Iodination in relation to thyroglobulin maturation and subunit aggregation

A thyroid peroxidase (TPO) mutation in dogs reveals a canid-specific gene structure

Congenital hypothyroidism with goiter (CHG) occurring as an autosomal recessive disorder is typically due to a defect of thyroid hormone synthesis (aka dyshormonogenesis). Thyroid peroxidase (TPO) is a multifunctional, heme-containing enzyme whose activity is required, and several inactivating TPO mutations causing CHG in humans and dogs have been described. Recently, two half-sib Spanish water dog (SWD) pups were diagnosed with CHG based on clinical signs, endocrine testing, and thyroid histology. TPO enzyme activity was absent, and immuno-cross-reactive TPO was undetectable in affected-dog thyroid tissue. A single guanosine insertion was observed in the first exon of the affected-dog TPO cDNA at a site not previously thought to be within the coding sequence. The insertion allele segregated with the deduced disease allele in the SWD breed and was not observed in unrelated dogs of various breeds. Comparison of the insertion site (an 8-nt poly-G tract) with the orthologous sequences of other mammalian reference genomes revealed that the octa-G tract obliterated the intron 1 splice acceptor site and the exon 2 translation initiation codon found at that position in other species. An in-frame ATG in strong Kozak consensus context was observed in the normal dog sequence 12 codons 5' of the usual mammalian start site, suggesting that dogs have lost the noncoding exon 1 demonstrated in human and mouse. A survey of TPO sequences in other carnivore species indicates that the poly-G tract necessitating an alternative translation initiation site is a canid-specific feature.

Congenital hypothyroidism with goiter in Tenterfield terriers

BACKGROUND

A cluster of cases of congenital hypothyroidism with goiter (CHG) in Tenterfield Terriers was identified and hypothesized to be dyshormonogenesis of genetic etiology with autosomal recessive inheritance.

OBJECTIVES

To describe the phenotype, thyroid histopathology, biochemistry, mode of inheritance, and causal mutation of CHG in Tenterfield Terriers.

ANIMALS

Thyroid tissue from 1 CHG-affected Tenterfield Terriers, 2 affected Toy Fox Terriers, and 7 normal control dogs. Genomic DNA from blood or buccal brushings of 114 additional Tenterfield Terriers.

METHODS

Biochemical and genetic segregation analysis of functional gene candidates in a Tenterfield Terrier kindred. Thyroid peroxidase (TPO) iodide oxidation activity was measured, and TPO protein and SDS-resistant thyroglobulin aggregation were assessed on western blots. TPO cDNA was amplified from thyroid RNA and sequenced. Exons and flanking splice sites were amplified from genomic DNA and sequenced. Variant TPO allele segregation was assessed by restriction enzyme digestion of PCR products.

RESULTS

Thyroid from an affected pup had lesions consistent with dyshormonogenesis. TPO activity was absent, but normal sized immunocrossreactive TPO protein was present. Affected dog cDNA and genomic sequences revealed a homozygous TPO missense mutation in exon 9 (R593W) that was heterozygous in all obligate carriers and in 31% of other clinically normal Tenterfield Terriers.

CONCLUSIONS

The mutation underlying CHG in Tenterfield Terriers was identified, and a convenient carrier test made available for screening Tenterfield Terriers used for breeding.

Radical directed dissociation for facile identification of iodotyrosine residues using electrospray ionization mass spectrometry

Iodination of tyrosine residues in proteins has many uses in chemistry, biology, and medicine. Site specific identification of the sites of iodination is important for many of these uses. Reported herein is a facile method employing photodissociation and mass spectrometry to localize sites of iodination in whole proteins. Absorption of ultraviolet photons by iodotyrosine results in loss of iodine via homolytic bond dissociation. The resulting protein radical fragments in the vicinity of the iodotyrosine upon collisional activation. Analysis of the fragments within the vicinity of each tyrosine residue in the protein enables quantitative evaluation of the likelihood for iodination at each site. The results are compared with both traditional bottom up and top down mass spectrometric methods. Radical directed dissociation yields results in agreement with traditional approaches but requires significantly less effort and is inherently more sensitive. One limitation occurs when multiple tyrosine residues are in close proximity, in which case the extent of iodination at each residue may be difficult to determine. This limitation is frequently problematic for traditional approaches as well.

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.

An endoplasmic reticulum storage disease causing congenital goiter with hypothyroidism

In humans, deficient thyroglobulin (Tg, the thyroid prohormone) is an important cause of congenital hypothyroid goiter; further, homozygous mice expressing two cog/cog alleles (linked to the Tg locus) exhibit the same phenotype. Tg mutations might affect multiple different steps in thyroid hormone synthesis; however, the microscopic and biochemical phenotype tends to involve enlargement of the thyroid ER and accumulation of protein bands of M(r) < 100. To explore further the cell biology of this autosomal recessive illness, we have examined the folding and intracellular transport of newly synthesized Tg in cog/cog thyroid tissue. We find that mutant mice synthesize a full-length Tg, which appears to undergo normal N-linked glycosylation and glucose trimming. Nevertheless, in the mutant, Tg is deficient in the folding that leads to homodimerization, and there is a deficiency in the quantity of intracellular Tg transported to the distal portion of the secretory pathway. Indeed, we find that the underlying disorder in cog/cog mice is a thyroid ER storage disease, in which a temperature-sensitive Tg folding defect, in conjunction with normal ER quality control mechanisms, leads to defective Tg export. In relation to quality control, we find that the physiological response in this illness includes the specific induction of five molecular chaperones in the thyroid ER. Based on the pattern of chaperone binding, different potential roles for individual chaperones are suggested in glycoprotein folding, retention, and degradation in this ER storage disease.

Differences in iodinated peptides and thyroid hormone formation after chemical and thyroid peroxidase-catalyzed iodination of human thyroglobulin

The distribution of iodine among the polypeptides of human goiter thyroglobulin (Tg) was examined. Tg was iodinated in vitro with 131I to levels of 2 to 84 gram atoms (g.a.)/mol using thyroid peroxidase (TPO) or a chemical iodination system. The samples were reduced, alkylated, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two low-molecular-weight peptides appeared preferentially in radioautograms of the sodium dodecyl sulfate (SDS) gels of TPO-iodinated samples. Iodination of these peptides increased sharply in the TPO-treated Tg as the level of total iodine/molecule rose. Radioiodine was incorporated into these same gel regions in the chemically treated Tg, but only after much higher levels of total iodination were reached. Differences in iodoamino acid distribution were also noted between the chemically and enzymatically iodinated thyroglobulins. In the chemically iodinated samples, little thyroxine (T4) was synthesized, even at high iodine levels. In the TPO-treated samples only small amounts of T4 were seen below 14 g.a. total I/mol, while at or above that level of iodination T4 formation increased sharply. To examine the coupling process, Tg was chemically iodinated, excess I- removed, and the samples treated with TPO and a H2O2-generating system in the absence of iodide. Radioautograms obtained from SDS-polyacrylamide gels of reduced and alkylated protein from such coupling assays showed an increase in the level of iodine in the low-molecular-weight peptides after TPO treatment. Thyroxine production also increased with TPO treatment. The addition of free DIT (a known coupling enhancer) to the [131I]Tg/TPO incubation increased both the production of T4 and the amount of iodine in the smaller polypeptides. Two-dimensional maps prepared from CNBr-digested TG showed differences between the coupled and uncoupled samples. Our observations confirm the importance of the low-molecular-weight peptides derived from Tg in thyroid hormone synthesis. At total iodine levels above 14 g.a./mol Tg in enzymatically treated samples there is selective incorporation of iodine into both the low-molecular-weight polypeptides and into thyroid hormone.

Electron microscopy of low iodinated thyroglobulin molecules

Thyroglobulin molecules were studied in the electron microscope with negative staining technique. In a first series of experiments samples of thyroglobulin varying in iodine content from 0.5 to 0.03% were prepared from the thyroids of mice and rats kept on iodine-poor diets. All samples contained thyroglobulin molecules of the normal ovoid shape, not deviating in size or shape from molecules obtained from normal thyroids. However, in addition, another type of molecule having a cylindrical shape was observed in all samples. The proportion of these cylindrical molecules increased from a few per cent in the moderately iodine-poor thyroglobulin samples to more than 80% in the highly iodine-deficient thyroglobulin (0.03%). In a second series of experiments extremely iodine-poor thyroglobulin (smaller than 0.005%) was obtained from propylthiouracil-treated rats. In these preparations practically all molecules had a cylindrical shape. These samples also contained smaller particles interpreted to be dissociation products. The cylindrical molecules were of two types, one appearing compact and measuring 250 times 135 A (length times diameter) and the other appearing porous and having a length of 145 and a diameter of 205 A. It is concluded that the cylindrical molecules represent non- or low-iodinated thyroglobulin and it is suggested that the porous cylindrical molecule is an unfolded form of the compact cylinder.