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

Defective Thyroglobulin: Cell Biology of Disease

The primary functional units of the thyroid gland are follicles of various sizes comprised of a monolayer of epithelial cells (thyrocytes) surrounding an apical extracellular cavity known as the follicle lumen. In the normal thyroid gland, the follicle lumen is filled with secreted protein (referred to as colloid), comprised nearly exclusively of thyroglobulin with a half-life ranging from days to weeks. At the cellular boundary of the follicle lumen, secreted thyroglobulin becomes iodinated, resulting from the coordinated activities of enzymes localized to the thyrocyte apical plasma membrane. Thyroglobulin appearance in evolution is essentially synchronous with the appearance of the follicular architecture of the vertebrate thyroid gland. Thyroglobulin is the most highly expressed thyroid gene and represents the most abundantly expressed thyroid protein. Wildtype thyroglobulin protein is a large and complex glycoprotein that folds in the endoplasmic reticulum, leading to homodimerization and export via the classical secretory pathway to the follicle lumen. However, of the hundreds of human thyroglobulin genetic variants, most exhibit increased susceptibility to misfolding with defective export from the endoplasmic reticulum, triggering hypothyroidism as well as thyroidal endoplasmic reticulum stress. The human disease of hypothyroidism with defective thyroglobulin (either homozygous, or compound heterozygous) can be experimentally modeled in thyrocyte cell culture, or in whole animals, such as mice that are readily amenable to genetic manipulation. From a combination of approaches, it can be demonstrated that in the setting of thyroglobulin misfolding, thyrocytes under chronic continuous ER stress exhibit increased susceptibility to cell death, with interesting cell biological and pathophysiological consequences.

Thyroid hormone synthesis continues despite biallelic thyroglobulin mutation with cell death

Complete absence of thyroid hormone is incompatible with life in vertebrates. Thyroxine is synthesized within thyroid follicles upon iodination of thyroglobulin conveyed from the endoplasmic reticulum (ER), via the Golgi complex, to the extracellular follicular lumen. In congenital hypothyroidism from biallelic thyroglobulin mutation, thyroglobulin is misfolded and cannot advance from the ER, eliminating its secretion and triggering ER stress. Nevertheless, untreated patients somehow continue to synthesize sufficient thyroxine to yield measurable serum levels that sustain life. Here, we demonstrate that TG^{W2346R/W2346R} humans, TG^cog/cog mice, and TG^rdw/rdw rats exhibited no detectable ER export of thyroglobulin, accompanied by severe thyroidal ER stress and thyroid cell death. Nevertheless, thyroxine was synthesized, and brief treatment of TG^rdw/rdw rats with antithyroid drug was lethal to the animals. When untreated, remarkably, thyroxine was synthesized on the mutant thyroglobulin protein, delivered via dead thyrocytes that decompose within the follicle lumen, where they were iodinated and cannibalized by surrounding live thyrocytes. As the animals continued to grow goiters, circulating thyroxine increased. However, when TG^rdw/rdw rats age, they cannot sustain goiter growth that provided the dying cells needed for ongoing thyroxine synthesis, resulting in profound hypothyroidism. These results establish a disease mechanism wherein dead thyrocytes support organismal survival.

p.L571P in the linker domain of rat thyroglobulin causes intracellular retention

Thyroglobulin (TG), a large glycosylated protein secreted by thyrocytes into the thyroid follicular lumen, plays an essential role in thyroid hormone biosynthesis. Rattus norvegicus TG (rTG) is encoded by a large single copy gene, 186-kb long, located on chromosome 7 composed of 48 exons encoding a 8461-kb mRNA. Although the TG gene displays sequence variability, many missense mutations do not impose any adverse effect on the TG protein, whereas other nucleotide substitutions may affect its TG stability and/or TG intracellular trafficking. In order to gain a further understanding of the protein domains regulating its intracellular fate, we cloned a full-length cDNA from rTG into the pcDNA6/V5-His B expression vector. However, transient expression of the cDNA in HEK293T cells showed that the encoded protein was not a wild-type molecule, as it was unable to be secreted in the culture supernatant. Sequencing analyses revealed three random mutations, which accidentally emerged during the course of cloning: c.1712T>C [p.L571P] in the linker domain (amino acid positions 360 to 604), c.2027A>G [p.Q676R] in TG type 1-6 repeat and c.2720A>G [p.Q907R] in the TG type 1-7 repeat. Expression of cDNAs encoding a combination of two mutations [p.Q676R-p.Q907R], [p.L571P-p.Q907R] or [p.L571P-p.Q676R] indicated that any TG bearing the p.L571P substitution was trapped intracellularly. Indeed, we expressed the single point mutant p.L571P and confirmed that this point mutation was sufficient to cause intracellular retention of mutant TG in HEK293T cells. Endo H analysis showed that the p.L571P mutant is completely sensitive to the enzyme, whereas the will-type TG acquires full N-glycan modifications in Golgi apparatus. This data suggest that the p.L571P mutant contains the mannose-type N-glycan, that was added at the first stage of glycosylation. Complex-type N-glycan formation in the Golgi apparatus does not occur, consistent with defective endoplasmic reticulum exit of the mutant TG. Moreover, predictive analysis of the 3D linker domain showed that the p.L571P mutation would result in a significant protein conformational change. In conclusion, our studies identified a novel amino acid residue within the linker domain of TG associated with its conformational maturation and intracellular trafficking.

Binding of thyroglobulin (Tg) to the low-density lipoprotein receptor-associated protein (RAP) during the biosynthetic pathway prevents premature Tg interactions with sortilin

OBJECTIVE

Sortilin, a Vps10p family member, is expressed by thyroid epithelial cells (TEC), where it binds to internalized thyroglobulin (Tg) molecules. Premature binding of Tg to sortilin during biosynthesis may cause intracellular retention of Tg. Such a premature interaction may be prevented by one or more inhibitor/s. Because both sortilin and Tg bind to the low-density lipoprotein receptor-associated protein (RAP), we investigated whether RAP serves such a function.

METHODS

Immunofluorescence staining for sortilin, Tg, and RAP was performed in FRTL-5 cells. Co-immunoprecipitation experiments were performed in extracts from FRTL-5 or COS-7 cells, the former co-transfected with Tg and/or RAP and/or sortilin, or in thyroid extracts from RAP KO mice.

RESULTS

Tg and sortilin did not co-localize in FRTL-5 cells following inhibition of protein synthesis, suggesting that newly synthesized, endogenous sortilin and Tg do not interact, in confirmation of which an anti-sortilin antibody did not co-precipitate Tg in FRTL-5 cells. In contrast, Tg co-localized with RAP in FRTL-5 cells. Co-immunoprecipitation of Tg with an anti-sortilin antibody in COS-7 cells transfected with sortilin and Tg was abolished when cells were co-transfected with RAP, indicating that RAP prevents binding of Tg to sortilin during biosynthesis, in confirmation of which an anti-sortilin antibody co-precipitated Tg in thyroid extracts from RAP KO mice to a greater extent than in thyroid extracts from WT mice.

CONCLUSIONS

Tg does not bind prematurely to sortilin because of its interaction with RAP during protein biosynthesis. These findings add new information to the knowledge of thyroid physiology.

3-Iodothyronamine Decreases Expression of Genes Involved in Iodide Metabolism in Mouse Thyroids and Inhibits Iodide Uptake in PCCL3 Thyrocytes

BACKGROUND

3-Iodothyronamine (3-T₁AM) is an endogenous decarboxylated thyroid hormone (TH) metabolite. Pharmacological doses of 3-T₁AM decrease heart rate, body temperature, and metabolic rate in rodents-effects that are contrary to classic TH excess. Furthermore, a single dose of 3-T₁AM was shown to suppress the hypothalamic-pituitary-thyroid (HPT) axis in rats. It was hypothesized that 3-T₁AM might play a role in the fine-tuning of TH action and might have a direct regulatory effect on the thyroid gland.

METHODS

This study tested whether repeated 3-T₁AM treatment interfered with thyroid function and the HPT axis in mice. Therefore, male C57BL/6 mice were intraperitoneally injected with 5 mg/kg of 3-T₁AM or vehicle daily for seven days. Additionally, the effects of 3-T₁AM on the differentiated rat thyrocyte cell line PCCL3 were analyzed.

RESULTS

Repeated administration of 3-T₁AM decreased thyroidal mRNA content of the sodium iodide symporter (Nis), thyroglobulin, and pendrin in mice. No interference with the HPT axis was observed, as determined by unaltered pituitary mRNA levels of triiodothyronine-responsive genes, including thyrotropin subunit β. Furthermore, 3-T₁AM treatment did not change transcript levels of hepatic triiodothyronine-responsive genes, such as deiodinase 1. In line with this, serum TH concentrations were not changed after the treatment period of seven days. In concordance with the in vivo findings, 3-T₁AM decreased the thyrotropin-dependent expression of Nis and functional iodide uptake in PCCL3 cells in vitro. Additionally, uptake and metabolism of 3-T₁AM by PCCL3 cells was observed, as well as 3-T₁AM-dependent changes in intracellular Ca²⁺ concentration that might be involved in mediating the reported effects.

CONCLUSIONS

In conclusion, 3-T₁AM application decreased expression of selected TH synthesis genes by acting directly on the thyroid gland, and it might therefore affect TH synthesis without involvement of the HPT axis.

Intracellular retention of thyroglobulin in the absence of the low-density lipoprotein receptor-associated protein (RAP) is likely due to premature binding to megalin in the biosynthetic pathway

OBJECTIVE

The low-density lipoprotein receptor associated protein (RAP) is expressed by thyroid epithelial cells (TEC) in a TSH-dependent manner. In the thyroid RAP functions as a molecular chaperone for the thyroglobulin (Tg) endocytic receptor megalin/LRP2, which is retained intracellularly in RAP KO mice rather than being expressed on the apical membrane of TEC, its usual location. RAP binds also to Tg, which is also retained intracellularly in RAP KO mice, thereby suggesting a role of RAP in Tg secretion. Here we investigated whether Tg intracellular retention in the absence of RAP is due to premature Tg-megalin interactions during the biosynthetic pathway or to a direct action of RAP on Tg secretion.

METHODS

We performed immunoprecipitation experiments in thyroid extracts from RAP KO and WT mice. In addition, we investigated Tg secretion in COS-7 cells co-transfected with human RAP (hRAP) and mouse Tg (mTg).

RESULTS

An anti-megalin megalin precipitated greater amounts of Tg in thyroid extracts from RAP KO than from WT mice, suggesting increased intracellular interactions between megalin and Tg in the absence of RAP. COS-7 cells transiently transfected with hRAP, mTg or both, expressed the two proteins accordingly. RAP was found almost exclusively in cell extracts, whereas Tg was found both in extracts and media, as expected from the knowledge that RAP is ER-resident and that Tg is secreted. Regardless of whether cells were transfected with mTg alone or were co-transfected with hRAP, similar proportions of the total Tg synthesized were detected in cell extracts and media.

CONCLUSIONS

The intracellular retention of Tg in the absence of RAP is likely due to its premature interaction with megalin, whereas RAP does not seem to affect Tg secretion directly.

Thyroglobulin Represents a Novel Molecular Architecture of Vertebrates

Thyroid hormones modulate not only multiple functions in vertebrates (energy metabolism, central nervous system function, seasonal changes in physiology, and behavior) but also in some non-vertebrates where they control critical post-embryonic developmental transitions such as metamorphosis. Despite their obvious biological importance, the thyroid hormone precursor protein, thyroglobulin (Tg), has been experimentally investigated only in mammals. This may bias our view of how thyroid hormones are produced in other organisms. In this study we searched genomic databases and found Tg orthologs in all vertebrates including the sea lamprey (Petromyzon marinus). We cloned a full-size Tg coding sequence from western clawed frog (Xenopus tropicalis) and zebrafish (Danio rerio). Comparisons between the representative mammal, amphibian, teleost fish, and basal vertebrate indicate that all of the different domains of Tg, as well as Tg regional structure, are conserved throughout the vertebrates. Indeed, in Xenopus, zebrafish, and lamprey Tgs, key residues, including the hormonogenic tyrosines and the disulfide bond-forming cysteines critical for Tg function, are well conserved despite overall divergence of amino acid sequences. We uncovered upstream sequences that include start codons of zebrafish and Xenopus Tgs and experimentally proved that these are full-length secreted proteins, which are specifically recognized by antibodies against rat Tg. By contrast, we have not been able to find any orthologs of Tg among non-vertebrate species. Thus, Tg appears to be a novel protein elaborated as a single event at the base of vertebrates and virtually unchanged thereafter.

Intrinsic regulation of thyroid function by thyroglobulin

BACKGROUND

The established paradigm for thyroglobulin (Tg) function is that of a high molecular weight precursor of the much smaller thyroid hormones, triiodothyronine (T3) and thyroxine (T4). However, speculation regarding the cause of the functional and morphologic heterogeneity of the follicles that make up the thyroid gland has given rise to the proposition that Tg is not only a precursor of thyroid hormones, but that it also functions as an important signal molecule in regulating thyroid hormone biosynthesis.

SUMMARY

Evidence supporting this alternative paradigm of Tg function, including the up- or downregulation by colloidal Tg of the transcription of Tg, iodide transporters, and enzymes employed in Tg iodination, and also the effects of Tg on the proliferation of thyroid and nonthyroid cells, is examined in the present review. Also discussed in detail are potential mechanisms of Tg signaling in follicular cells.

CONCLUSIONS

Finally, we propose a mechanism, based on experimental observations of Tg effects on thyroid cell behavior, that could account for the phenomenon of follicular heterogeneity as a highly regulated cycle of increasing and decreasing colloidal Tg concentration that functions to optimize thyroid hormone production through the transcriptional activation or suppression of specific genes.

Maturation of thyroglobulin protein region I

In vertebrates, the thyroglobulin (Tg) gene product must be exported to the lumen of thyroid follicles for thyroid hormone synthesis. In toto, Tg is composed of multiple type-1 repeats connected by linker and hinge (altogether considered as "region I," nearly 1,200 residues); regions II-III (~720 residues); and cholinesterase-like (ChEL) domain (~570 residues). Regions II-III and ChEL rapidly acquire competence for secretion, yet regions I-II-III require 20 min to become a partially mature disulfide isomer; stabilization of a fully oxidized form requires ChEL. Transition from partially mature to mature Tg occurs as a discrete "jump" in mobility by nonreducing SDS-PAGE, suggesting formation of at most a few final pairings of Cys residues that may be separated by significant intervening primary sequence. Using two independent approaches, we have investigated which portion of Tg is engaged in this late stage of its maturation. First, we demonstrate that this event is linked to oxidation involving region I. Introduction of the Tg-C1245R mutation in the hinge (identical to that causing human goitrous hypothyroidism) inhibits this maturation, although the Cys-1245 partner remains unidentified. Second, we find that Tg truncated after its fourth type-1 repeat is a fully independent secretory protein. Together, the data indicate that final acquisition of secretory competence includes conformational maturation in the interval between linker and hinge segments of region I.

Cis and trans actions of the cholinesterase-like domain within the thyroglobulin dimer

Thyroglobulin (Tg, precursor for thyroid hormone synthesis) is a large secreted glycoprotein composed of upstream regions I-II-III, followed by the approximately 570 residue cholinesterase-like (ChEL) domain. ChEL has two identified functions: 1) homodimerization, and 2) binding to I-II-III that facilitates I-II-III oxidative maturation required for intracellular protein transport. Like its homologs in the acetylcholinesterase (AChE) family, ChEL possesses two carboxyl-terminal alpha-helices. We find that a Tg-AChE chimera (swapping AChE in place of ChEL) allows for dimerization with monomeric AChE, proving exposure of the carboxyl-terminal helices within the larger context of Tg. Further, we establish that perturbing trans-helical interaction blocks homodimerization of the Tg ChEL domain. Additionally, ChEL can associate with neuroligins (a related family of cholinesterase-like proteins), demonstrating potential for Tg cross-dimerization between non-identical partners. Indeed, when mutant rdw-Tg (Tg-G2298R, defective for protein secretion) is co-expressed with wild-type Tg, the two proteins cross-dimerize and secretion of rdw-Tg is partially restored. Moreover, we find that AChE and soluble neuroligins also can bind to the upstream Tg regions I-II-III; however, they cannot rescue secretion, because they cannot facilitate oxidative maturation of I-II-III. These data suggest that specific properties of distinct Tg ChEL mutants may result in distinct patterns of Tg monomer folding, cross-dimerization with wild-type Tg, and variable secretion behavior in heterozygous patients.

The cholinesterase-like domain, essential in thyroglobulin trafficking for thyroid hormone synthesis, is required for protein dimerization

The carboxyl-terminal cholinesterase-like (ChEL) domain of thyroglobulin (Tg) has been identified as critically important in Tg export from the endoplasmic reticulum. In a number of human kindreds suffering from congenital hypothyroidism, and in the cog congenital goiter mouse and rdw rat dwarf models, thyroid hormone synthesis is inhibited because of mutations in the ChEL domain that block protein export from the endoplasmic reticulum. We hypothesize that Tg forms homodimers through noncovalent interactions involving two predicted alpha-helices in each ChEL domain that are homologous to the dimerization helices of acetylcholinesterase. This has been explored through selective epitope tagging of dimerization partners and by inserting an extra, unpaired Cys residue to create an opportunity for intermolecular disulfide pairing. We show that the ChEL domain is necessary and sufficient for Tg dimerization; specifically, the isolated ChEL domain can dimerize with full-length Tg or with itself. Insertion of an N-linked glycan into the putative upstream dimerization helix inhibits homodimerization of the isolated ChEL domain. However, interestingly, co-expression of upstream Tg domains, either in cis or in trans, overrides the dimerization defect of such a mutant. Thus, although the ChEL domain provides a nidus for Tg dimerization, interactions of upstream Tg regions with the ChEL domain actively stabilizes the Tg dimer complex for intracellular transport.

Perchlorate and ethylenethiourea induce different histological and molecular alterations in a non-mammalian vertebrate model of thyroid goitrogenesis

Despite evidence for a conserved role of thyroid-stimulating hormone (TSH) in regulating vertebrate thyroid function, molecular data on thyroid responses to TSH are mainly limited to mammalian species. In this study, we examined histological and molecular changes in the thyroid of Xenopus laevis tadpoles during a 12-day treatment with 20mg/l perchlorate (PER) and 50mg/l ethylenethiourea (ETU). Inhibition of thyroid hormone (TH) synthesis by PER and ETU was evident from developmental retardation, reduced expression of TH-regulated genes and up-regulation of tshb-A mRNA. Thyroid histopathology revealed goiters with strikingly different follicular morphologies following PER and ETU treatment. Using real-time PCR, we analyzed thyroids sampled on day 12 for differential expression of 60 candidate genes. Further temporal analyses were performed for a subset of 14 genes. Relative to the control, PER and ETU treatment modulated the expression of 51 and 49 transcripts, respectively. Particularly genes related to TH synthesis and protein metabolism were similarly affected by PER and ETU. However, several genes were differentially expressed in PER- and ETU-treated tadpoles. Specifically, goiter formation in the PER treatment was associated with low expression of genes related to DNA replication but high expression of negative growth regulators. Results from this work provide for the first time a characterization of gene expression profiles during goitrogenesis in a non-mammalian vertebrate model. Overall, our data suggest that, in addition to TSH over-stimulation, further mechanisms related to the mode of goitrogen action contribute to the regulation of thyroid gene expression.

The cholinesterase-like domain of thyroglobulin functions as an intramolecular chaperone

Thyroid hormonogenesis requires secretion of thyroglobulin, a protein comprising Cys-rich regions I, II, and III (referred to collectively as region I-II-III) followed by a cholinesterase-like (ChEL) domain. Secretion of mature thyroglobulin requires extensive folding and glycosylation in the ER. Multiple reports have linked mutations in the ChEL domain to congenital hypothyroidism in humans and rodents; these mutations block thyroglobulin from exiting the ER and induce ER stress. We report that, in a cell-based system, mutations in the ChEL domain impaired folding of thyroglobulin region I-II-III. Truncated thyroglobulin devoid of the ChEL domain was incompetent for cellular export; however, a recombinant ChEL protein ("secretory ChEL") was secreted efficiently. Coexpression of secretory ChEL with truncated thyroglobulin increased intracellular folding, promoted oxidative maturation, and facilitated secretion of region I-II-III, indicating that the ChEL domain may function as an intramolecular chaperone. Additionally, we found that the I-II-III peptide was cosecreted and physically associated with secretory ChEL. A functional ChEL domain engineered to be retained intracellularly triggered oxidative maturation of I-II-III but coretained I-II-III, indicating that the ChEL domain may also function as a molecular escort. These insights into the role of the ChEL domain may represent potential therapeutic targets in the treatment of congenital hypothyroidism.

Measuring thyroglobulin and thyroglobulin autoantibody in patients with differentiated thyroid cancer

Measurement of serum thyroglobulin is primarily used as a tumor marker in the postoperative management of patients with differentiated thyroid cancer. Unfortunately, the technical quality of current thyroglobulin assay methods varies and influences the clinical utility of this test. Two different methodologic approaches are used to measure serum thyroglobulin: the original competitive radioimmunoassay methodology and noncompetitive immunometric assay methods. Although the newer immunometric assays offer the technical benefits of eliminating the use of isotopes, using smaller specimen volumes, and having higher sensitivity potential, shorter turnaround times and the convenience of automation, immunometric assays also have a higher propensity for interference from both thyroglobulin autoantibodies and heterophilic antibodies, if present in the specimen. It is critical that physicians understand the technical limitations inherent in thyroglobulin measurement in order to effectively use this test for the postoperative management of patients with differentiated thyroid cancers.

Defective protein folding and intracellular retention of thyroglobulin-R19K mutant as a cause of human congenital goiter

It has been suggested that a thyroglobulin (Tg)-R19K missense mutation may be a newly identified cause of human congenital goiter, which is surprising for this seemingly conservative substitution. Here, we have examined the intracellular fate of recombinant mutant Tg expressed in COS-7 cells. Incorporation of the R19K mutation largely blocked Tg secretion, and this mutant was approximately 90% degraded intracellularly over a 24-h period after synthesis. Before its degradation, the Tg-R19K mutant exhibited abnormally increased association with molecular chaperones BiP, calnexin, and protein disulfide isomerase, and was unable to undergo anterograde advance from the endoplasmic reticulum (ER) through the Golgi complex. Inhibitors of proteasomal proteolysis and ER mannosidase-I both prevented ER-associated degradation of the Tg-R19K mutant and increased its association with ER molecular chaperones. ER quality control around Tg residue 19 is not dependent upon charge but upon side-chain packing, because Tg-R19Q was efficiently secreted. Whereas a Tg mutant truncated after residue 174 folds sufficiently well to escape ER quality control, introduction of the R19K point mutation blocked its secretion. The data indicate that the R19K mutation induces local misfolding in the amino-terminal domain of Tg that has global effects on Tg transport and thyroid hormonogenesis.

Defective thyroglobulin storage in LDL receptor-associated protein-deficient mice

The molecular chaperone receptor-associated protein (RAP) is required for biosynthesis of megalin, an endocytic receptor for follicular thyroglobulin (Tg), the thyroid hormone precursor. RAP also binds to Tg itself, suggesting that it may affect Tg trafficking in various manners. To elucidate RAP function, we have studied the thyroid phenotype in RAP-knockout (RAP-KO) mice and found a reduction of Tg aggregates into thyroid follicles. Serum Tg levels were significantly increased compared with those of wild-type (WT) mice, suggesting a directional alteration of Tg secretion. In spite of these abnormalities, hormone secretion was maintained as indicated by normal serum thyroxine levels. Because Tg in thyroid extracts from RAP-KO mice contained thyroxine residues as in WT mice, we concluded that in RAP-KO mice, follicular Tg, although reduced, was nevertheless sufficient to provide normal hormone secretion. Serum TSH was increased in RAP-KO mice, and although no thyroid enlargement was observed, some histological features resembling early goiter were present. Megalin was decreased in RAP-KO mice, but this did not affect thyroid function, probably because of the concomitant reduction of follicular Tg. In conclusion, RAP is required for the establishment of Tg reservoirs, but its absence does not affect hormone secretion.

ERp29, an unusual redox-inactive member of the thioredoxin family

Oxidative folding in the endoplasmic reticulum is accomplished by a group of oxidoreductases where the protein disulfide isomerase (PDI) plays a key role. Structurally, redox-active PDI domains, like many other enzymes utilizing cysteine chemistry, adopt characteristic thioredoxin folds. However, this structural unit is not necessarily associated with the redox function and the current review focuses on the interesting example of a loss-of-function PDI-like protein from the endoplasmic reticulum, ERp29. ERp29 shares a common predecessor with PDI; however in the course of divergent evolution it has lost a hallmark active site motif of redox enzymes but retained the characteristic structural fold in one of its domains. Although the functional characterization of ERp29 is far from completion, all available data point to its important role in the early secretory pathway and allow tentative categorization as a secretion factor/escort protein of a broad profile.

ERp29 is an essential endoplasmic reticulum factor regulating secretion of thyroglobulin

ERp29 is a ubiquitously expressed endoplasmic reticulum (ER) protein, which is found in the folding complexes of several secretory proteins in the ER. In our previous work, it was suggested that ERp29 function is critical for the folding/secretion of thyroglobulin (Tg), a major secretory product of thyroid cells. Current work is an attempt to substantiate this assumption by answering the question whether the secretion of Tg can be regulated through the manipulation of ERp29 expression in the FRTL-5 rat thyroid cells. Indeed, transient overexpression of ERp29 resulted in twofold enhancement of the Tg secretion whereas the RNAi-mediated ERp29 silencing led to the attenuation of the Tg export. Mutational analysis has suggested two loci that might be involved in the ERp29-Tg interactions: the interdomain linker including Cys157, an amino acid, which is important for the structural integrity of the C-terminal domain and an uncharged surface on the N-terminal domain flanked by Tyr64 and Gln70.

Naturally occurring mutations in the thyroglobulin gene

Thyroglobulin (Tg) is a large glycoprotein dimer secreted into the follicular lumen. It serves as the matrix for the synthesis of thyroxine (T4) and triiodothyronine (T3), and the storage of thyroid hormone and iodide. In response to demand for thyroid hormone secretion, Tg is internalized into the follicular cell and digested in lysosomes. Subsequently, the thyronines T4 (approximately 80%) and T3 (approximately 20%) are released into the blood stream. Biallelic mutations in the Tg gene have been identified in several animal species and human patients presenting with goiter and overt or compensated hypothyroidism. In untreated patients, goiters are often remarkably large and display continuous growth. In most instances, the affected individuals have related parents and are homozygous for inactivating mutations in the Tg gene. More rarely, compound heterozygous mutations lead to a loss of function of both alleles. Molecular analyses indicate that at least some of these alterations result in a secretory defect and an endoplasmic reticulum storage disease (ERSD). This review discusses the nature and consequences of naturally occurring Tg gene mutations in humans and several animal species. Recent recommendations for the nomenclature of mutations have led to different numbering systems, an aspect that is discussed in order to clarify discrepancies between different publications.

Androgen-dependent expression of FcgammaRIIB2 by thyrocytes from patients with autoimmune Graves' disease: a possible molecular clue for sex dependence of autoimmune disease

Thyrocyte expression of HLA class I and class II antigens and related accessory molecules would convert these epithelial cells into functional antigen-presenting cells. Here we show that whereas normal thyrocytes express FcRn, Graves' disease thyrocytes also express FcgammaRIIB2. We further find that expression of FcgammaRIIB2, but not FcRn, is repressed by dihydrotestosterone. By mediating the uptake and transport of autoantibodies, we suggest that these IgG Fc receptors contribute in various ways to the onset and/or progression of autoimmune thyroid diseases. The androgen-mediated decrease of FcgammaRIIB2 expression in Graves' disease thyrocytes also provides a rationale for the predominant susceptibility of women to develop an autoimmune thyroid disease. Our findings open up a new prospect to autoimmunity, linking the role of the target organ to the sex dependence in autoimmune disease.

Identification of ERp29, an endoplasmic reticulum lumenal protein, as a new member of the thyroglobulin folding complex

Folding and post-translational modification of the thyroid hormone precursor, thyroglobulin (Tg), in the endoplasmic reticulum (ER) of the thyroid epithelial cells is facilitated by several molecular chaperones and folding enzymes, such as BiP, GRP94, calnexin, protein disulfide isomerase, ERp72, and others. They have been shown to associate simultaneously and/or sequentially with Tg in the course of its maturation, thus forming large heterocomplexes in the ER of thyrocytes. Here we present evidence that such complexes include a novel member, an ER-resident lumenal protein, ERp29, which is present in all mammalian tissues with exceptionally high levels of expression in the secretory cells. ERp29 was induced upon treatment of FRTL-5 rat thyrocytes with the thyroid-stimulating hormone, which is essential for the maintenance of thyroid cells and Tg biosynthesis. Chemical cross-linking followed by the cell lysis and immunoprecipitation of ERp29 or Tg revealed association of these proteins and additionally, immunocomplexes that also included major ER chaperones, BiP and GRP94. Sucrose density gradient analysis indicated co-localization of ERp29 with Tg and BiP in the fractions containing large macromolecular complexes. This was supported by immunofluorescent microscopy showing co-localization of ERp29 with Tg in the putative transport vesicular structures. Affinity chromatography using Tg as an affinity ligand demonstrated that ERp29 might be selectively isolated from the FRTL-5 cell lysate or purified lumenal fraction of rat liver microsomes along with the other ER chaperones. Preferential association with the urea-denatured Tg-Sepharose was indicative of either direct or circuitous ERp29/Tg interactions in a chaperone-like manner. Despite the presence of the C-terminal ER-retrieval signal, significant amounts of ERp29 were also recovered from the culture medium of stimulated thyrocytes, indicating ERp29 secretion. Based on these data, we suggest that the function of ERp29 in thyroid cells is connected with folding and/or secretion of Tg.

Polarized trafficking of thyrocyte proteins in MDCK cells

Recent studies suggest striking similarities between polarized protein sorting in thyrocytes and MDCK epithelial cells, including apical trafficking of thyroglobulin (Tg), thyroid peroxidase, and aminopeptidase N; as well as basolateral targeting of heparan sulfate proteoglycans, thrombospondin 1 (TSP1), type 1 5'-deiodinase, sodium-potassium ATPase, and the thyrotropin receptor. In this report, we have firstly expressed in stably transfected MDCK II cells a range of truncation mutants lacking up to 78% of the C-terminus of TSP1; these studies indicate that the N-terminal region containing the heparin binding domain is sufficient for basolateral targeting of TSP1. Secondly, we have stably transfected MDCK II cells with both Tg and sodium-iodide symporter (NIS) cDNAs, obtaining clones that simultaneously express both thyroid-specific proteins at the apical and basolateral cell surfaces, respectively. These studies represent promising early steps towards designing artificial thyrocytes by thyroid gene transfer into MDCK cells.

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.

Thyroglobulin is selected as luminal protein cargo for apical transport via detergent-resistant membranes in epithelial cells

Thyroid hormone synthesis by thyrocytes depends upon apical secretion of thyroglobulin (Tg), the glycoprotein prohormone. In stably transfected MDCK cells, recombinant Tg is also secreted apically. All secreted Tg has undergone Golgi carbohydrate modification, whereas most intracellular Tg (which is slow to exit the endoplasmic reticulum) is sensitive to digestion with endoglycosidase H. However, in MDCK cells and PC Cl3 thyrocytes, a subpopulation of newly synthesized recombinant and endogenous Tg, respectively, is recovered in a Triton X-100 insoluble, glycosphingolipid/cholesterol-enriched (GEM/raft) fraction, and this small subpopulation is overwhelmingly endoglycosidase H resistant. Upon apical secretion, Tg solubility is restored. Apical secretion of Tg is inhibited by cellular cholesterol depletion. In FRT cells, recombinant Tg becomes Triton X-100 insoluble within 60 min after synthesis and a portion is actually endoglycosidase H-sensitive, suggesting early Tg entry into GEMs/rafts. Interestingly in FRT cells, Tg remains associated with the apical plasma membrane upon exocytosis, and all surface Tg is GEM/raft-associated. Thus, Tg is the first secretory protein demonstrated to enter Triton X-100 insoluble membranes en route to the apical surface of epithelial cells. The data imply that Tg utilizes a cargo-selective mechanism for apical sorting.

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.

Protein-disulfide isomerase (PDI) in FRTL5 cells. pH-dependent thyroglobulin/PDI interactions determine a novel PDI function in the post-endoplasmic reticulum of thyrocytes

Thyroglobulin (TG) is secreted by the thyrocytes into the follicular lumen of the thyroid. After maturation and hormone formation, TG is endocytosed and delivered to lysosomes. Quality control mechanisms may occur during this bidirectional traffic since 1) several molecular chaperones are cosecreted with TG in vivo, and 2) lysosomal targeting of immature TG is thought to be prevented via the interaction, in acidic conditions, between the Ser(789)-Met(1172) TG hormonogenic domain (BD) and an unidentified membrane receptor. We investigated the secretion and cell surface expression of PDI and other chaperones in the FRTL5 thyroid cell line, and then studied the characteristics of the interaction between TG and PDI. We demonstrated that PDI, but also other chaperones such as calnexin and KDEL-containing proteins are exposed at the cell surface. We observed on living cells or membrane preparations that PDI specifically binds TG in acidic conditions, and that only BD is involved in binding. Surface plasmon resonance analysis of TG/PDI interactions indicated: 1) that PDI bound TG but only in acidic conditions, and that it preferentially recognized immature molecules, and 2) BD is involved in binding even if cysteine-rich modules are deleted. The notion that PDI acts as an "escort" for immature TG in acidic post-endoplasmic reticulum compartments is discussed.

Polarized secretion of transgene products from salivary glands in vivo

Previously (Kagami et al. Hum. Gene Ther. 1996;7:2177-2184) we have shown that salivary glands are able to secrete a transgene-encoded protein into serum as well as saliva. This result and other published data suggest that salivary glands may be a useful target site for vectors encoding therapeutic proteins for systemic delivery. The aim of the present study was to assess in vivo if transgene-encoded secretory proteins follow distinct, polarized sorting pathways as has been shown to occur "classically" in cell biological studies in vitro. Four first-generation, E1-, type 5 recombinant adenoviruses were used to deliver different transgenes to a rat submandibular cell line in vitro or to rat submandibular glands in vivo. Subsequently, the secretory distribution of the encoded proteins was determined. Luciferase, which has no signal peptide, served as a cell-associated, negative control and was used to correct for any nonspecific secretory protein release from cells. The three remaining transgene products tested, human tissue kallikrein (hK1), human growth hormone (hGH), and human alpha1-antitrypsin (halpha1AT), were predominantly secreted (>96%) in vitro. Most importantly, in vivo, after a parasympathomimetic secretory stimulus, both hK1 and hGH were secreted primarily in an exocrine manner into saliva. Conversely, halpha1AT was predominantly secreted into the bloodstream, i.e., in an endocrine manner. The aggregate results are consistent with the recognition of signals encoded within the transgenes that result in specific patterns of polarized protein secretion from rat submandibular gland cells in vivo.

Polarized targeting of epithelial cell proteins in thyrocytes and MDCK cells

Polarized trafficking signals may be interpreted differently in different cell types. In this study, we have compared the polarized trafficking of different proteins expressed endogenously in primary porcine thyroid epithelial cells to similar proteins expressed in MDCK cells. As in MDCK cells, NH4Cl treatment of filter-grown thyrocytes caused mis-sorted soluble proteins to exhibit enhanced secretion to the apical medium. In independent studies, thrombospondin 1 (a thyroid basolaterally secreted protein) was secreted basolaterally from MDCK cells. Likewise, the 5′-deiodinase (a thyroid basolateral membrane protein) encoded by the DIO1 gene was also distributed basolaterally in transfected MDCK cells. Consistent with previous reports, when the secretion of human growth hormone (an unglycosylated regulated secretory protein) was examined from transfected MDCK cells, the release was nonpolarized. However, transfected thyrocytes secreted growth hormone apically in a manner dependent upon zinc addition. Moreover, two additional regulated secretory proteins expressed in thyrocytes, thyroglobulin (the major endogenous glycoprotein) and parathyroid hormone (an unglycosylated protein expressed transiently), were secreted apically even in the absence of zinc. We hypothesize that while cellular mechanisms for interpreting polarity signals are generally similar between thyrocytes and MDCK cells, thyrocytes allow for specialized packaging of regulated secretory proteins for apical delivery, which does not require glycosylation but may involve availability of certain ions as well as appropriate intracellular compartmentation.

Tyrosine transport into melanosomes is increased following stimulation of melanocyte differentiation

A variety of physiological factors can stimulate differentiation of melanocytes to increase pigmentation, and critical to this process is the transport of the melanogenic substrate (tyrosine) into melanosomes. In this study, we examined whether stimulation of melanogenesis affects melanosomal tyrosine transport. Tyrosine uptake increased almost 2-fold in melanosomes derived from melanocytes treated with melanocyte-stimulating hormone (MSH), which acts to increase intracellular cAMP levels, resulting in the up-regulation of many genes involved in melanogenesis. Stimulation of melanoma cells with dibutyryl cAMP increased melanosomal tyrosine transport 2- to 3-fold after 24 to 48 hrs, with peak levels occurring after 3 to 5 days of treatment, suggesting that de novo gene expression may be required. The cAMP-induced increase in melanosomal tyrosine transport could be effectively competed with phenylalanine or tryptophan, but not with dopamine or proline, suggesting either that a pool of transporters with greater tyrosine transporting ability pre-exists, or that a greater number of tyrosine transporters reside within the melanosomal membrane. These results illustrate a rare example of hormonal plasma membrane stimulation which transduces a signal for increased vesicular transport of an amino acid.

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.