Generation and characterization of murine models to study the molecular mechanisms of G protein signaling in the thyroid and their role in thyroid diseases
Final Report Abstract
The aim of our study was to provide novel information with respect to the molecular mechanism of thyroid diseases using genetically modified animal models. Specifically hyperthyroidism, which presents considerable morbidity and mortality, if not treated promptly. Constitutively activating mutations (CAMs) of the thyroid stimulating hormone receptor (TSHR) can lead to non-autoimmune hyperthyroidism (NAH) in humans through a permanent activation of the G-protein Gs/cAMP- or more rarely Gq/11-mediated signaling resulting in uncontrolled release of thyroid hormones. However, the detailed mechanisms of G-protein action in the course of NAH remain unclear. Functional characterization of TSHR mutations as the molecular cause for hyperthyroidism is exclusively carried out by in vitro experiments using heterologous cell systems. However, the understanding of the molecular mechanisms, ontogeny and development of NAH requires an in vivo approach. Therefore, we have generated a mouse model with activating TSHR mutation D633H, which activates both G- protein signaling cascades (Gs and Gq/11) independently from the natural ligand thyrotropin. This model enables us to investigate the contribution of different signaling pathways in thyroid function and disease and gives us for the first time the opportunity to study the impact of extra-thyroidal TSHR expression in bone, lipid metabolism or cardiac function. Furthermore, it represents an in vivo model for pharmacological testing of newly identified small molecular ligands for the TSHR with inverse agonistic properties. The phenotypic characterization of mice expressing TSHR mutation D633H revealed overt hyperthyroidism in homozygous (HOZ) females at young age and latent hyperthyroidism in HOZ males and heterozygous (HEZ) females. Interestingly, this phenotype was not persistent as indicated by normalized thyroid hormone levels after six months in HOZ females. This finding suggests a strong compensatory mechanism, which has to be explored in more detail in future studies. Another striking observation concerns the histology of the thyroid, which was changing over the investigated time period. At two months of age we observed only minor alterations in HEZ and HOZ mice when compared with the wild type, whereas as six months the predominant histology can be described as a colloid goiter with flattened thyrocytes suggesting an inactive thyroid. Most strikingly, at nine months of age about 50% of HOZ mice develop papillary thyroid cancer. Whether this phenotype is solely mediated by the TSHR D633H mutation or other mechanisms are involved remains to be elucidated. Furthermore, we identified various parameters, which show a clear dependence on sex, e.g. overt hyperthyroidism was determined in HOZ females but not in males, thyroid growth was more pronounced in female mice and changes in the bone structure of the skull were predominantly observed in males. Taken together, our mouse model with TSHRD633H mutation offers a new disease model to investigate the development of non-autoimmune hyperthyroidism and to study novel treatment options. While, HOZ female mice could serve as a model for overt hyperthyroidism at young age, HOZ males and HEZ female mice offer a tool to study latent hyperthyroidism. Unexpectedly, this mouse model can also serve a suitable instrument to study the development and progression of papillary thyroid cancer. Moreover, TSHRD633H mice are a perfect model to investigate sex-related differences in the background of thyroid disorders.
