Thyroglobulin internalized by thyrocytes passes through early and late endosomes

Serum thyroglobulin is associated with orbitopathy in Graves' disease

PURPOSE

Serum thyroglobulin levels are often elevated in Graves' disease (GD) and in most cases decrease during treatment. Its relation to Graves' orbitopathy (GO) has not been clarified. Previously, a risk of GO has been linked to smoking, TSH receptor stimulation, high TSH-receptor antibodies (TRAb), low thyroid peroxidase and thyroglobulin antibodies (TPOAb, TgAb).

METHODS

We examined Tg levels in 30 consecutive patients with GD were given drug therapy (methimazole + thyroxine) for up to 24 months. GO was identified by clinical signs and symptoms. 17 patients had GO, 11 of whom had it at diagnosis while 6 developed GO during treatment. During the study, 5 subjects were referred to radioiodine treatment, 3 to surgery. The remaining 22 subjects (GO n = 12, non-GO n = 10) completed the drug regimen.

RESULTS

At diagnosis, Tg levels in GO patients (n = 11) were higher (84, 30-555 µg/L, median, range) than in non-GO patients (n = 19) (38, 3.5-287 µg/L), p = 0.042. Adding the 6 subjects who developed eye symptoms during treatment to the GO group (n = 17), yielded p = 0.001 vs. non-GO (n = 13). TRAb tended to be higher, while TPOAb and TgAb tended to be lower in the GO group. For the 22 patients who completed the drug regimen, Tg levels were higher in GO (n = 12) vs. non-GO (n = 10), p = 0.004, whereas TRAb levels did not differ.

CONCLUSION

The data may suggest that evaluation of thyroglobulin levels in GD could contribute to identify patients at increased risk of developing GO. Possibly, thyroidal release of Tg in GD reflects a disturbance that also impacts retroorbital tissues.

ABSENCE OF A THYROID PHENOTYPE IN SORTILIN-DEFICIENT MICE

OBJECTIVE

The Vps10p family member sortilin is expressed in thyroid epithelial cells where it contributes to recycling of the thyroid hormone precursor thyroglobulin (Tg), a process that is thought to render hormone release more effective. Here we investigated the functional impact of sortilin in the thyroid gland using sortilin-deficient mice.

METHODS

We measured free T4, thyroid-stimulating hormone (TSH) and Tg serum levels and studied thyroid morphology in 14 sortilin-deficient (Sort1)(-/-)and 12 wildtype (WT) mice.

RESULTS

Serum free T4 levels did not differ between Sort1(-/-)and WT females but were significantly lower in Sort1(-/-)males compared with WT (P = .0424). Neither serum TSH nor Tg levels differed between Sort1(-/-)and WT mice, regardless of sex. On the same line, no thyroid histology differences were observed.

CONCLUSION

Our findings seem to exclude a role of sortilin in thyroid hormone secretion, although it is possible that the absence of sortilin may result in a thyroid phenotype if combined with other molecular defects of thyroid hormone synthesis and secretion or under iodine deficiency.

P-Type Lectins: Cation-Dependent Mannose-6-Phosphate Receptor

In eukaryotic cells, post-translational modification of secreted proteins and intracellular protein transport between organelles are ubiquitous features. One of the most studied systems is the N-linked glycosylation pathway in the synthesis of secreted glycoproteins (Schrag et al. 2003). The N-linked glycoproteins are subjected to diverse modifications and are transported through ER and Golgi apparatus to their final destinations in- and outside the cell. Incorporation of cargo glycoproteins into transport vesicles is mediated by transmembrane cargo receptors, which have been identified as intracellular lectins. For example, mannose 6-phosphate receptors (Ghosh et al. 2003) function as a cargo receptor for lysosomal proteins in the trans-Golgi network, whereas ERGIC-53 (Zhang et al. 2003) and its yeast orthologs Emp46/47p (Sato and Nakano 2002) are transport lectins for glycoproteins that are transported out of ER.

Cathepsin C and plasma glutamate carboxypeptidase secreted from Fischer rat thyroid cells liberate thyroxin from the N-terminus of thyroglobulin

The release of a thyroid hormone from thyroglobulin is controlled by a complex regulatory system. We focused on the extracellular action of two lysosomal enzymes, cathepsin C (catC, dipeptidyl peptidase I) and PGCP (lysosomal dipeptidase), on thyroglobulin, and their ability to liberate the hormone thyroxin. Cathepsin C, an exopeptidase, removes dipeptides from the N-terminus of substrates, and PGCP hydrolyses dipeptides to amino acids. In vitro experiments proved that cathepsin C removes up to 12 amino acids from the N-terminus of porcine thyroglobulin, including a dipeptide with thyroxin on position 5. The newly formed N-terminus, Arg-Pro-, was not hydrolysed further by cathepsin C. Cell culture experiments with FRTL-5 cell line showed localization of cathepsin C and PGCP and their secretion into the medium. Secretion of the active cathepsin C from FRTL-5 cells is stimulated by TSH, insulin, and/or somatostatin. The released enzymes liberate thyroxin from porcine thyroglobulin added to media. The hormone liberation can be reduced by synthetic inhibitors of cysteine proteinases and metalloproteinases. Additionally, we show that TSH, insulin, and/or somatostatin induce up-regulation of N-acetylglucosaminyltransferase 1, the enzyme responsible for the initiation of biosynthesis of hybrid and complex N-glycosylation of proteins.

Binding, uptake, and degradation of internalized thyroglobulin in cultured thyroid and non-thyroid cells

Thyroid hormone release requires degradation of thyroglobulin (Tg) by thyroid epithelial cells, which occurs mainly in the lysosomal pathway following Tg endocytosis. Non-specific fluid-phase endocytosis is thought to be the main route of Tg uptake leading to degradation, whereas receptor- mediated endocytosis is believed to lead to post-endocytic pathways other than degradation. To gain more insights into these issues, we investigated handling of Tg by various cell types. Tg bound similarly to thyroid (FRTL-5, FRT) and non-thyroid (COS-7, IRPT) cells, indicating the presence of membrane-binding sites, presumably receptors, in both cell types. Tg was internalized and degraded by all cells and degradation paralleled uptake, with the exception of FRTL- 5 cells, in which a lower proportion of Tg was degraded, suggesting that in FRTL-5 cells mechanisms that target Tg to the various post-endocytic pathways (either receptors or postreceptorial factors) are differently represented. Immunoelectronmicroscopy showed a common path of endocytosis in FRTL-5, COS-7, and IRPT cells, namely the formation of pseudopods engulfing Tg, followed by internalization and accumulation of Tg in cytoplasmic vesicles and lysosomes. The fastest rate was observed in COS-7 cells, probably reflecting a lower impact of endocytic receptors. Our findings suggest that Tg uptake and degradation are not thyroid-specific, that Tg binding sites exist in different cell types, and that uptake and/or degradation are differently regulated in differentiated thyroid cells, presumably because of a different impact of endocytic receptors or post-endocytic mechanisms, which are probably responsible for the regulation of hormone release.

Sortilin is a putative postendocytic receptor of thyroglobulin

The Vps10p family member sortilin is involved in various cell processes, including protein trafficking. Here we found that sortilin is expressed in thyroid epithelial cells (thyrocytes) in a TSH-dependent manner, that the hormone precursor thyroglobulin (Tg) is a high-affinity sortilin ligand, and that binding to sortilin occurs after Tg endocytosis, resulting in Tg recycling. Sortilin was found to be expressed intracellularly in thyrocytes, as observed in mouse, human, and rat thyroid as well as in FRTL-5 cells. Sortilin expression was demonstrated to be TSH dependent, both in FRTL-5 cells and in mice treated with methimazole and perchlorate. Plasmon resonance binding assays showed that Tg binds to sortilin in a concentration-dependent manner and with high affinity, with Kd values that paralleled the hormone content of Tg. In addition, we found that Tg and sortilin interact in vivo and in cultured cells, as observed by immunoprecipitation, in mouse thyroid extracts and in COS-7 cells transiently cotransfected with sortilin and Tg. After incubation of FRTL-5 cells with exogenous, labeled Tg, sortilin and Tg interacted intracellularly, presumably within the endocytic pathway, as observed by immunofluorescence and immunoelectron microscopy, the latter technique showing some degree of Tg recycling. This was confirmed in FRTL-5 cells in which Tg recycling was reduced by silencing of the sortilin gene and in CHO cells transfected with sortilin in which recycling was increased. Our findings provide a novel pathway of Tg trafficking and a novel function of sortilin in the thyroid gland, the functional impact of which remains to be established.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Recombinant human thyroid-stimulating hormone: pharmacology, clinical applications and potential uses

The major functions of pituitary thyroid-stimulating hormone (TSH) are to maintain the biosynthesis and secretion of the thyroid hormones L-thyroxine (T4) and L-3,5,3'triidothyronine (T3). The TSH core contains two apoproteins, the alpha and beta subunits. The alpha subunit is identical to that of pituitary follitropin, pituitary lutropin and placental chorionic gonadotropin, whereas the beta subunit is unique. TSH is a glycoprotein; the glycoprotein components of the alpha and beta subunits account for more than 10% of their mass and are essential for normal thyrotropic action and intravascular kinetics. The hypothalamic tripeptide, TSH-releasing hormone (TRH) is required for optimum TSH biosynthesis, particularly as far as addition of the glycoprotein components is concerned. TRH deficiency is associated with secretion of TSH molecules that are appropriately measured in most assays but have reduced bioactivity. In previous years the TSH used in clinical practice was obtained and purified from bovine pituitaries. Bovine TSH was used to test thyroid function and to augment the uptake of radioiodine in patients with thyroid cancer. Bovine TSH has been largely abandoned as a clinical agent because of adverse immune reactions. A recombinant human TSH (rhTSH; Thyrogen), has been approved by the US FDA for diagnostic use in patients with thyroid cancer. The alpha and beta subunits of Thyrogen are identical to those of human pituitary TSH. Thyrogen has a specific activity of approximately 4 IU/mg and is a potent stimulator of T4, T3 and thyroglobulin (Tg) secretion in healthy volunteers. It also increases thyroid iodide uptake in patients with thyroid cancer or multinodular goitre and in volunteers, even those exposed to large amounts of stable iodide. Thyroid cancer patients who have been treated by thyroidectomy and radioiodine ablation but are at risk of harbouring residual thyroid cancer are candidates for Thyrogen administration to prepare them for whole body iodide scans and serum Tg measurements. In thyroidectomised thyroid cancer patients who are unable to secrete pituitary TSH upon thyroid hormone withdrawal, Thyrogen is the only acceptable method to prepare them for these procedures. Thyrogen has been used on a compassionate basis to prepare patients for radioiodine ablation. rhTSH, in addition to being useful in the management of patients with thyroid cancer, is potentially useful to test thyroid reserve and to aid in thyroid-related nuclear medicine procedures. In the future, TSH analogues that have superagonist or antagonist properties may become available as therapeutic agents.

Update on intrathyroidal iodine metabolism

The thyroid concentrates iodide from the serum and oxidizes it at the apical membrane, attaching it to tyrosyl residues within thyroglobulin (Tg) to make diiodotyrosine and monoiodotyrosine. Major players in this process are Tg, thyroperoxidase (TPO), hydrogen peroxide, pendrin, and nicotinamide adenine dinucleotide phosphate (NADPH). Further action of TPO, hydrogen peroxide (H2O2), and iodinated Tg produce thyroxine (T4) and triiodothyronine (T3). Hormone-containing Tg is stored in the follicular lumen, then processed, most commonly by micropinocytosis. The lysosomal enzymes cathepsins B, L, and D are active in Tg proteolysis. Tg digestion leaves T4 and T3 intact, to be released from the cell, while the 3,5'-diiodotyrosine (DIT) and 3-iodotyrosine (MIT) are retained and deiodinated for recycling within the thyroid. Some areas of especially active recent research include: (1) the role of molecular chaperones in directing properly folded TPO and Tg to the apical membrane; (2) details of proteolytic pathways; (3) modulation of iodine metabolism, not only by thyrotropin (TSH) but by iodine supply and by feedback effects of Tg, glutathione, and inhibitory elements in the N-terminal region of Tg; and (4) details of Tg structure and iodotyrosyl coupling. Despite general agreement on the major steps in intrathyroidal iodine metabolism, new details of mechanisms are constantly being uncovered and are greatly improving understanding of the overall process.

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.

Binding of rat thyroglobulin to heparan sulfate proteoglycans

We previously showed that rat thyroglobulin (Tg) is a heparin-binding protein and that heparin inhibits Tg binding to megalin (gp330), an endocytic Tg receptor found on the apical surface of thyrocytes. Cooperation between cell surface receptors and heparin-like molecules, namely heparan sulfate proteoglycans (HSPGs), can facilitate cell surface binding of some heparin-binding proteins. Based on our previous findings indicating that heparin and megalin-binding sites of rat Tg are functionally related, here we investigated whether rat Tg binds to HSPGs, which are expressed by thyroid cells. We showed in solid phase assays that unlabeled rat Tg binds to a heparan sulfate (HS) preparation in a dose-dependent, saturable manner, with moderately high affinity (Kd approximately 19 nM, Ki approximately 25 nM). Binding was inhibited by heparin and by HS itself. We then studied the role of HSPGs in Tg binding to FRTL-5 cells, a differentiated Fisher rat thyroid cell line. As previously reported, after incubation of FRTL-5 cells with unlabeled rat Tg at 4 degrees C, heparin released virtually all the cell-bound Tg. Co-incubation of Tg with HS or with a preparation of HSPGs resulted in a reduction of binding by 35%-40%. When FRTL-5 cells were preincubated with heparitinase or heparinase I, which released 20%-30% of cell surface HSPGs, Tg binding was reduced to a similar extent. An antibody against a Tg heparin-binding site functionally related to a major megalin-binding site virtually abolished Tg binding to HS and to FRTL-5 cells, supporting the hypothesis that combined interactions of Tg with HSPGs and with megalin are involved in Tg binding to rat thyroid cells.

Selective endocytosis of thyroglobulin: a review of potential mechanisms for protecting newly synthesized molecules from premature degradation

In 1976 Cortese, Schneider and Salvatore (Eur. J. Biochem. 68 (1976) 121-129) showed that the thyroid gland protects newly synthesized, iodine and hormone poor thyroglobulin from immediate degradation. Since then there has been substantial progress in understanding the mechanism by which this selectivity of degradation occurs. Thyroglobulin in the follicular lumen is internalized mainly by receptor-specific endocytosis. Recycling of immature, poorly iodinated thyroglobulin back to the follicular lumen is the pathway most likely responsible for selectivity. Since additional carbohydrate groups are added to the immature thyroglobulin, it appears that this recycling occurs via the Golgi compartment. The molecular signal for recycling most likely involves the complex carbohydrates and probably is exposed GlcNAc groups. A thyroid-specific GlcNAc receptor has been identified and cloned. Other Tg-binding sites have been identified in the thyroid, but their physiological role remains to be determined.

Regulation of the three-dimensional organization of thyroid epithelial cells into follicle structures by the matricellular protein, thrombospondin-1

Thyroid epithelial cells in primary culture have the capacity to organize into thyroid-specific three-dimensional structures, the follicles, in response to TSH. We studied whether thrombospondin 1 (TSP1), which represents, besides thyroglobulin, the main protein secreted by thyroid cells, could play a role in the process of folliculogenesis. TSH promoted follicle formation and inhibited TSP1 production. On the contrary, the phorbol ester, 12-O-tetradecanoyl-phorbol 13-acetate (1-100 nM) prevented TSH-induced follicle formation and strongly increased the synthesis of TSP1. Activation of TSP1 synthesis was dependent upon messenger RNA synthesis. Transforming growth factor-beta, like 12-O-tetradecanoyl-phorbol 13-acetate, increased TSP1 synthesis and prevented TSH-induced follicle formation. Thus, signaling molecules that depressed or conversely activated TSP1 production, respectively promoted or prevented thyroid folliculogenesis. TSP1, purified from platelets, was devoid of effect on cell substratum attachment, but exerted a concentration-dependent inhibition of the TSH-activated reconstitution of thyroid follicles (half-inhibition at 40 microg/ml). TSP1 exhibited the same effect when added to thyroid cell aggregates representing primitive follicle structures. Our data suggest that the control of thyroid follicle formation may operate at least in part through regulation of the production of the matricellular protein TSP1, which acts as a negative modulator of the cell-cell adhesion process involved in thyroid follicle morphogenesis.

Calcium is transported into the lumen of pig thyroid follicles by fluid phase basolateral to apical transcytosis

The lumen of thyroid follicles contains a high concentration of thyroglobulin, the thyroid prohormone and a high concentration of calcium (Ca2+). As thyroglobulin binds Ca2+, intraluminal Ca2+ is expected to be in free and protein-bound forms. In the present work, we have investigated the mechanism(s) by which Ca2+ could enter the lumen of thyroid follicles. 45Ca2+ uptake studies were carried out on reconstituted pig thyroid follicles (RTF) and pig thyroid cell monolayers (TCM) in primary culture, representing experimental systems with two compartments (cells + lumina) and one compartment, respectively. 45Ca2+ accumulation in RTF was rapid during the first hour of incubation and then slowly increased. Analysis of the uptake data with a "two compartments" model gave two kinetic constant values: k = 1.71 +/- 0.28 hr(-1) and k(-2) = 0.20 +/- 0.05 hr(-1) (n = 10). The slow uptake process accounted for 20-50% of the total RTF-associated Ca2+ after 24 hr. 45Ca2+ uptake by TCM was rapid and reached a stable level within 1-2 hr. Experimental data fitted with a "single compartment" model and gave a k(-1) value of 1.64 +/- 0.15 hr(-1) (n = 10) which was not statistically different from the k(-1) obtained for 45Ca2+ uptake by RTF. We then compared the kinetics of 45Ca2+ uptake by RTF with the kinetics of transport of fluid phase markers: [14C]-sucrose and Lucifer Yellow from the medium to the lumen of RTF. [14C]-sucrose and Lucifer Yellow uptakes by RTF appeared as slow processes compatible with the entry in a single compartment with k values of 0.32 +/- 0.06 hr(-1) (n = 3) and 0.23 +/- 0.015 hr(-1) (n = 3), respectively. These values were not significantly different from the k(-2) value obtained for 45Ca2+ uptake by RTF. These data suggest that thyroid follicles would possess two independent Ca2+ compartments: cells and lumen, and that the entry of Ca2+ into the lumen of follicles probably could take place by fluid phase basolateral to apical transcytosis.

Effects of Propyithiouracil (PTU) Administration on the Synthesis and Secretion of Thyroglobulin in the Rat Thyroid Gland: A Quantitative Immuno-electron Microscopic Study Using Immunogold Technique

To clarify the effects of an antithyroid drug on the kinetics of thyroglobulin synthesis, secretion, and reabsorption in the thyroid follicles, propylthiouracil (PTU) was administered to rats and the thyroid glands were examined by a refined post-embedding immunogold technique during and after withdrawal of PTU. Seven-wk-old male Wistar rats were administered with S mg of PTU/d through a gastric tube, and sacrificed at 1 and 2 wk of administration and at 1, 2, and 3 d, and 1, 2, 3, and 4 wk, after discontinuation. The administration of PTU caused a remarkable dilatation of the rER and Golgi apparatus, but these areas gradually recovered after withdrawal of PTU. During the experiment, no significant change in the density of thyroglobulin (Tg) was observed except for a transient increase immediately after withdrawal of PTU. The expression of Tg on subapical vesicles (SV) and follicular colloid took a relatively parallel course; increasing during administration of PTU and decreasing with a transient peak immediately after treatment was discontinued. In contrast to the remarkable changes in the morphology of compartments involved in Tg synthesis, the development of colloid droplets and formation of secondary lysosomes were suppressed during and after discontinuing administration of PTU. However, the basic pattern of the gradient of Tg density among the cellular compartments was essentially retained in the experimental group. Thus the present immunoelectron-microscopic study provided evidence that administration of PTU stimulates the synthesis and secretion of Tg in the follicular epithelium in vivo, and, also, suppresses reabsorption and degradation of Tg. Further, it was speculated that the density gradient of Tg among the compartments involved in Tg synthesis, secretion and storage is regulated by an unknown constitutive mechanism and not by the thyroid-stimulating hormone (TSH)-TSH receptor-mediated system.

Involvement of a membrane-bound form of glutamate dehydrogenase in the association of lysosomes to microtubules

A 50-kDa membrane protein corresponding to a membrane-bound isoform of glutamate dehydrogenase was proposed as a molecular species that could mediate lysosome-microtubule interactions. This protein, isolated from purified lysosome membranes, is a peripheral membrane protein with an ATP-dependent microtubule binding activity. We have produced antibodies against the purified 50-kDa protein to investigate its role in the association of lysosomes to microtubules using a cell-free reconstitution assay and cell microinjection. Pretreatment of purified lysosomes with the antibodies inhibited the association of these vesicles to microtubules. The blocking effect of antibodies was demonstrated by a differential sedimentation method and negative staining electron microscopy, allowing us to quantify the amount of microtubules interacting with lysosomes and the proportion of lysosomes bound to microtubules, respectively. Affinity-purified antibodies microinjected into intact cells altered the distribution of lysosomes that appeared less clustered in the vicinity of nuclei. The antibody-induced lysosome dispersion was assessed by quantitative videomicroscope analyses. These data show that the 50-kDa membrane protein could act, through its microtubule binding activity, as a molecule of attachment of lysosomes to microtubules. This membrane-bound isoform of glutamate dehydrogenase could be involved in the microtubule-dependent perinuclear localization of lysosomes.

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

Thyroglobulin (Tg) proteolytic steps are central phenomena in Tg processing and thyroid hormone release in thyrocytes. Based on recent literature data, we propose that the type-1 repetitive units present in the Tg sequence could act as binders and reversible inhibitors of the proteases implicated in Tg processing. The pH-dependent interactions of proteases with the repeats could permit (i) protection from degradation of low iodinated Tg to be recycled; (ii) restriction of early proteolytic attacks to N- and C-terminal hormone formation sites; (iii) increase of the half-time of acidic proteases necessary for the final, extensive degradation steps of Tg.

Intracellular routing of GLcNAc-bearing molecules in thyrocytes: selective recycling through the Golgi apparatus

Previous experiments led us to speculate that thyrocytes contain a recycling system for GlcNAc-bearing immature thyroglobulin molecules which prevents these molecules from lysosomal degradation (Miquelis, R., C. Alquier, and M. Monsigny. 1987. J. Biol. Chem. 262:15291-15298). To confirm this hypothesis, the fate of GlcNAc-bearing proteins after internalization by thyrocytes was monitored and compared to that of fluid phase markers. Kinetic internalization studies were performed using 125I-GlcNAc-BSA and 131I-Man-BSA. We observed that the apparent intake rate as well as the amount of hydrolyzed GlcNAc-BSA are smaller than the corresponding values for Man-BSA. These differences were reduced by GlcNAc competitors (thyroglobulin and ovomucoid) or a weak base (chloroquine). Part of the internalized GlcNAc-BSA was released into the extracellular milieu at a higher rate and shorter half life (t1/2 = approximately 30 min) than the Man-BSA (t1/2 = approximately 8 h). Subcellular homing was first studied by cell fractionation after internalization using 125I-ovomucoid and 131I-BSA. During Percoll density gradient fractionation, endogenous thyroperoxidase was used to separate subsets of organelles involved in the biosynthetic exocytotic pathway. Incubation of the cell homogenate in the presence of DAB and H2O2 before cell fractionation give rise to a shift in the density of organelles containing 3.5 times more ovomucoid than BSA. Discontinuous sucrose gradient showed that: (a) thyroperoxidase was colocalized with galactosyltransferase-contraining organelles in Golgi-rich subfractions; and (b) that at every time studied from 10 to 100 min, the ovomucoid/BSA ratio was higher in these organelles than in other subfractions. Finally we also observed that: (a) ovomucoid sequestered in the Golgi-rich subfraction incorporated [3H]galactose; and (b) that part of internalized ovomucoid was localized on the Golgi stacks as well as elements of the trans-Golgi, as revealed by immunogold labeling on ultrathin cryosections. These data prove that in thyrocytes GlcNAc accessible sugar moieties on soluble internalized molecules are sufficient to trigger their recycling via the Golgi apparatus.

Dynamic analysis of drug action on in vitro reconstituted thyroid follicle by microinjection of tracer molecules and videomicroscopy

Thyroid cells isolated from the gland by trypsinization are capable in culture of reconstituting histiotypic structures, the thyroid follicles. This morphological differentiation requires the presence of the main thyroid regulator; thyrotropin. We have analyzed some structural and functional aspects of in vitro reconstituted thyroid follicles (RTF) using microinjection of fluorescent probes and videomicroscopy. This experimental approach allowed to visualize biological processes and actions of drugs, signalling factors, etc. in living cells. We describe here some examples of what can be studied with this powerful still-undervalued method. Microinjection of a cell-impermeant fluorescent probe of either high or low molecular mass into the lumen of RTF allowed to check the tightness of this compartment and therefore to analyze the control of tight junctions assembly. A small cell-impermeant probe like Lucifer Yellow microinjected into a cell was used to demonstrate and then to study the regulation of cell to cell communication via gap junctions. The presence of calcium in the lumen of RTF was detected by microinjection of a properly designed probe: Calcium Green which becomes fluorescent in the presence of the ligand. The lumen to cell transport or endocytosis of thyroglobulin, the thyroid prohormone, which is stored into the lumen of the follicles, is currently studied by microinjection of TRITC-labeled thyroglobulin. Coupled to image processing and videorecorder systems, kinetic analysis and quantitative measurements can be performed.

Effects of epidermal growth factor, phorbol ester, and retinoic acid on hormone synthesis and morphology in porcine thyroid follicles cultured in collagen gel

Epidermal growth factor (EGF), phorbol esters (PEs), and retinoic acid (RA) inhibit differentiated functions of thyrocytes. In the present study the inhibitory effects of these growth-promoting factors on hormone synthesis were studied in thyroid follicles cultured in type-I collagen gel, and morphologic alteration by these factors was examined by light and electron microscopy (EM). Porcine open thyroid follicles obtained by treatment with 0.1% collagenase were embedded in collagen gel and cultured in Ham's F12 medium supplemented with 6H (insulin, hydrocortisone, somatostatin, transferrin, glycyl-his-lys, and thyrotropin) + 0.5% fetal bovine serum (FBS). After 1 week these open follicles developed to closed follicles, and the medium was changed to one containing 6H + 0.5% FBS + 0.1 microM sodium iodide (NaI). Some media were supplemented with either EGF, phorbol 12-myristate 13-acetate (PMA), or all-trans RA. The closed follicles retained ability for hormone synthesis for 2 weeks after the medium change in the presence of 6H + FBS + NaI. The amounts of T4 and T3 secreted into the culture medium from day 9 to day 12 after the medium change were 60% and 45% of those from day 0 to day 4, respectively. EGF reduced production of T4 and T3 by 61% and 69%, respectively; PMA, by 87% and 99%; and RA, by 55% and 44%. In the medium supplemented with 6H + 0.5% FBS, the follicles exhibited intact polarity. Apical surfaces with microvilli were oriented to the follicular lumen and tight junctions were on the apical side of cell-to-cell contacts. Desmosomes were found on both the apical and basal halves of the cell contacts.(ABSTRACT TRUNCATED AT 250 WORDS)