Characterization of thyroid hormone transport in a human choroid plexus papilloma cell line (HIBCPP) as an in vitro blood-cerebrospinal fluid barrier model

Abstract

Adequate levels of thyroid hormones (THs) in the fetal brain are vital for early neurodevelopment. Most of the TH in fetal brain is derived from circulating thyroxine (T4), which gets locally converted into the biologically active triiodothyronine (T3) by deiodinase enzymes. One of the major routes of TH into the brain is through the blood-cerebrospinal fluid barrier (BCSFB). The BCSFB is maintained by the choroid plexus (CP), which separates the blood from the cerebrospinal fluid (CSF). TH transport across the BCSFB is facilitated by TH transmembrane transporters (THTMTs) present in the CP. However, TH transport across the BCSFB is still poorly understood as there is a lack of human representative BCSFB models. Previously, a human choroid plexus papilloma cell line (HIBCPP) has been established, which exhibits certain key characteristics of the human CP. In this study, the suitability of the HIBCPP cell line as a human in vitro BCSFB model for TH transport was evaluated. For this, HIBCPP cells were grown on transwell inserts and the gene and protein expression of several THTMTs was assessed using qPCR and immunohistochemistry. Additionally, the transport of T4 across a HIBCPP monolayer was assessed using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS), with a special focus on the role of transthyretin (TTR), a TH binding protein produced by the CP involved in TH transport across the BCSFB. Finally, inhibition studies were performed with various THTMT inhibitors, to conclude which THTMT drive TH transport across the BCSFB. Gene and protein expression data showed that several THTMTs were expressed in the HIBCPP model, however HIBCPP cells lacked key THTMTs, notably monocarboxylate transporter 8 (MCT8) and organic anion transporter polypeptide 1C1 (OATP1C1), known to be highly expressed in the human BCSFB. Moreover, TH transport across the HIBCPP model was low and addition of TTR did not increase this transport. Two TTR-binding chemicals, TBBPA and F21388, significantly decreased the transport of T4 across the HIBCPPs cells, suggesting a possible role of intracellular TTR in the transport of T4 across the BCSFB. The transport of TTR-T4 complex might be mediated through SR-B1, indicated by the decreased T4 transport after BLT-1 exposure. However, the poor expression of several important THTMTs, together with the low amount of TH transport, indicate that the HIBCPP cells lack key features that drive TH transport in the BCSFB. The HIBCPP cells could serve as a model to further study the mechanisms of TTR driven TH transport, but for the identification of THTMTs more in vivo accurate BCSFB models are necessary.