Thyroid hormone synthesis continues despite homozygous thyroglobulin mutation with cell death

CINTIA E. CITTERIO; YOSHIAKI MORISHITA; PETER ARVAN; AARON P. KELLOG ; DENNIS LARKIN; VIVIANA A. BALBI; XIOHAN ZHANG; HAO ZHANG ; HÉCTOR M. TARGOVNIK

Ann Arbor, Michigan

Congreso; 7th Annual Protein Folding Diseases Initiative Symposium; 2020

University of Michigan Medical School

Most patients bearing 2-allele TG mutations have no follicularcolloid ? which is where T4 is made. Rdw/rdw rats have no follicularcolloid, yet, untreated, they can achieve a serum total T4about half that of normal controls even without T4 treatment. BecauseTg secretion (absent in most individuals with 2-allele TGmutations) provides the only known protein substrate for efficientthyroid hormone synthesis, how do untreated subjects synthesizethyroid hormone?We studied homozygous rdw/rdw rats expressingmutant Tg-G2300R. We examined thyroid gland size; serum totalT4 levels, thyroid gland histology by H&E and DAPI staining; celldeath by TUNEL staining, and thyroid immunofluorescence withanti-Tg and anti-T4 antibodies. We find that rdw/rdw rats not supplementedT4 with exhibit a small goiter during young adulthood?corresponding to an age when serum total T4 is significant (albeit inthe hypothyroid range). Histological examination reveals no detectablesecreted Tg but instead yields profiles of ??cell ghosts?? withTUNEL-positive staining, indicating the presence of dead cells inthe lumina of many thyroid follicles. DAPI staining demonstrateskaryolysis (i.e., ??exploded?? nuclei) that expand in the lumen toreach the epithelial surface of living follicular cells. Anti-Tgstaining establishes that the dead cells are of thyrocyte origin. Remarkably,proteinaceous T4 occurs primarily if not exclusively inthe lumina of thyroid follicles in which dead thyrocytes are detected.Many follicle lumina are TUNEL and T4 double-positive, orDAPI and T4 double-positive, with T4 staining over cell ghosts.(Ultimately, animals >30 weeks old have been published to have ahypoplastic thyroid gland and unmeasureably low serum T4.) Ourdata establish that despite the complete absence of Tg secretion,rdw/rdw rats engage in thyroid hormone synthesis through the iodinationof the detritus of dead, disintegrating thyrocytes, followedby the endocytic reabsorption of the follicular contents into theliving thyroid epithelial monolayer. In 2-allele TG mutations, goitermay provide substrate, such that significant hormonogenesis cancontinue as long as an abundant supply of dead cells is available inthe thyroid follicle lumen.