P28-01 Human Choroid Plexus Organoid Model to Test Inhibition of Thyroid Hormone Transport across the Blood-Cerebrospinal fluid-Barrier

Thyroid hormones (THs) are essential for brain development, and deficiencies of TH during development are associated with lower IQ and delayed motor function in childrenIn the brain, the prohormone thyroxine (T4) is converted by deiodinase 2 (DIO2) into the active hormone triiodothyronine (T3), or by DIO3 into the inactive form reverse T3 (rT3) [2] . During early fetal development, transport of TH to the brain is regulated in the ventricles of the brain at the Choroid Plexus (ChP) by the Blood-Cerebrospinal fluid-Barrier (BCS-FB) [3] . The BCSFB consists of ChP epithelial cells that carry out the barrier function and stromal tissue that contains fenestrated capillaries and immune cells [4] . The importance of the BCSFB in TH transport is highlighted by the high expression of the TH distributor protein transthyretin (TTR), that peaks during early brain development. TTR is distributed to the cerebrospinal fluid (CSF), that is produced by the ChP and provides the brain with nutrients, hormones and signalling molecules [5] . The transport of TH is further regulated by thyroid hormone transmembrane transporters (THTMTs), of which several are expressed at the BCSFB 6 , 7 . The function of THTMTs and TTR might be inhibited by endocrine disrupting chemicals (EDCs), but this needs more investigation. Therefore, we developed a human pluripotent stem cell based- ChP organoid model that we used to study TH transport at the BCSFB and the possible influence of EDCs on this TH transport over the BCFSB. ChP organoids form vesicles containing a fluid that is very similar to embryonic CSF based on protein composition [8] . The protein expression of TTR, THTMTs and deiodinases, stromal tissue and embryonic CSF in the ChP organoids was confirmed with immunofluorescence staining. Furthermore, LC/ MS analysis of the organoid fluid confirmed the uptake of T4 into the vesicles after a 4 hour incubation as well as deiodination of T4 to rT3. Silychristin, Lesinurad, TBBPA, F21388 and Triclosan concentration-dependently inhibited TH transport in the ChP organoids. In conclusion, we have established a human-relevant stem cell based- ChP organoid model that can be used to study the inhibition of TH transport at the BCSFB. Further studies can utilize the ChP organoids to screen for novel possible THTMT inhibitors, as well as possible TTR binders at the developing BCSFB.