Role of connexins and their channels in cholestasis

Connexin (Cx) proteins are key players in communication between cells, which is indispensable to maintain tissue homeostasis. Specifically, Cx proteins are the building blocks of hemichannels that can merge together to form gap junctions. While gap junctions establish communication between two adjacent cells under physiological circumstances, hemichannels facilitate communication between the cell and the extracellular environment in pathological conditions. In healthy conditions, hepatocytes typically express Cx32, though in several liver pathologies, they switch to a Cx43-based mode. The aim of this PhD project, which revolves around Cxbased communication in liver, was twofold. The first goal was to specifically address the role of these Cx proteins in cholestasis, which is a pathological condition resulting from bile acid accumulation in the liver. The second goal was to search for in vitro biomarkers for drug-induced cholestasis. The first study within this doctoral project characterized Cx proteins in both an in vitro model (human hepatoma HepaRG cell cultures exposed to different cholestatic drugs) and an in vivo model (bile-duct ligated mice) of cholestasis. While for Cx26 and Cx43 inconclusive effects have been noticed, Cx32 was generally downregulated in both models. The second study aimed to find novel in vitro biomarkers of drug-induced cholestasis with a specific focus on alterations in bile transporters and Cxs in sandwich-cultured primary human hepatocytes exposed to cholestatic drugs. Despite no changes in gene expression levels of bile transporters and Cxs, alterations in functionality of the foremost were observed. Furthermore, certain gene markers were similarly regulated among the different cholestatic drugs that could serve as potential new in vitro biomarkers. The third study in this PhD project tested the effects of drugs, used in the treatment of coronavirus disease 2019, on Cx43. None of the tested drugs showed an effect on the functionality of Cx43 hemichannels.
Overall, it can be concluded that Cx proteins, specifically Cx32, could be biomarkers for cholestasis which also suggests a role for this Cxs as therapeutic drug targets but this needs further investigation. Furthermore, they should be used preferentially in conjunction with other targets but not as a stand-alone biomarker.