The Thyroid Hormone Transporter Mct8 Restricts Cathepsin-Mediated Thyroglobulin Processing in Male Mice through Thyroid Auto-Regulatory Mechanisms That Encompass Autophagy

Abstract

The thyroid gland is both a thyroid hormone (TH) generating as well as a TH responsive organ. It is hence crucial that cathepsin-mediated proteolytic cleavage of the precursor thyroglobulin is regulated and integrated with the subsequent export of TH into the blood circulation, which is enabled by TH transporters such as monocarboxylate transporters Mct8 and Mct10. Previously, we showed that cathepsin K-deficient mice exhibit the phenomenon of functional compensation through cathepsin L upregulation, which is independent of the canonical hypothalamus-pituitary-thyroid axis, thus, due to auto-regulation. Since these animals also feature enhanced Mct8 expression, we aimed to understand if TH transporters are part of the thyroid auto-regulatory mechanisms. Therefore, we analyzed phenotypic differences in thyroid function arising from combined cathepsin K and TH transporter deficiencies, i.e., in Ctsk-/-/Mct10-/-, Ctsk-/-/Mct8-/y, and Ctsk-/-/Mct8-/y/Mct10-/-. Despite the impaired TH export, thyroglobulin degradation was enhanced in the mice lacking Mct8, particularly in the triple-deficient genotype, due to increased cathepsin amounts and enhanced cysteine peptidase activities, leading to ongoing thyroglobulin proteolysis for TH liberation, eventually causing self-thyrotoxic thyroid states. The increased cathepsin amounts were a consequence of autophagy-mediated lysosomal biogenesis that is possibly triggered due to the stress accompanying intrathyroidal TH accumulation, in particular in the Ctsk-/-/Mct8-/y/Mct10-/- animals. Collectively, our data points to the notion that the absence of cathepsin K and Mct8 leads to excessive thyroglobulin degradation and TH liberation in a non-classical pathway of thyroid auto-regulation.

The Amino Acid Transporter Mct10/Tat1 Is Important to Maintain the TSH Receptor at Its Canonical Basolateral Localization and Assures Regular Turnover of Thyroid Follicle Cells in Male Mice

“…Recently, we have reported that mice lacking Mct8 and Mct10 on a cathepsin K-deficient background exhibit autophagy-induced excessive cathepsin-mediated Tg proteolysis [ 6 ]. The resulting enhanced intrathyroidal TH accumulation, due to the lack of exporting TH transporters, leads to self-toxicity in thyrocytes of Ctsk −/− / Mct8 −/y / Mct10 −/− mice.…”

“…The resulting enhanced intrathyroidal TH accumulation, due to the lack of exporting TH transporters, leads to self-toxicity in thyrocytes of Ctsk −/− / Mct8 −/y / Mct10 −/− mice. Such a phenotype is also observed in mice lacking Mct8 and cathepsin K, but not in a combined Mct10- and cathepsin K-deficient genotype [ 6 ]. This indicates that while Tg utilization is possibly induced in Ctsk −/− / Mct8 −/y and Ctsk −/− / Mct8 −/y / Mct10 −/− , it appears unaffected in the Ctsk −/− / Mct10 −/− mice, where indeed cathepsin-mediated Tg degradation remains unchanged from wild-type (WT) controls.…”

“…However, Ctsk −/− / Mct10 −/− mice show reduced total Tg amounts and the extent of Tg glycosylation is diminished when compared to WT controls. Consequently, Ctsk −/− / Mct10 −/− thyroid follicles display enhanced amounts of soluble Tg, hinting at deregulated Tg biosynthesis, where reduced glycosylation results in less stable Tg deposits in the thyroid follicle lumen [ 6 ]. Since Tg biosynthesis is regulated via Gα s , while Tg proteolysis is mediated by Gα q , we suggest that intrathyroidal TSH receptor signaling might be altered in TH transporter deficiencies, specifically on a cathepsin K-deficient background, resulting in altered thyroid functional phenotypes.…”

“…An important determinant of the outcomes of TSH receptor signaling for maintenance of normal thyroid gland function is the size of the thyroid gland, which is attributed to several parameters, including the number of thyroid follicles, area and number of thyrocytes, epithelial height, area occupied by the follicle lumen, and the area occupied by or numbers of other cell types residing within the thyroid gland [ 6 , 7 , 8 ]. Therefore, understanding thyroid gland structure-to-function relationships is critical for proper evaluation and treatment of thyroid dysfunctional states [ 9 ].…”

“…Then, epithelial extensions and distribution of intra- vs. extracellular cathepsins were determined as a proxy of Gα q signaling, while the determination of follicle numbers, follicle and lumen areas, thyrocyte areas and numbers, and follicular cathepsin amounts were studied as a proxy of Gα s -mediated signaling. Because some of these parameters, for instance cathepsin-mediated Tg utilization, were previously analyzed in Ctsk −/− , Mct10 −/− , Mct8 −/y , and Mct8 −/y / Mct10 −/− mice [ 6 , 8 , 13 , 14 ], here we focused on inspecting the Ctsk −/− / Mct10 −/− , Ctsk −/− / Mct8 −/y , and Ctsk −/− / Mct8 −/y / Mct10 −/− genotypes more comprehensively than before [ 6 ]. Our data indicate a possible role of Mct10 in maintaining the TSH receptor at its canonical basolateral localization.…”