Thyroglobulin structure and function: recent advances

Morphological and functional changes in rat thyroid gland after a year following chronic exposure to low and intermediate doses of γ-radiation

INTRODUCTION

Thyroid function depends on iodine uptake by the body as well as on exposure to various harmful environmental hazards (stress, ionizing radiation).

AIM

The aim of the work was to assess the effect of exposure to low and intermediate doses of external γ-radiation on the thyroid structure and function in young female rats at remote periods after radiation.

MATERIALS AND METHODS

Forty female rats were used to study remote effects of external γ-radiation exposure during 20 d (at daily doses of 0.1, 0.25 and 0.5 Gy) on the functional activity (levels of thyroid hormones, iodine metabolism) and the morphological structure of the rat thyroid) after 12 months following the radiation exposure.

RESULTS

An increase in thyroid mass and a decrease in total thyroid protein concentration along with a reduction of blood T3 and T4 was shown only in rat groups exposed to 0.25 and 0.5 Gy. Both the concentration of total iodine and its protein-bound fraction (1.2-1.4 fold, p < .01) and the protein-bound to total iodine ratio were decreased in the thyroids of all irradiated animals. The 0.1-Gy group showed elevated thyroperoxidase (TPO) activity along with increased catalase activity, which may indicate the activation of iodine oxidation by thyrocytes. Only the 0.5-Gy group demonstrated reduced urinary excretion of iodine (2.1 fold, p < .01).The reduction of thyroid function at radiation doses of 0.25 and 0.5 Gy was characterized by a microfollicular structure and the development of atrophic changes in the parenchyma, desquamation of thyroid epithelium and an increase in epithelium proliferation. The diameter of the thyrocyte nuclei was increased in rats exposed to 0.25 and 0.5 Gy, which indicates functional tension of thyrocytes.

CONCLUSION

Our research shows that after a year, the exposure to external γ-radiation of 0.1, 0.25 and 0.5-Gy caused changes in the structure and function of the rat thyroid which are manifested by the development of hypothyroiditis (0.5 Gy), 'subclinical' hypothyroiditis (0.25 Gy) and functional tension of thyrocytes. The mechanisms of thyroid dysfunction - impaired- uptake of iodine and its organification against the background of activation of free radical processes - suggest disturbances in the function of the sodium/iodide symporter (NIS), TPO and thyroglobulin synthesis. In contrast to the intermediate doses, the effects of the 0.1-Gy dose were mostly found at the remote periods compared to the earlier periods (180 days).

Cellular and molecular distribution of thyroid-specific proteins, thyroid-stimulating hormone receptor (TSH-R) and thyroglobulin (TG) in the central nervous system

The widespread extra-thyroidal localisation of thyroid-specific proteins, thyroid-stimulating hormone receptor (TSH-R) and thyroglobulin (TG), has been well documented. However, more recent years has seen the focus of this research area shift to the distribution of these thyroid-specific proteins, in the central nervous system (CNS). This is largely attributed to the well-known associations between thyroid auto-immunity and neuro-psychiatric disorders. Although these associations have not yet been well defined, there are several studies that demonstrate the presence of TSH-R and TG proteins in CNS regions and its cellular structures. In addition, there is an emerging body of evidence to describe the potential functional roles of these thyroid proteins in various regions of the CNS. In this review, the neural distribution of TSH-R and TG as well as their possible physiological implications in various regions of human and non-human brain is discussed.

Genetic Variants in the ST6GAL1 Gene Are Associated with Thyroglobulin Plasma Level in Healthy Individuals

Background: Thyroglobulin (Tg) is a 660 kDa iodoglycoprotein that serves as a scaffold for thyroid hormone synthesis. Although a twin study showed that variability of serum Tg levels has a substantial genetic basis, no genome-wide association study (GWAS) of serum/plasma Tg levels has been performed to date. The aim of this study was to identify genetic variants associated with plasma Tg levels among healthy individuals. Methods: A GWAS was conducted on two Croatian cohorts, and a combined analysis was performed. The analyses included 1094 individuals. A total of 7,597,379 variants, imputed using the 1000 Genomes reference panel, were analyzed for association. GWAS was performed under an additive model, controlling for age, sex, and relatedness within each data set. Combined analysis was conducted using the inverse-variance fixed-effects method. Results: Sixteen variants located on chromosome 3, within the ST6GAL1 gene, reached genome-wide significance. The lead SNP was rs4012172 ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$p = 1.29 \times {10^{ - 10}}$$ \end{document} ), which explained 3.19% of the variance in Tg levels. ST6GAL1 belongs to the sialyltransferase protein family, which has a fundamental role in the synthesis of specific sialylated structures on various glycoproteins, including Tg. It is known that only immature Tg (poorly sialylated or desialylated) can be transferred to the bloodstream. Conclusions: A highly biologically plausible locus was identified that could have a role in the regulation of plasma Tg levels in healthy individuals.

Thyroglobulin From Molecular and Cellular Biology to Clinical Endocrinology

Thyroglobulin (Tg) is a vertebrate secretory protein synthesized in the thyrocyte endoplasmic reticulum (ER), where it acquires N-linked glycosylation and conformational maturation (including formation of many disulfide bonds), leading to homodimerization. Its primary functions include iodide storage and thyroid hormonogenesis. Tg consists largely of repeating domains, and many tyrosyl residues in these domains become iodinated to form monoiodo- and diiodotyrosine, whereas only a small portion of Tg structure is dedicated to hormone formation. Interestingly, evolutionary ancestors, dependent upon thyroid hormone for development, synthesize thyroid hormones without the complete Tg protein architecture. Nevertheless, in all vertebrates, Tg follows a strict pattern of region I, II-III, and the cholinesterase-like (ChEL) domain. In vertebrates, Tg first undergoes intracellular transport through the secretory pathway, which requires the assistance of thyrocyte ER chaperones and oxidoreductases, as well as coordination of distinct regions of Tg, to achieve a native conformation. Curiously, regions II-III and ChEL behave as fully independent folding units that could function as successful secretory proteins by themselves. However, the large Tg region I (bearing the primary T4-forming site) is incompetent by itself for intracellular transport, requiring the downstream regions II-III and ChEL to complete its folding. A combination of nonsense mutations, frameshift mutations, splice site mutations, and missense mutations in Tg occurs spontaneously to cause congenital hypothyroidism and thyroidal ER stress. These Tg mutants are unable to achieve a native conformation within the ER, interfering with the efficiency of Tg maturation and export to the thyroid follicle lumen for iodide storage and hormonogenesis.

ERp29 deficiency affects sensitivity to apoptosis via impairment of the ATF6-CHOP pathway of stress response

Endoplasmic reticulum protein 29 (ERp29) belongs to the redox-inactive PDI-Dβ-subfamily of PDI-proteins. ERp29 is expressed in all mammalian tissues examined. Especially high levels of expression were observed in secretory tissues and in some tumors. However, the biological role of ERp29 remains unclear. In the present study we show, by using thyrocytes and primary dermal fibroblasts from adult ERp29(-/-) mice, that ERp29 deficiency affects the activation of the ATF6-CHOP-branch of unfolded protein response (UPR) without influencing the function of other UPR branches, like the ATF4-eIF2α-XBP1 signaling pathway. As a result of impaired ATF6 activation, dermal fibroblasts and adult thyrocytes from ERp29(-/-) mice display significantly lower apoptosis sensitivities when treated with tunicamycin and hydrogen peroxide. However, in contrast to previous reports, we could demonstrate that ERp29 deficiency does not alter thyroglobulin expression levels. Therefore, our study suggests that ERp29 acts as an escort factor for ATF6 and promotes its transport from ER to Golgi apparatus under ER stress conditions.

A novel transgenic mouse model of growth plate dysplasia reveals that decreased chondrocyte proliferation due to chronic ER stress is a key factor in reduced bone growth

Disease mechanisms leading to different forms of chondrodysplasia include extracellular matrix (ECM) alterations and intracellular stress resulting in abnormal changes to chondrocyte proliferation and survival. Delineating the relative contribution of these two disease mechanisms is a major challenge in understanding disease pathophysiology in genetic skeletal diseases and a prerequisite for developing effective therapies. To determine the influence of intracellular stress and changes in chondrocyte phenotype to the development of chondrodysplasia, we targeted the expression of the G2320R mutant form of thyroglobulin to the endoplasmic reticulum (ER) of resting and proliferating chondrocytes. Previous studies on this mutant protein have shown that it induces intracellular aggregates and causes cell stress and death in the thyroid gland. The expression and retention of this exogenous mutant protein in resting and proliferating chondrocytes resulted in a chronic cell stress response, growth plate dysplasia and reduced bone growth, without inducing any alterations to the architecture and organization of the cartilage ECM. More significantly, the decreased bone growth seemed to be the direct result of reduced chondrocyte proliferation in the proliferative zone of growth plates in transgenic mice, without transcriptional activation of a classical unfolded protein response (UPR) or apoptosis. Overall, these data show that mutant protein retention in the ER of resting and proliferative zone chondrocytes is sufficient to cause disrupted bone growth. The specific disease pathways triggered by mutant protein retention do not necessarily involve a prototypic UPR, but all pathways impact upon chondrocyte proliferation in the cartilage growth plate.

Classical and alternative components of the mitochondrial respiratory chain in pathogenic fungi as potential therapeutic targets

The frequency of opportunistic fungal infection has increased drastically, mainly in patients who are immunocompromised due to organ transplant, leukemia or HIV infection. In spite of this, only a few classes of drugs with a limited array of targets, are available for antifungal therapy. Therefore, more specific and less toxic drugs with new molecular targets is desirable for the treatment of fungal infections. In this context, searching for differences between mitochondrial mammalian hosts and fungi in the classical and alternative components of the mitochondrial respiratory chain may provide new potential therapeutic targets for this purpose.

Revisiting iodination sites in thyroglobulin with an organ-oriented shotgun strategy

Thyroglobulin (Tg) is secreted by thyroid epithelial cells. It is essential for thyroid hormonogenesis and iodine storage. Although studied for many years, only indirect and partial surveys of its post-translational modifications were reported. Here, we present a direct proteomic approach, used to study the degree of iodination of mouse Tg without any preliminary purification. A comprehensive coverage of Tg was obtained using a combination of different proteases, MS/MS fragmentation procedures with inclusion lists and a hybrid mass high-resolution LTQ-Orbitrap XL mass spectrometer. Although only 16 iodinated sites are currently known for human Tg, we uncovered 37 iodinated tyrosine residues, most of them being mono- or diiodinated. We report the specific isotopic pattern of thyroxine modification, not recognized as a normal peptide pattern. Four hormonogenic sites were detected. Two donor sites were identified through the detection of a pyruvic acid residue in place of the initial tyrosine. Evidence for polypeptide cleavages sites due to the action of cathepsins and dipeptidyl proteases in the thyroid were also detected. This work shows that semi-quantitation of Tg iodination states is feasible for human biopsies and should be of significant medical interest for further characterization of human thyroid pathologies.

Evaluation of a new method for the diagnosis of alterations of Lens culinaris agglutinin binding of thyroglobulin molecules in thyroid carcinoma

BACKGROUND

The measurement of serum thyroglobulin (Tg) is widely used as a marker for recurrence of thyroid carcinoma following total thyroidectomy. However, this method cannot differentiate between benign and malignant disease. We focused on the sugar chain in the Tg molecule and investigated the usefulness of Lens culinaris agglutinin (LCA)-reactive Tg ratios in sera and wash fluids obtained during fine-needle aspiration (FNA) for the detection of thyroid carcinoma.

METHODS

The study was performed using 203 serum samples (115 from patients with benign thyroid disease and 88 from patients with thyroid carcinomas) and 176 wash fluid samples (143 benign, 21 malignant, and 12 inconclusive). LCA-reactive Tg ratios were determined using an enzyme-linked immunosorbent assay, and a comparison was made between malignant and benign lesions.

RESULTS

In serum, the ratio in patients with malignancy was 79.5+/-6.0 [mean+/-standard deviation (SD)], significantly lower than in patients with benign lesions (84.9+/-3.5). The ratios in wash fluid from malignant lesions (75.8+/-18.9) were also significantly lower than those from benign lesions (85.6+/-3.9).

CONCLUSIONS

These results suggest that this method could distinguish between benign and malignant lesions and may be useful for screening serum and wash samples.

Two decades of thyroglobulin type-1 domain research

Thyroglobulin type-1 repeats are primarily found in thyroglobulin and several other functionally unrelated proteins. Because a few of them exhibit inhibitory activity against cysteine proteases they were named thyropins (thyroglobulin type-1 domain protease inhibitors). In contrast to cystatins, the best-characterized group of papain-like protease inhibitors, they exhibit greater selectivity in their interactions with target proteases. Interestingly, a few members inhibit aspartic protease cathepsin D and metalloproteases. In contrast to the inhibitory fragment of the major histocompatibility complex class II-associated p41 form of invariant chain, whose structural integrity appears mandatory for its inhibitory properties, short polypeptides derived from insulin-like growth factor-binding proteins exhibit the same activity as the structure of the whole fragment. Taken together, the results indicate that the thyroglobulin type-1 repeat is a structural motif occasionally employed as an inhibitor of proteases.

Variable influences of iodine on the T-cell recognition of a single thyroglobulin epitope

We have previously shown that iodotyrosyl formation within certain innocuous thyroglobulin (Tg) peptides confers on them immunopathogenic properties. In this report, we generated a panel of T-cell hybridoma clones specific for the immunogenic 16 mer Tg peptide p179 (amino acids 179-94) or its iodinated analogue (I-p179), with a view to examining the effects of a single iodine atom at the Y192 amino acid residue on T-cell recognition. We found that the peptide p179 was subdominant, and its binding to both A(k) and E(k) molecules was not significantly influenced by iodine. T-cell receptor (TCR) engagement was unaffected by the bulky iodine atom in two clones that responded to both analogues but it was sterically hindered in two other clones that recognized only p179. One clone was reactive only to I-p179, suggesting that the iodine atom is an integral part of its TCR ligand. Truncation analysis localized the determinant seen by all clones within the 11 mer peptide p184 (amino acids 184-194), suggesting that the cross-reactive clones were not activated by a minimal epitope lacking Y192 and that the negative influence of iodine was not the result of a flanking residue effect. These results demonstrate, at the clonal level, variable influences of a single iodine atom on the recognition of a single Tg peptide. Iodination of tyrosyl-containing, immunopathogenic Tg peptides may have unpredictable effects at the polyclonal level, depending on the extent of iodination at the particular site, and the relative number or effector function of autoreactive T-cell clones that are switched on or off by the neoantigenic determinant.

Antibodies cross-reacting with thyroglobulin and thyroid peroxidase are induced by immunization of rabbits with an immunogenic thyroglobulin 20mer peptide

Thyroglobulin (Tg) and thyroid peroxidase (TPO) are two major autoantigens in autoimmune thyroid diseases (AITD). Cross-reactive anti-Tg/TPO antibodies have been identified in patients with AITD and in mice immunized with Tg or TPO. In the present study, we investigated the production of anti-Tg/TPO antibodies in rabbits immunized with human Tg and with a highly immunogenic Tg peptide (namely TgP41, sequence 2651-2670 of human Tg), by noncompetitive and competitive ELISA. TgP41 was found previously to induce intramolecular epitope spreading. We found that Tg-immunized rabbits developed a serological immune response to TPO due to cross-reactivity with Tg, since serum TPO reactivity was inhibited by soluble Tg and affinity-purified anti-Tg antibodies cross-reacted with TPO. Moreover, TgP41-immunized rabbits responded to Tg and TPO. This serological response was attributed to anti-Tg/TPO antibodies, based on the observation that serum TPO reactivity was again inhibited by soluble Tg, and affinity-purified anti-Tg antibodies, induced by TgP41-immunization, cross-reacted with TPO. Purified anti-TgP41 antibodies did not react with TPO, suggesting that a putative common antigenic determinant is not included in the peptide sequence. We propose that intermolecular spreading of reactivity to TPO observed after administration of the Tg-peptide is a result of intramolecular epitope spreading to determinant(s) responsible for Tg/TPO cross-reactivity.

The Acetylcholinesterase Homology Region Is Essential for Normal Conformational Maturation and Secretion of Thyroglobulin

Secretion of thyroglobulin (Tg, a large homodimeric glycoprotein) is essential to deliver Tg to its site of iodination for thyroxine biosynthesis. An L2263P missense mutation in Tg has been proposed as the molecular defect causing congenital goitrous hypothyroidism in cog/cog mice due to perturbed Tg homodimerization, resulting in its retention within the endoplasmic reticulum. The mutation falls within a carboxyl-terminal region of Tg with high structural similarity to the entirety of acetylcholinesterase (AChE), a secretory protein that also forms homodimers. We provide new evidence that authentic AChE and the cholinesterase-like domain of Tg share a common tertiary structure. Moreover, we find that a Tg truncation, deleted of the cholinesterase-like region (but not a comparably sized deletion of internal Tg regions), blocks Tg export. Appending to this truncation a cDNA encoding authentic AChE results in translation of a chimeric protein in which AChE is present in a native, enzymatically active (albeit latent) conformation, and this fully rescues Tg secretion. Introduction of the cog mutation inhibits AChE enzyme activity, and established denaturing mutations of AChE block secretion of the Tg. Additional studies show that the native structure of the AChE region functions as a "dimerization domain," facilitating intracellular transport of Tg to the site of thyroid hormonogenesis.

Hypothyroid Goitre in a Ram: Chemical Analysis gives Indirect Evidence for a Structurally Altered Type of Ovine Thyroglobulin

The thyroglobulin of a ram of the East Friesian milk sheep breed suffering from goitre was investigated by physico- and immunochemical methods. The respective ram was the only animal amongst the other sheep of the flock, that exhibited severe goitre, additionally showing depressed behaviour. Results of the thyroid-stimulating hormone response test were indicative of hypothyroidism. The dysfunction of the thyroid gland could be treated by additional iodine supplementation quite successfully, although all sheep had been given iodinated cattle salt throughout the course of the history. Without reducing conditions sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of isolated thyroglobulin molecules of the ram and control sheep did not reveal different band patterns, but under reducing conditions different band patterns were evident for the respective animals: the ram's thyroglobulin displayed two main bands, those of healthy reference sheep only one. Both bands reacted equally with anti-thyroglobulin antibodies, even with those produced by immunizing rabbits with single bands. The reduced single thyroglobulin band of healthy sheep corresponded to a truncated form of that molecule, whereas the additional main band of the ram was a more resistant, intact thyroglobulin subunit, as was shown by mass spectrometry. In conclusion, results of physico- and immunochemical investigations gave evidence of a modification of thyroglobulin with suspected different iodine binding properties in the ram. The latter finding may have clinical relevance in similar cases in other species, as it is an example of the impact that a minor change in a protein molecule may have on a complete metabolic pathway. Additionally, it could be shown, that in the ovine species the generally found single main band of thyroglobulin after reduction is a truncated form and not an intact subunit. This truncation seems to be induced in vitro by the reductive sample pretreatment prior to SDS-PAGE.

Evidence for intramolecular B-cell epitope spreading during experimental immunization with an immunogenic thyroglobulin peptide

Thyroglobulin (Tg) is a target autoantigen in autoimmune thyroid diseases, such as Graves' disease (GD) and Hashimoto's thyroiditis. In a previous study we identified three 20mer Tg peptides bearing epitopes of autoantibodies associated with GD (TgP15, TgP26 and TgP41: sequences 2339-2358, 2471-2490 and 2651-2670 of human Tg, respectively). In the present study, we investigated the antigenicity of the above peptides in experimental immunization with Tg, the immunogenicity of antigenic peptides and the possibility of intramolecular B-cell epitope spreading during peptide immunization. For this purpose, two rabbits were injected with human Tg in CFA six times, every three weeks. Two control animals were injected only with CFA. Testing of antisera and of affinity-purified antibodies, by ELISA against the three peptides, revealed reactivity only to TgP41. This synthetic peptide was subsequently administered to two rabbits, in its free form (100 micro g in CFA six times, every two weeks). A strong serological response was developed not only against TgP41, but also to intact human and rabbit Tg. Immunization with TgP41 induced intramolecular B-cell epitope spreading, i.e. production of antibodies to sites on Tg other than that corresponding to TgP41, as revealed by immunoadsorption and competitive ELISA. Histopathological studies did not reveal any infiltration in thyroid glands. We conclude that peptide TgP41 encompasses not only an epitope of disease-associated autoantibodies, but also a dominant immunogenic epitope of experimentally induced Tg-specific antibodies, able to drive B-cell epitope spreading.

Individual recombinant thyroglobulin type-1 domains are substrates for lysosomal cysteine proteinases

Thyroglobulin contains 11 repeats of a motif called thyroglobulin type-1 domain that show sequence similarity to some proteins exhibiting inhibitory activity against cysteine proteinases. Here we report that thyroglobulin decreases the activity of cathepsins B, H, L, and papain. To examine the possible involvement of particular type-1 domains in that decrease of activity, some individual thyroglobulin type-1 domains were expressed in E. coli. These recombinant domains proved to be substrates for cathepsins B, H, L, and papain instead of inhibitors. The cleavage points with cathepsins B and L on the second and the fourth domains were determined. The possible reasons for degradation are discussed.

The hormonogenic tyrosine 5 of porcine thyroglobulin is sulfated

Our previous results showed that sulfated tyrosines of thyroglobulin (Tg), the molecular support of thyroid hormonosynthesis, are involved in the hormonogenic process. Moreover, the consensus sequence required for tyrosine sulfation is present in most of the hormonogenic sites. These observations suggest that tyrosine sulfation might play a critical role in the hormonogenic process. In this paper we studied the putative sulfation of tyrosine 5 contained in the preferential hormonogenic site. Porcine thyrocytes were cultured with thyrotropin but without iodide to preserve the sulfation state of tyrosine 5 and then incubated or not with [35S]sulfate. Secreted Tg was purified and submitted to peptide sequence analysis which confirmed the known peptide sequence of the NH(2) extremity of Tg:NIFEYQV. The treatment of [35S]sulfate-labeled Tg by leucine aminopeptidase, which sequentially digested its amino-terminal extremity, released the same amino acids and further analysis by thin layer chromatography showed that the tyrosine was sulfated. We concluded that tyrosine 5 is sulfated but the role of sulfate group in the hormonogenic process remains to be elucidated.

The Fox and the thyroid: the amphioxus perspective

The evolutionary origins of several vertebrate organs are still controversial. The thyroid is classically thought to derive directly from the endostyle (a pharyngeal organ found in urochordates, cephalochordates and lampreys). Several molecular and biochemical lines of evidence agree with this scenario. However, a recent paper,1 describing the expression of a FoxE ortholog in amphioxus, suggests that some molecular pathways might actually have been recruited from an adjacent region of the pharynx. Although additional data from urochordates and lamprey are needed to confirm this hypothesis; these results propose an interesting new scenario for thyroid evolution that involved the reorganisation of genetical and morphological features in the pharyngeal endoderm in order to give rise to a entirely new organ. They also give an indication that the ancestral role of the FoxE gene family was probably limited to the differentiation of part of the pharynx.

Ligand-regulated secretion of recombinant annexin V from cultured thyroid epithelial cells

The exposure of anionic phospholipids on the external surface of injured endothelial cells and activated platelets is a primary biological signal to initiate blood coagulation. Disease conditions that promote the formation of ectopic thrombi result in tissue ischemia. Annexins, Ca2+-dependent anionic phospholipid binding proteins, are potential therapeutic agents for the inhibition of coagulation. We have designed a transgene that targets secretion of annexin V from cultured thyroid cells under the control of doxycycline. Our results indicate that annexin V in the endoplasmic reticulum (ER)/Golgi lumen does not affect the synthesis, processing, and secretion of thyroglobulin. ER luminal Ca2+ was moderately increased and can be released by inositol 1,4,5-trisphosphate. Our study demonstrates that targeting and secretion of annexin V through the secretory pathway of mammalian cells does not adversely affect cellular function. Regulated synthesis and release of annexin V may exert anticoagulatory and anti-inflammatory effects systemically and may prove useful in further developing therapeutic strategies for conditions including antiphospholipid syndrome.

Shape of large bound polysomes in cultured fibroblasts and thyroid epithelial cells

Large bound polysomes were observed by conventional electron microscopy in surface or en face views of the rough endoplasmic reticulum (RER) in two cultured cell types. Cultured thyroid follicular epithelial cells and dermal fibroblasts, both from rats, were prepared for electron microscopy. Ultrathin sections were cut in the plane of the flattened cells to maximize the incidence of RER surface views. Some observations were also made on tissue sections of rat thyroid. Most of the large, RER-bound polysomes in both cell types appeared as two parallel rows of ribosome, thus resembling the shape of long hairpins, although probably closed at both ends. The two parallel rows of ribosomes were about 14 nm apart, and the center-to-center distance between ribosomes in the strands averaged 25 nm. Most of the large bound polysomes in thyroid epithelial cells were presumably making thyroglobulin subunits (330 kDa), while a majority of those in the fibroblasts were probably making prepro-alpha chains of collagen I (150 kDa). It was not possible in this material to see complete large polysomes, because their size usually caused them to extend out of the plane of section. In addition to the hairpin polysomes, there were smaller numbers of other forms. A characteristic large spiral polysome was seen occasionally in both cell types and contained as many as 31 ribosomes. One or two dense particles were sometimes seen in the center of spiral or circular polysomes. The consistent hairpin shape of most large bound polysomes observed in this study suggests that their shape is quite stable.

Activated thyroglobulin possesses a transforming growth factor-beta activity

Thyroglobulin (Tg), the thyroid hormone precursor, is a major protein component in the thyroid gland and may have other important functions. Here, we show that bovine Tg inhibited 125I-labeled transforming growth factor-beta1 (125I-TGF-beta1) binding to cell-surface TGF-beta receptors in mink lung epithelial cells with an IC50 of approximately 300 nM. After disuccinimidyl suberate (DSS) modification, reduction/alkylation, treatment with 8 M urea, 0. 1% SDS, or acidic pH (pH 4-5), Tg exhibited a approximately 5-10-fold increase of 125I-TGF-beta1 binding inhibitory activity with IC50 of approximately 30-60 nM. This inhibitory activity was an intrinsic property of the Tg and could not be segregated from Tg protein by 5% SDS-polyacrylamide gel electrophoresis or by immunoprecipitation using antiserum to Tg. Untreated Tg did not affect DNA synthesis but blocked the TGF-beta-induced inhibition of DNA synthesis in mink lung epithelial cells. After DSS activation, Tg possessed TGF-beta agonist activity and inhibited DNA synthesis of mink lung epithelial cells and rat thyroid cells. The activated Tg also exerted a small but significant TGF-beta agonist activity in transcriptional activation of plasminogen activator inhibitor-1. These results suggest that Tg possesses an authentic TGF-beta activity which can be induced by chemical modifications and treatments with denaturing agents and acidic pH.

Enhanced binding to the molecular chaperone BiP slows thyroglobulin export from the endoplasmic reticulum

To examine how binding of BiP (a molecular chaperone of the hsp70 family that resides in the endoplasmic reticulum) influences the conformational maturation of thyroglobulin (Tg, the precursor for thyroid hormone synthesis), we have developed a system of recombinant Tg stably expressed in wild-type Chinese hamster ovary (CHO) cells and CHO-B cells genetically manipulated for selectively increased BiP expression. The elevation of immunoreactive BiP in CHO-B cells is comparable to that seen during the unfolded protein response in the thyrocytes of certain human patients and animals suffering from congenital hypothyroid goiter with defective Tg. However, in CHO-B cells, we expressed Tg containing no mutations that induce misfolding (i.e. no unfolded protein response), so that levels of all other endoplasmic reticulum chaperones were normal. Increased availability of BiP did not accelerate Tg secretion; rather, the export of newly synthesized Tg was delayed. Tg detained intracellularly was concentrated in the endoplasmic reticulum. By coimmunoprecipitation, BiP exhibited enhanced binding to Tg in CHO-B cells. Moreover, two-dimensional gel analysis showed that BiP associated especially well with intracellular Tg containing mispaired disulfide bonds, thought to represent early Tg folding intermediates. An endoplasmic reticulum chaperone of the hsp90 family, GRP94, was also associated in Tg-chaperone complexes. The results suggest that increased binding of BiP to Tg leads to its delayed conformational maturation in the endoplasmic reticulum.

Thyroglobulin transport along the secretory pathway. Investigation of the role of molecular chaperone, GRP94, in protein export from the endoplasmic reticulum

GRP94 serves as a molecular chaperone in the endoplasmic reticulum (ER). In normal thyrocytes, GRP94 interacts transiently with thyroglobulin (Tg), and in thyrocytes of animals suffering from congenital hypothyroid goiter with defective thyroglobulin, GRP94 and thyroglobulin associate in a protracted fashion. In order explore possible consequences of GRP94 binding, we have studied recombinant nonmutant thyroglobulin expressed in control Chinese hamster ovary (CHO) cells in comparison to that produced in CHO cells genetically manipulated for selectively increased GRP94 expression. Levels of ER chaperones other than GRP94 did not detectably differ, and thyroglobulin achieved transport competence in both kinds of CHO cells. However, increased availability of GRP94 caused the residence time of Tg in the ER to be remarkably prolonged. This was accompanied by a major increase in Tg directly associated with GRP94 and an increase in the ER pool size of Tg. Importantly, co-immunoprecipitation analysis revealed disulfide-linked Tg complexes (previously reported as an early Tg-folding intermediate) especially associated with GRP94. Indeed, non-native Tg, GRP94, and a 78-kDa protein likely to be BiP, appeared in ternary complexes. Under these conditions, GRP94 association appears directly involved in prolongation of Tg folding and export, consistent with a role in quality control in the ER.

Identification of a new thyroglobulin variant: a guanine-to-adenine transition resulting in the substitution of arginine 2510 by glutamine

We analyzed thyroglobulin (Tg) reverse transcription polymerase chain reaction (RT-PCR) products from three congenital goiters and three normal thyroid tissues by Taq I digestion. Tg coding sequences were amplified from position 57 to 8448 in 12 amplification fragments. A Taq I restriction fragment length polymorphism was detected in the most 3' RT-PCR product (nt 7584 through 8448). Data from the sequence showed a G-->A transition (nt 7627) causing the disappearance of the Taq I site in position 7625. It produced the substitution of arginine for a glutamine at position 2510. Afterwards, we established that the glutamine allele is present in normal unrelated individuals, with an allelic frequency of 62%. This Tg variant is thus widely represented in the human population. The available sequence information from rat and bovine Tg showed the presence, in both, of glutamine at position 2510.

Antigenic mapping of human thyroglobulin--topographic relationship between antigenic regions and functional domains

We characterized 26 mAb to human thyroglobulin to obtain a topographic map of the thyroglobulin antigenic surface. Among these mAb, three bind thyroglobulin peptides that are located in the primary sequence of thyroglobulin at either the N terminus or in the middle part of the molecule, three bind thyroglobulin via epitopes comprising the thyroid-hormone moiety, and three bind thyroglobulin through epitopes involved in the recognition of the molecule by its receptor. The 18 remaining mAb bind thyroglobulin through undetermined epitopes; most of these epitopes are resistant to trypsinization. We used two methods to map the antigenic regions of thyroglobulin: all 26 mAb were grouped, by means of cross-inhibition experiments, in 11 clusters corresponding to 11 antigenic regions of the thyroglobulin surface; by means of thyroglobulin peptides of decreasing size, obtained by time-controlled tryptic digestion, we analyzed the relative distance between pairs of epitopes in sandwich immunoassays. By combining these two methods, we organized most of the 11 antigenic regions on a topographic representation of the thyroglobulin surface. This new topographic map of thyroglobulin led us to some unexpected features of the thyroglobulin structure. First, antigenic region 8 located far from the N-terminal region is in close contact with two remote N-terminal antigenic regions (1 and 4), both involved in hormone formation. This antigenic region is likely to play a role in the correct positioning of hormonogenic tyrosines so as to optimize iodination-coupling reactions. Secondly, the domain involved in the binding of thyroglobulin to its receptor, probed by three mAb, is shared by two distinct mid-molecule antigenic regions, one being the main autoantigenic region of thyroglobulin.

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.

Congenital hypothyroid goiter with deficient thyroglobulin. Identification of an endoplasmic reticulum storage disease with induction of molecular chaperones

Recent advances in understanding the molecular pathogenesis of congenital hypothyroid goiter in cog/cog mice, have raised important questions concerning the maturation of thyroglobulin (the thyroid prohormone) in certain human kindreds with congenital goiter. We have now examined affected siblings from two unrelated families that synthesize an apparently normally glycosylated, > 300 kD immunoreactive thyroglobulin, yet have a reduced quantity of intraglandular thyroglobulin and that secreted into the circulation. From thyroid tissues of the four patients, light microscopic approaches demonstrated presence of intracellular thyroglobulin despite its absence in thyroid follicle lumina, while electron microscopy indicated abnormal distention of the endoplasmic reticulum (ER). We have confirmed biochemically that most intrathyroidal thyroglobulin fails to reach the (Golgi) compartment where complex carbohydrate modification takes place. Moreover, the disease in the affected patients is associated with massive induction of specific ER molecular chaperones including the hsp90 homolog, GRP94, and the hsp70 homolog, BiP. The data suggest that these patients synthesize a mutant thyroglobulin which is defective for folding/assembly, leading to a markedly reduced ability to export the protein from the ER. Thus, these kindreds suffer from a thyroid ER storage disease, a cell biological defect phenotypically indistinguishable from that found in cog/cog mice.

Thyroglobulin exon 10 gene point mutation in a patient with endemic goiter

Iodine deficiency is the most relevant etiologic factor in endemic goiter. However, the fact that not all residents in the same area eventually develop goiter suggests that individual factors might also be involved in the etiology of endemic goiter. We have previously reported a point mutation in thyroglobulin exon 10 associated with nonendemic simple goiter. In an attempt to determine whether the mutation in thyroglobulin exon 10 might be linked to endemic goiter, we studied the genomic organization of thyroglobulin exon 10 in 36 patients diagnosed with endemic goiter by Southern blot, PCR, and sequencing analysis. We also analyzed by Southern blot the organization of the genomic region that contains thyroglobulin exons 1 to 11. In one case, we observed a point mutation in thyroglobulin exon 10. Sequencing analysis revealed a mutation at position 2610 of the cDNA, which implies a G to T substitution. This single base change results in a glutamine to histidine substitution and is the same as that previously reported by our group in patients with nonendemic goiter. To our knowledge, this is the first time that a mutation in the thyroglobulin gene has been described in a patient with endemic simple goiter and further confirms the association between the exon 10 mutation and development of goiter.

Thyrotropin controls dolichol-linked sugar pools and oligosaccharyltransferase activity in thyroid cells

We previously showed that thyroglobulin (Tg) glycosylation is enhanced 1.5-fold under thyrotropin (TSH) stimulation, corresponding to an increased number of oligosaccharide chains per molecule of Tg. Now the steps involving dolichol components and oligosaccharyltransferase activity have been studied. Porcine thyroid cells were cultured on porous bottom filters with or without TSH and incubated with [14C]mevalonate. Under TSH regulation, the level of the whole of dolichol components was increased 1.25-fold without modifying their distribution. Dolichol, and free and monosaccharide-linked dolichyl-phosphate, represented respectively 40% and 45% of total dolichol components while dolichyl-pyrophosphate-oligosaccharide represented 3% only. A marked enhancement (4.2-fold) of oligosaccharyltransferase activity occurred in stimulated cells, which could correspond to the addition of the two TSH effects: stimulation of Tg synthesis (3-fold) and of Tg glycosylation (1.5-fold). The amount of lipid carriers appeared to be insufficiently increased but no component is a limiting step, suggesting that the turnover of dolichol derivatives may be increased under TSH control through their use by higher amounts of Tg.

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.

Hormonal regulation of some steps of thyroglobulin synthesis and secretion in bicameral cell culture

Porcine thyroid cells were cultured for 15 days on porous bottom chambers with or without different mixtures of hormones added to serum-free basal medium. Assays with 10% serum were also performed for comparison with previously published results. The effects of the hormones, particularly insulin, TSH and hydrocortisone, were studied on total RNA content, thyroglobulin mRNA level, the amount of thyroglobulin secreted into the apical medium and on glycosylation. Insulin and TSH similarly increased the total RNA content, and their effects were additive. Thyroglobulin mRNA content was increased twofold by insulin and threefold by TSH. When they were added simultaneously, the maximal level of thyroglobulin mRNA was reached, showing that TSH and insulin effects on thyroglobulin gene expression were additive. Hydrocortisone alone did not modify total RNA or thyroglobulin mRNA content but the hormone amplified total RNA when insulin and TSH were present together. The basal level of thyroglobulin secreted into the apical medium was increased threefold by insulin and fourfold by TSH. The effects of these two hormones added together appeared to be additive. Hydrocortisone had no effect alone or even when combined with insulin or TSH. However, when the three hormones were added together, the hormonal response was amplified. TSH effect and insulin effect on the incorporation of 3H-mannose into thyroglobulin as well as on the anionic residue content of the molecule were additive.

Preferential sites of proteolytic cleavage of bovine, human and rat thyroglobulin. The use of limited proteolysis to detect solvent-exposed regions of the primary structure

The sites and the sequence of the proteolytic cleavages of bovine, human and rat thyroglobulin, during the limited proteolysis with thermolysin and trypsin, were determined by sequencing the NH2 termini of the peptides produced and comparing them to the cDNA-derived sequences of bovine, human and rat thyroglobulin. Major cleavage sites of bovine thyroglobulin included residues 240, 502, 993, 1218, 1784 with thermolysin, and 240, 520, 1142, 1783, 2515 with trypsin. Cleavage sites of human thyroglobulin included residues 503, 982, 990, 1405, 1831 with thermolysin, and 522, 1627, 2513 with trypsin. Those of rat thyroglobulin included residues 501, 1776, 1784 with thermolysin, and 522, 1771, 1825, 2515 with trypsin (numbered as in bovine thyroglobulin). Thus, thyroglobulin from various species presents well localized and conserved regions particularly sensitive to proteolysis. The most sensitive region extended for 30 residues after residue 500. Another major cluster of cleavages was centered around residue 1800; this region was only partially sensitive in human thyroglobulin. A conserved tryptic site lay at the COOH terminus of the molecule. Most cleavage sites occurred within the inserted sequences that disrupt the Cys-rich, tandem repeats of thyroglobulin and either contain or are located near exon-intron junctions. Several cleavage sites lay in proximity of early iodinated or hormonogenic tyrosyl residues or of putative N-linked glycosylation sites. While a predominantly beta-type secondary structure and a rigid three-dimensional structure were predicted for the Cys-rich repeats, stretches of predicted alpha-helices, beta-strands and irregular structure were interspersed in the regions surrounding the cleavage sites. These data demonstrate the existence of conserved regions of thyroglobulin inherently sensitive to proteolysis, which most likely represent solvent-exposed regions of the primary structure, possibly forming loops at the surface of thyroglobulin.

Thyrotropin regulates thyroglobulin mRNA splicing and differential processing

Rat thyroid tissue and cultured rat thyrocyte lines contain two thyroglobulin (Tg) mRNAs: a 9 kb rTg-1 mRNA encoding the 330, kDa Tg monomer and a recently described 0.95 kb rTg-2 mRNA. These transcripts have identical 5' coding sequences (641 nucleotides); however, the 3' end of rTg-2 is comprised of coding and non-coding sequences not present in rTg-1. To determine if a single Tg gene encoded both mRNA species, a genomic clone was isolated which spanned the full-length rTg-2 cDNA sequence. The promoter sequence and restriction map were the same as for the previously characterized rTg-1 gene, indicating that rTg-1 and rTg-2 mRNAs are splicing variants derived from the same Tg gene. The unique 3' end of rTg-2 mRNA comprised a single exon which was intronic with respect to rTg-1 mRNA formation. The level of rTg-2 in cultured rat thyrocytes was more sensitive to thyrotropin (TSH) regulation than was rTg-1. rTg-2 mRNA was rapidly (and reversibly) depleted to nearly undetectable levels after TSH removal, unlike rTg-1. Conversely, TSH rapidly restored control levels of rTg-2 mRNA in such depleted cells. The data thus support a model of TSH-induced splicing and regulation of the two Tg mRNAs in the rat.

Thyroglobulin gene point mutation associated with non-endemic simple goitre

Simple goitre is defined as an enlargement of the thyroid gland that is not the result of an inflammatory or neoplastic process and is not associated with thyrotoxicosis or myxoedema; the cause is unknown in most cases. Structural or regulatory defects in the proteins involved in thyroid metabolism might be involved in the functional abnormality that brings about the disorder. We have found a mutation within exon 10 of the thyroglobulin gene in 25 of 56 members of three families affected by simple goitre; 14 of the gene carriers had the disorder. DNA sequencing showed a mis-sense mutation within thyroglobulin gene exon 10, resulting in a glutamine to histidine substitution. Thus, some cases of non-endemic simple goitre are associated with a mutation at the thyroglobulin locus.

Transient aggregation of nascent thyroglobulin in the endoplasmic reticulum: relationship to the molecular chaperone, BiP

Because of its unusual length, nascent thyroglobulin (Tg) requires a long time after translocation into the endoplasmic reticulum (ER) to assume its mature tertiary structure. Thus, Tg is an ideal molecule for the study of protein folding and export from the ER, and is an excellent potential substrate for molecular chaperones. During the first 15 min after biosynthesis, Tg is found in transient aggregates with and without interchain disulfide bonds, which precede the formation of free monomers (and ultimately dimers) within the ER. By immunoprecipitation, newly synthesized Tg was associated with the binding protein (BiP); association was maximal at the earliest chase times. Much of the Tg released from BiP by the addition of Mg-ATP was found in aggregates containing interchain disulfide bonds; other BiP-associated Tg represented non-covalent aggregates and unfolded free monomers. Importantly, the immediate precursor to Tg dimer was a compact monomer which did not associate with BiP. The average stoichiometry of BiP/Tg interaction involved nearly 10 BiP molecules per Tg molecule. Cycloheximide was used to reduced the ER concentration of Tg relative to chaperones, with subsequent removal of the drug in order to rapidly restore Tg synthesis. After this treatment, nascent Tg aggregates were no longer detectable. The data suggest a model of folding of exportable proteins in which nascent polypeptides immediately upon translocation into the ER interact with BiP. Early interaction with BiP may help in presenting nascent polypeptides to other helper molecules that catalyze folding, thereby preventing aggregation or driving aggregate dissolution in the ER.

Identification of a minor Tg mRNA transcript in RNA from normal and goitrous thyroids

Polymerase chain reaction (PCR) amplification of nt 4502 to nt 5184 of the thyroglobulin (Tg) mRNA from several patients, with or without elevated serum thyrotropin (TSH), showed a predominant fragment of the expected size (683 bp) and a minor fragment of 512 bp. The sequence of this minor fragment revealed that 171 bp were missing between position 4567 and 4737. It is highly probable that the deleted sequence corresponds to a complete exon, suggesting an alternative splicing as mechanism for the generation of the minor transcript.

A 3' splice site mutation in the thyroglobulin gene responsible for congenital goiter with hypothyroidism

A case of congenital goiter with defective thyroglobulin synthesis has been studied in molecular terms. The patient is the fifth of a kindred of six, three of which have a goiter. The parents are first cousins. Segregation of thyroglobulin alleles in the family was studied by Southern blotting with a probe revealing a diallelic restriction fragment length polymorphism (RFLP). The results demonstrated that the three affected siblings were homozygous for the RFLP. Northern blotting analysis of the goiter RNA with a thyroglobulin probe suggested that thyroglobulin mRNA size was slightly reduced. Polymerase chain reaction amplification of the 8.5-kb thyroglobulin mRNA as overlapping cDNA fragments demonstrated that a 200-bp segment was missing from the 5' region of the goiter mRNA. Subcloning and sequencing of the cDNA fragments, and of the patient genomic DNA amplified from this region, revealed that exon 4 is missing from the major thyroglobulin transcript in the goiter, and that this aberrant splicing is due to a C to G transversion at position minus 3 in the acceptor splice site of intron 3. The presence in exon 4 of a putative donor tyrosine residue (Tyrosine nr 130) involved in thyroid hormone formation provides a coherent explanation to the hypothyroid status of the patient.

Demonstration of a heterogeneous transcription pattern of thyroglobulin mRNA in human thyroid tissues

Previous reports on human thyroglobulin (hTg) modifications in thyroid carcinomas prompted us to study hTg mRNA in thyroid adenomas and carcinomas. The quantification of hTg mRNA showed a decrease in its levels of expression in both pathological conditions which differed by a factor of 2 between adenomas and carcinomas. Furthermore, PCR was used to analyse the characteristics of hTg mRNA by amplifying 4 regions of the hTg mRNA. When applied to 2 normal, 17 benign and 13 malignant pathological tissue specimens, PCR showed no modification in the size of Tg mRNA. However, abnormal sized cDNAs appeared in all tissues with no distinction between the pathologies; the Restriction Fragment Length Polymorphism study of these cDNAs suggests the existence of alternate splicing patterns in thyroglobulin mRNAs.

Immunohistochemical study of thyroid peroxidase in normal, hyperplastic, and neoplastic human thyroid tissues

An immunohistochemical study using two monoclonal antibodies (MoAb 30 and MoAb 47) against thyroid peroxidase (TPO) was performed on surgical specimens of human thyroid carcinoma (n = 65), adenoma (n = 70) and Graves' disease (n = 10). Normal adjacent thyroid tissue was used as positive control. Monoclonal antibody 30 reacted significantly with all adenoma and most carcinoma, whereas MoAb 47 reacted with 66 adenoma but only two carcinoma. Of the four adenomas that did not react with MoAb 47, three were of the fetal type. Both carcinoma reacting with MoAb 47 were of the well-differentiated follicular type. These findings further confirm the hypothesis that thyroid carcinoma is associated with changes in the quantity and antigenic properties of TPO. Although the alterations in antigenic behavior revealed by MoAb 47 are not 100% specific, they may allow more accurate diagnosis of malignancy in thyroid tumors.

Trace element deficiencies in cattle

Deficiency of cobalt, copper, iron, iodine, manganese, selenium, or zinc can cause a reduction in production. Reduced production occurs most commonly when a deficiency corresponds to the phases of growth, reproduction, or lactation. Because of environmental, nutrient, disease, genetic, and drug interactions, deficiencies of single or multiple elements can occur even when the levels recommended by the National Research Council for these nutrients are being fed. Additionally, random supplementation of trace elements above National Research Council recommendations is not justified because of the negative interaction among nutrients and potential toxicosis. Evaluation of trace element status can be difficult because many disease states will alter blood analytes used to evaluate nutrient adequacy. Proper dietary and animal evaluation, as well as response to supplementation, are necessary before diagnosing a trace element deficiency.

Epitope mapping of human thyroglobulin reveals a central immunodominant region

Thyroglobulin is the thyroid hormone precursor and the major antigen frequently involved in autoimmune diseases. The primary structure of human thyroglobulin is known but the spatial structure remains largely undetermined. By using fusion protein produced in prokaryotic system we have characterized seven short immunoreactive peptides carrying at least one epitope. None of them includes hormonogenic sites, but five are concentrated in the central part of the monomeric molecule, which thus emerges as the major immunogenic region of this protein.