In vitro human brain barrier models for studying thyroid hormone transport.

Early brain development is dependent on the supply of thyroid hormone (TH) to the fetal brain. Disruption of TH concentrations in early brain development is associated with lower IQ and delayed motor development in children. How TH system disruption may affect brain development has mainly been studied in animal models that are not always relevant to humans and do not reflect the TH system in the developing brain. Furthermore, using animal models for safety assessments also raises ethical concerns, is still low-throughput and associated with high costs. All these reasons stress the need to develop new approach methodologies (NAMs), including in vitro methods that help to improve human relevant risk assessment. Initiatives are taken to develop in vitro assays for important key events in the fetal brain, but before TH can enter the fetal brain, it has to pass the developing blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). During brain development, the composition of the barriers change over time, as well as the interplay between the two different barriers. Therefore, barrier models need to be included in testing strategies for TH system disruption in the developing brain and these models should take the timepoint of development into account. Barriers are crucial for the supply of TH in the brain. TH is actively transported through these barriers via TH transmembrane transporters (THTMTs) such as MCT8 and OATP1C1, but alternatively, other THTMTs may be involved too. Furthermore, transport of TH across the brain barriers can be disrupted by chemicals. Currently, the extent of THTMT inhibition and its subsequent adverse effects on brain development is largely undiscovered. To further investigate TH transport across the BBB and BCSFB, human cell-based NAMs are being developed that more closely resemble the human brain barriers. These models take the complex cellular composition of the brain barriers into account and in case of organ-on-chip models, the blood/cerebrospinal fluid flow as well. In this review, aspects of accurate in vitro models ranging from simple mono-cultures to extended 3D cultures of the brain barriers are discussed as well as how (a combination of) these in vitro models can be utilized to study TH transport and its disruption in the brain.