Biomedicine of Proteolysis — Quantitative Cell Biology

There are numerous regulatory mechanisms that ensure that thyroid hormones are produced in sufficient amounts to direct the cells in the body. Deciphering thyroid-gland regulation is vital to understand healthy thyroid states and how to maintain them. Thyroid gland function depends upon a highly interwoven network of special enzymes, transporter molecules, and receptor signaling. To decipher the various contributions that ensure healthy thyroid function, we have used thyroid tissue phenotyping by quantitative cell biology.

• Thyroid Gland Auto-Regulation — As a continuation of our previous studies on thyroid gland self-regulation by non-classical mechanisms, we conducted a series of studies. It became obvious that two different G protein-coupled receptors — the trace amine-associated receptor 1 (Taar1) and the thyroid stimulating hormone (TSH) receptor — co-regulate thyroid function. In the absence of a specific thyroid hormone transporter, Mct8, thyroid hormone thyroxine cannot be released into circulation. Mct8 also affects the localization and signaling of the TSH receptor. We achieved these results on non-classical and classical thyroid regulation using mouse models. It is important to note that patients with a rare disease, known as Allan Herndon Dudley Syndrome, partly exhibit similar phenotypes. 
• Aging — We investigated the link between thyroid dysfunctions and the effects of thyroid hormones on target organs upon aging. In particular, we characterized proteases (a special type of enzyme) in the central nervous system and the thyroid gland itself, and found a range of changes to them upon aging. Moreover, we studied anti-oxidant response regulators and thyroid gland functionality in aging. 
• Cancer and Protease Trafficking — In specific cell lines, we observed alternative transport pathways for proteases such as the cysteine cathepsins B and V in human thyroid epithelial cell lines. Carcinoma cells featured additional mis-sorting and altered protease trafficking.
• Cross-talk of GPCRs and Proteases at Primary Cilia — One of our most astonishing findings of recent years was the observation that proteolytically active cysteine cathepsins are essential to keep primary cilia intact and extended from the apical surface of polarized epithelial cells. Because cilia serve as signaling platforms, such as via Taar1 of thyroid epithelial cells, we developed a cellular model to allow detailed future investigations on the underlying molecular mechanisms.

In 2019–2022, the research group headed by Klaudia Brix consisted of two PhD students, a coordination team assistant, and a lab technician supervising the research projects of 18 BSc students. DAAD and DFG funded some of the group’s research activities. In addition, we received funding from the European Thyroid Association, the DFG, and the HAORI Foundation for COVID-19 related projects, which are ongoing. We cooperated with international groups such as at ETH Zürich in Switzerland and Stanford University in the United States, as well as local groups at University of Bremen and Constructor University. Klaudia Brix was the main responsible principal investigator for the recent acquisition of a high-end laser scanning microscopy system for Constructor University.

• Szumska, J., Batool, Z., Al-Hashimi, A., Venugopalan, V., Skripnik, V., Schaschke, N., Bogyo, M., and Brix, K. (2019) Treatment of rat thyrocytes in vitro with cathepsin B and L inhibitors results in disruption of primary cilia leading to redistribution of the trace amine associated receptor 1 to the endoplasmic reticulum. Biochimie 166, 270–285.
• Al-Hashimi, A., Venugopalan, V., Sereesongsaeng, N., Tedelind, S., Pinzaru, A.M., Hein, Z., Springer, S., Weber, E., Führer, D., Scott, C.J., Burden, R.E., and Brix, K. (2020) Significance of nuclear cathepsin V in normal thyroid epithelial and carcinoma cells. BBA — Molecular Cell Research 1867, 118846. 
• Venugopalan, V., Al-Hashimi, A., Rehders, M., Golchert, J., Reinecke, V., Homuth, G., Völker, U., Manirajah, M., Touzani, A., Weber, J., Bogyo, M.S., Verrey, F., Wirth, E.K., Schweizer, U., Heuer, H., Kirstein, J., and Brix, K. (2021) The Thyroid Hormone Transporter Mct8 Restricts Cathepsin-Mediated Thyroglobulin Processing in Male Mice through Thyroid Auto-Regulatory Mechanisms that Encompass Autophagy. Int. J. Mol. Sci. 22, 462.
• Qatato, M., Venugopalan, V., Al-Hashimi, A., Rehders, M. Valentine, A.D., Hein, Z., Dallto, U., Springer, S., and Brix, K. (2021) Trace Amine-Associated Receptor 1 Trafficking to Cilia of Thyroid Epithelial Cells. Cells 10, 1518.
• Yu, D.M.T., Dauth, S., Margineanu, M.B., Snetkova, V., Rehders, M., Jordans, S., and Brix, K. (2022) Characterization of cysteine cathepsin expression in the central nervous system of aged wildtype and cathepsin-deficient mice. Appl. Sci. 12, 2608.