Thyroglobulin molecules internalized by thyrocytes are sorted in early endosomes and partially recycled back to the follicular lumen.

Abstract

Thyroglobulin (Tg) molecules stored in thyroid follicle lumens are heterogeneous in terms of iodine and hormone contents. It has been suggested that thyroid hormone is preferentially produced from the most highly iodinated Tg molecules and that thyrocytes are capable of selecting these molecules. The cellular localization as well as the molecular basis of such a selection process are not known. The present work was undertaken to determine whether there is selectivity at the step of endocytosis and, if not, to discover other possible mechanisms. Studies were conducted on reconstituted thyroid follicles (RTF) in culture. We compared the ability of thyrocytes to internalize Tg and an exogenous protein, BSA, which is neither iodinated nor glycosylated. To identify the protein, Tg and BSA were coupled to gold particles of different size and microinjected in a fixed ratio into the lumen of RTF. Neither of the two protein gold probes detected by transmission electron microscope bound at the cell surface, and both entered the cells at a similar rate and were concentrated in early endosomes. After 20 min, both Tg-G and BSA-G were segregated into distinct vacuolar structures. At 60 min, the intracellular content of BSA-G (mainly in prelysosomes and lysosomes) was 2- to 3-fold higher than that of Tg-G. At the same time, there was a marked reduction in the BSA-G/Tg-G ratio in the lumen. The differences between the Tg-G and BSA-G distribution patterns that were amplified in TSH-treated RTF are in keeping with a back-transfer of internalized Tg toward the lumen. The existence of a cell to lumen transport of previously endocytosed Tg was further documented using intralumenal [125I]Tg as a marker. RTF pulse labeled with tracer amounts of [125I]iodide were shortly incubated with TSH to induce [125I]Tg endocytosis, and the fate of internalized [125I] Tg was studied in a chase incubation period of up to 4 h. At 20 C, where the degradation of internalized Tg is blocked, we observed a time-dependent decrease in intracellular [125I]Tg and a corresponding increase in the lumenal [125I]Tg content. This cell to lumen [125I]Tg transfer was inhibited by primaquine. In conclusion, our data show that 1) the thyroid apical endocytic process does not exhibit selectivity for Tg; 2) the thyrocyte possesses a sorting machinery for endocytosed ligands; and 3) internalized Tg molecules can be recycled back to the follicular lumen.(ABSTRACT TRUNCATED AT 400 WORDS)

The type‐1 repeats of thyroglobulin regulate thyroglobulin degradation and T3, T4 release in thyrocytes

“…1, upper panel, steps A, B, C). Incompletely iodinated Tg goes back into the lumen for recycling without degradation [22]. In a late and long (1-4 h) second step ( Fig.…”

“…The Tg-associated cathepsins are thus inhibited and stabilized, so as to increase their half-life, and remain available for the late degradation steps. In addition, the immature incompletely iodinated Tg could be more resistant to degradation in its way back for recycling in the lumen [22]. The drawing in Fig.…”

Megalin (gp330) Is an Endocytic Receptor for Thyroglobulin on Cultured Fisher Rat Thyroid Cells

“…There is evidence that micropinocytosis of Tg can take place both by nonselective fluid phase uptake and receptor-mediated endocytosis, but the relative importance of these two mechanisms is uncertain (1). Although evidence has been obtained of low affinity receptors for Tg on thyroid cells, a receptor capable of mediating Tg endocytosis has not been fully characterized (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). We have previously obtained evidence suggesting that megalin (gp330) may function as a receptor for Tg (13).…”

Culture of Dendritic Cells from a Nonlymphoid Organ, the Thyroid Gland: Evidence for TNFα-Dependent Phenotypic Changes of Thyroid-Derived Dendritic Cells

“…BSA-gold and thyroglobulin-gold complexes were prepared from 9-nm and 15-nm gold particles, respectively, as previously described (Kostrouch et al, 1993). Cells were incubated with a 1 to 1 mixture of BSA-gold and thyroglobulin-gold complexes (10 13 to 10 14 particles/ml) in Ham's F-12 medium containing BSA (1 mg/ml) for 60 minutes at 37°C as previously described (Gire et al, 1996).…”