Intracellular iodination of thyroglobulin in filter-polarized thyrocytes leads to the synthesis and basolateral secretion of thyroid hormone

Iodide organification defects resulting from cosegregation of mutated and null thyroid peroxidase alleles

This report describes an intriguing combination of the thyroid peroxidase (TPO) alleles resulting in an iodide organification defect. Sequence analysis of the patient's TPO gene showed the presence of T-deletion in exon 14 of the TPO gene (T2512del). From the sequencing pattern, this new mutation of the TPO gene was thought to be homozygous. mRNA transfection studies in which mutated mRNA was transfected to CHO-K(1) cells by electroporation showed that the cells transfected with mutated mRNA expressed smaller TPO molecules than those of cells transfected with wild-type mRNA and that they had TPO activity. However, the smaller TPO molecules could not translocate onto the cell surface. To investigate T2512del in the parents, their genomic DNAs were sequenced. Results showed that the mother had T2512del but the father did not. However, when seven polymorphic positions reported earlier were analyzed, the mother showed two kinds of nucleotides at four positions but the patient and father showed only one nucleotide at all seven positions. We suspected a deletion of the TPO gene (2p25) in one of two second chromosomes, and analyzed the patient's chromosomes by FISH using TPO cDNA and N-myc genomic DNA as probes. N-myc genomic DNA exhibited two signals and TPO cDNA only one signal, although the G-band showed no morphological abnormalities. T2512-deleted and 2p25-deleted null alleles cosegregated from her parents, resulting in iodide organification defect in the patient.

Molecular and cellular mechanisms of transepithelial iodide transport in the thyroid

Thyroid hormone is an essential regulator of developmental growth and metabolism in vertebrates. Iodine is a necessary constituent of thyroid hormone. Due to the scarcity and uneven distribution of iodine on the Earth's crust, the structure of the thyroid gland is adjusted to collect and store this element in order to secure a continuous supply of thyroid hormone throughout life. Still, disease resulting from hypothyroidism due to iodine deficiency is a global health problem, illustrating the great biological significance that iodine saving mechanisms have evolved. Iodide is accumulated together with prohormone (thyroglobulin) in the lumen of the thyroid follicles. The rate-limiting step of this transport is the sodium/iodide symporter located in the basolateral plasma membrane of the thyroid follicular cells. Iodide is also transferred across the apical plasma membrane into the lumen where hormonogenesis takes place. In this review, recent progress in the understanding of transepithelial iodide transport in the thyroid is summarized.

Iodination of mature cathepsin D in thyrocytes as an indicator for its transport to the cell surface

Thyrocytes are known for their ability to iodinate thyroglobulin from which the thyroid hormones are generated. In the intact thyroid gland the iodination process is almost exclusively executed at the apical plasma membrane of thyroid epithelial cells. Here, we show that freshly isolated thyrocytes iodinated polypeptides other than thyroglobulin and that one of the major iodinated polypeptides was the mature form of the lysosomal protease cathepsin D (CD). The detection of mature CD as an iodinated polypeptide suggested that a fraction of the lysosomally maturated enzyme was delivered to the apical plasma membrane where it became available for iodination. After labeling of thyrocytes with [35S]methionine/cysteine overnight part of the mature CD was released into the culture medium. This was abolished by inhibiting maturation of CD with NH4Cl, indicating that mature CD appeared in the medium after its proteolytic maturation in an acidic compartment. Besides CD other soluble lysosomal polypeptides like the beta-N-acetylhexosaminidase and the sphingolipid-activating protein D (Sap D) were iodinated and partially secreted as mature polypeptides. In contrast, the membrane-associated lysosomal ceramidase was iodinated and partially secreted as immature single-chain enzyme and not as fully maturated two-chain enzyme. These data indicate that a portion of mature CD and other soluble lysosomal enzymes is delivered from lysosomes to the cell surface whereas some membrane-associated enzymes from the terminal lysosomal compartment are efficiently excluded from this process.

Human thyroperoxidase in its alternatively spliced form (TPO2) is enzymatically inactive and exhibits changes in intracellular processing and trafficking

Thyroid peroxidase (TPO1) is a membrane-bound heme-containing glycoprotein that catalyzes the synthesis of thyroid hormones. We generated stable cell lines expressing TPO1 and the alternatively spliced isoform TPO2. Pulse-chase studies showed that TPO2 half-life was dramatically decreased as compared with TPO1. The sensitivity of TPO2 to endo-beta-N-acetylglucosaminidase H indicated that the protein is processed through the endoplasmic reticulum and bears high mannose-type structures. Cell surface biotinylation experiments showed that the two isoforms also differ in their intracellular trafficking. TPO2 was totally retained in the cell, whereas 15% of TPO1 reached the cell surface. The inability of TPO2 to come out of the intracellular compartments was related to structural changes in the molecule. Evidence of these changes was obtained through the lack of recognition of TPO2 by half of the 13 TPO monoclonal antibodies tested in immunoprecipitation experiments. Our data suggest that because of an improper folding, TPO2 is trapped in the endoplasmic reticulum and rapidly degraded. The failure of incorporation of [14C]aminolevulinic acid in the cultured cells showed that TPO2 did not bind to heme, whereas TPO1 did. This result was confirmed through a guaiacol assay showing that TPO2 is enzymatically inactive.

Carbohydrate analysis of porcine thyroglobulin isoforms with different iodine contents

To further validate the relationship between thyroid hormone formation and the carbohydrate structure of thyroglobulin (Tg), we reinvestigated the relationship between the iodine content and the asparagine-linked oligosaccharide structures of porcine Tg. Purified porcine Tg was further separated into isoforms (Tg-F1, -F2 and -F3) with a DEAE-cellulose ion-exchange chromatography column. The iodine residues, neutral sugar and sialic acid were analyzed for the separated Tg isoforms and their asparagine-linked oligosaccharide structures were analyzed. The asparagine-linked oligosaccharides were released from Tg-F1, -F2 and -F3 by hydrazinolysis and each oligosaccharide was labeled with p-aminobenzoic acid octyl ester (ABOE). The ABOE-labeled oligosaccharides from Tg-F1, -F2 and -F3 were analyzed for their relative content in oligosaccharides of each structure type by chemical methods and DEAE- and ConA high-performance liquid chromatography (HPLC) columns. As a result, it was revealed that the Tg fraction eluted at higher ionic strength from a DEAE-cellulose column is apt to contain more of each iodoamino acid, as well as total content of iodine, larger negative zeta-potential, conforming to sialic acid content in the Tg molecule and to a higher content of di-sialo-bi-antennary complex and to high mannose type oligosaccharides. These results support the conclusion that iodine organification of the Tg molecule is correlated with asparagine-linked oligosaccharide completion.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

N-glycans modulate in vivo and in vitro thyroid hormone synthesis. Study at the N-terminal domain of thyroglobulin

Thyroglobulin (Tg) is the substrate for thyroid hormone biosynthesis, which requires tyrosine iodination and iodotyrosine coupling and occurs at the apical membrane of the thyrocytes. Tg glycoconjugates have been shown to play a major role in Tg routing through cellular compartments and recycling after endocytosis. Here we show that glycoconjugates also play a direct role in hormonosynthesis. The N-terminal domain (NTD; Asn1-Met171) of human Tg, which bears the preferential hormonogenic site, brings two N-glycans (Asn57 and Asn91). NTD preparations were purified from Tg with low and mild iodine content in vivo and from poorly iodinated Tg after in vitro iodination and coupling. NTD separated from poorly iodinated Tg was also submitted to iodination and coupling after desialylation and deglycosylation. The various NTD isoforms were analyzed for their N-glycan structures and hormone contents. Our results show that 1) in vivo as well as in vitro unglycosylated isoforms did not synthesize hormones, whereas fully or partially (at Asn91) glycosylated isoforms did; 2) high mannose type structures enhanced the hormone content; and 3) desialylation did not affect in vitro hormone synthesis. Evidence of a direct involvement in hormonosynthesis adds to the role of N-glycans in Tg function and opens the way to new mechanisms for regulation (e.g. TSH modulation of N-glycan) or alteration (e.g. Asn91 mutation) of thyroid hormone synthesis.

Polarized distribution and delivery of plasma membrane proteins in thyroid follicular epithelial cells

Thyroid follicular cells coordinate several oppositely located surface enzyme activities. Recent studies have raised questions about the basic mechanisms used to achieve thyroid surface polarity. We investigated these mechanisms in primary thyroid epithelial monolayers cultured on porous filters. In the steady state, most Na+/K(+)-ATpase and aminopeptidase N were available for surface biotinylation, and these proteins exhibited physiological distributions (basolateral and apical, respectively). Glycosylphosphatidylinositol-anchored proteins were also apically distributed. By pulse-chase, newly synthesized transmembrane proteins exhibited polarized surface delivery that was oriented similarly to that observed at steady state. Little time elapsed between acquisition of Golgi-specific processing and cell surface arrival. Interestingly, when either newly synthesized or steady state-labeled thyroid peroxidase was similarly analyzed, only approximately 30% of the enzyme was ever detected at the cell surface. Of this, the majority was localized apically. The data suggest that most thyroid peroxidase remains intracellular in these monolayers, consistent with the possibility of intracellular iodination activity in addition to apical extracellular iodination. Nevertheless, in filter-polarized thyrocytes, most newly synthesized plasma membrane proteins appear to be sorted in the Golgi complex for direct delivery to apical and basolateral domains.