ABSTRACT
Thyroid hormones play an important role in early development and growth, as well as adult phenotypes affecting
oxidative metabolism, cardiac output, energy balance, fertility, CNS function, and others. Complete absence of
thyroid hormone is incompatible with life. Thyroglobulin (Tg) is the precursor protein used in the biosynthesis of
thyroxine (T4) and tri-iodothyronine (T3). Tg is synthesized in the endoplasmic reticulum (ER) of thyrocytes and
undergoes intracellular trafficking and secretion to the apical (extracellular) lumen of thyroid follicles, where it is
iodinated, triggering formation of T4 and T3. The function of thyrocytes is regulated by pituitary-derived Thyroid
Stimulating Hormone (TSH), which stimulates thyrocyte transcription and translation of hormonogenic genes, as
well as thyrocyte proliferation resulting in gland growth. Humans with biallelic TG mutation (and murine models)
have congenital hypothyroidism; in this disease, the mutant Tg protein misfolds in the ER and fails to undergo
anterograde trafficking in the secretory pathway, resulting in insufficient thyroid hormone synthesis with an
increase of circulating TSH. Additionally, misfolding of mutant Tg triggers severe ER stress, which is
accompanied by thyrocyte cell death (although growth can outpace death, favoring goiter development).
Notably, the dead thyrocytes are found in the apical follicle lumen. The finding of dead thyrocytes in the follicle
lumen either signifies cell death caused by the severe ER stress followed by extrusion into the apical lumen, or
hyperstimulation by TSH triggering cell growth into the follicle interior that exposes thyrocytes to an inhospitable
luminal environment, which causes cell death. In my F30 project, I will study homozygous TG-KO mice (created
via targeted gene disruption). My preliminary data indicate that the animals have severe hypothyroidism, yet
they survive to adulthood despite expressing no Tg mRNA (by qPCR) or Tg protein (by Western blotting). I seek
to address two general questions. First, since Tg is by far the most highly expressed thyroid secretory protein
of the ER, do the thyrocytes of TG-KO animals also experience thyrocyte cell death, and if so, does this involve
ER stress or occur independently of ER stress? The result of this question can help us understand the real
driving force behind the thyroid cell death observed in hypothyroid humans synthesizing mutant Tg. Second, as
some level of thyroid hormone synthesis is required for survival, how do animals lacking Tg protein make thyroid
hormone? Altogether, this project affords me significant educational opportunity, as I will develop the techniques
and tools of basic cell biology (ER stress analysis, proliferation and cell death analysis) with a deeper
understanding of the growth and function of the thyroid gland (a critical endocrine system), with clear clinical
implications.