Pulmonary fibrosis (PF) is a chronic progressive lung disease without effective medical treatment options leading to respiratory failure and death within 3-5years of diagnosis. The pathological process of PF is driven by aberrant wound-healing involving fibroblasts and myofibroblasts differentiated by secreted profibrotic transforming growth factor β (TGF-β1). Classical transient receptor potential 6 (TRPC6), a Na- and Ca-permeable cation channel, is able to promote myofibroblast conversion of primary rat cardiac and human dermal fibroblasts and TRPC6-deficiency impaired wound healing after injury. To study a potential role of TRPC6 in the development of PF we analyzed lung function, gene and protein expression in wild-type (WT) and TRPC6-deficient (TRPC6-/-) lungs utilizing a bleomycin-induced PF-model. Fibrotic WT-mice showed a significant higher death rate while bleomycin-treated TRPC6-deficient mice were partly protected from fibrosis as a consequence of a lower production of collagen and an almost normal function of the respiratory system (reduced resistance and elastance compared to fibrotic WT-mice). On a molecular level TGF-β1 induced TRPC6 up-regulation, increased Ca influx and nuclear NFAT localization in WT primary murine lung fibroblasts (PMLFs) resulting in higher stress fiber formation and accelerated contraction rates as compared to treated TRPC6-deficient fibroblasts. Therefore, we conclude that TRPC6 is an important determinant for TGF-β1-induced myofibroblast differentiation during fibrosis and specific channel inhibitors might be beneficial in a future treatment of PF.

These results confirm the finding that dynamic autoregulation is disturbed in severe head injury and that moderate transient hyperventilation can temporarily improve the efficiency of the autoregulatory response, probably as a result of a transient increase in vascular tone.

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.

This mini-review asks how self-regulation of the thyroid gland is realized at the cellular and molecular levels by canonical and non-canonical means. Canonical pathways of thyroid regulation comprise thyroid stimulating hormone-triggered receptor signaling. As part of non-canonical regulation, we hypothesized an interplay between protease-mediated thyroglobulin processing and thyroid hormone release into the circulation by means of thyroid hormone transporters like Mct8. We proposed a sensing mechanism by different thyroid hormone transporters, present in specific subcellular locations of thyroid epithelial cells, selectively monitoring individual steps of thyroglobulin processing, and thus, the cellular thyroid hormone status. Indeed, we found that proteases and thyroid hormone transporters are functionally inter-connected, however, in a counter-intuitive manner fostering self-thyrotoxicity in particular in Mct8- and/or Mct10-deficient mice. Furthermore, the possible role of the G protein-coupled receptor Taar1 is discussed, because we detected Taar1 at cilia of the apical plasma membrane of thyrocytes in vitro and in situ. Eventually, through pheno-typing Taar1-deficient mice, we identified a co-regulatory role of Taar1 and the thyroid stimulating hormone receptors. Recently, we showed that inhibition of thyroglobulin-processing enzymes results in disappearance of cilia from the apical pole of thyrocytes, while Taar1 is re-located to the endoplasmic reticulum. This pathway features a connection between thyrotropin-stimulated secretion of proteases into the thyroid follicle lumen and substrate-mediated self-assisted control of initially peri-cellular thyroglobulin processing, before its reinternalization by endocytosis, followed by extensive endo-lysosomal liberation of thyroid hormones, which are then released from thyroid follicles by means of thyroid hormone transporters.