous TSHR gene. The appearance of GFP-positive cells was dependent on the formation of embryoid bodies from undifferentiated ES cells and was greatly enhanced by TSH treatment during the first 2-4 d of differentiation. With the support of Matrigel, highly enriched ES cell-derived GFP-positive cells formed thyroid follicle-like clusters in a serum-free medium supplemented with TSH. Importantly, these clusters display the characteristics of thyroid follicular cells. Immunofluorescent studies confirmed the colocalization of TSHR with the Na+/I- symporter in the clusters and indicated that Na+/I- symporter was expressed exclusively in the plasma membrane. In addition, I- uptake activity was observed in these cells. Our results indicate that ES cells can be induced to differentiate into thyroid follicular cells, providing a powerful tool to study embryonic thyroid development and function.

Committing embryonic stem cells to differentiate into thyrocyte-like cells in vitro

The derivation of thyrocyte-like cells in culture is of importance in the basic study of early thyroid embryogenesis and the generation of an unlimited clinical source of thyrocytes for genetic manipulation and cell transplantation. We have established an experimental system, which shows that 6-d-old embryoid bodies (EBs) differentiated from mouse embryonic stem (ES) cells expressed a set of genes traditionally associated with thyroid cells. The genes analyzed included the thyroid transcription factor PAX8, the Na(+)/I(-) symporter, thyroperoxidase, thyroglobulin, and the TSH receptor (TSHR). Immunofluorescent analysis demonstrated the presence of TSHR-positive cells as outgrowths from 8-d-old EBs cultured on chamber slides. Accordingly, this area of cells also expressed PAX8 and another thyroid transcription factor TTF2. Of importance, TSH, the main regulator of the thyroid gland, was necessary to maintain the expression of PAX8 and TSHR genes during EB differentiation. Furthermore, thyroid-specific function, such as cAMP generation by TSH, was maintained in this model. Together, these results suggested that the developmental program associated with thyrocyte development is recapitulated in the ES/EB model system. The differentiation of mouse ES cells into thyrocyte-like cells provides a powerful model for the study of thyrocyte developmental diseases associated with this lineage and contributes to the development of thyroid hormone-secreting cell lines.

Megalin in thyroid physiology and pathology

Megalin, a member of the low density lipoprotein endocytic receptor family, is expressed on the apical surface of thyroid epithelial cells, directly facing the follicle lumen, where colloid is stored in high concentrations. Studies in vivo and with cultured thyroid cells have provided evidence that megalin expression on thyroid cells is TSH-dependent. Thyroglobulin (Tg), the major protein component of the colloid and the precursor of thyroid hormones, binds to megalin with high affinity and megalin mediates in part its uptake by thyrocytes. Tg internalized by megalin avoids the lysosomal pathway and is delivered by transepithelial transport (transcytosis) to the basolateral membrane of thyrocytes, from which it is released into the bloodstream. This process competes with pathways leading to thyroid hormone release from Tg molecules, which occurs following internalization of Tg molecules from the colloid by other means of uptake (fluid phase endocytosis or endocytosis mediated by low affinity receptors) that result in proteolytic cleavage in the lyosomes. During transcytosis of Tg, a portion of megalin (secretory component) remains complexed with Tg and enters the circulation, where its detection may serve as a tool to identify the origin of serum Tg in patients with thyroid diseases. Tg endocytosis via megalin is facilitated by the interaction of Tg with cell surface heparan sulfate proteoglycans, which occurs via a carboxyl terminal heparin binding site of Tg functionally related with a major megalin binding site. Although autoantibodies against megalin can be found in the serum of approximately 50% of patients with autoimmune thyroiditis, a role of megalin in this and other thyroid diseases remains to be established.

Role of thyroglobulin endocytic pathways in the control of thyroid hormone release

Thyroglobulin (Tg), the thyroid hormone precursor, is synthesized by thyrocytes and secreted into the colloid. Hormone release requires uptake of Tg by thyrocytes and degradation in lysosomes. This process must be precisely regulated. Tg uptake occurs mainly by micropinocytosis, which can result from both fluid-phase pinocytosis and receptor-mediated endocytosis. Because Tg is highly concentrated in the colloid, fluid-phase pinocytosis or low-affinity receptors should provide sufficient Tg uptake for hormone release; high-affinity receptors may serve to target Tg away from lysosomes, through recycling into the colloid or by transcytosis into the bloodstream. Several apical receptors have been suggested to play roles in Tg uptake and intracellular trafficking. A thyroid asialoglycoprotein receptor may internalize and recycle immature forms of Tg back to the colloid, a function also attributed to an as yet unidentified N-acetylglucosamine receptor. Megalin mediates Tg uptake by thyrocytes, especially under intense thyroid-stimulating hormone stimulation, resulting in transcytosis of Tg from the colloid to the bloodstream, a function that prevents excessive hormone release.

Identification of the exon structure and four alternative transcripts of the thyroglobulin-encoding gene

The human thyroglobulin (hTg)-encoding gene is a single-copy gene of more than 300 kb and composed of at least 42 exons. After studying the 5' portion of the Tg cDNA (from nucleotides (nt) 1 to 842), we have previously suggested the existence of alternative splicing of the hTg pre-mRNA in human thyroid tissue. Here, we describe four alternative splicings in the central region of the hTg pre-mRNA between nt 3597 and 4653. We have deduced from these results the exonic structure of this region (exons 16 to 22). Finally, the identification of alternative splicings has also allowed us to demonstrate the existence of a strong 5' splice site inside intron 16 of Tg.