Nuclear receptor ligand screening in an iPSC-derived in vitro blood-brain barrier model identifies new contributors to leptin transport

BACKGROUND

The hormone leptin exerts its function in the brain to reduce food intake and increase energy expenditure to prevent obesity. However, most obese subjects reflect the resistance to leptin even with elevated serum leptin. Considering that leptin must cross the blood-brain barrier (BBB) in several regions to enter the brain parenchyma, altered leptin transport through the BBB might play an important role in leptin resistance and other biological conditions. Here, we report the use of a human induced pluripotent stem cell (iPSC)-derived BBB model to explore mechanisms that influence leptin transport.

METHODS

iPSCs were differentiated into brain microvascular endothelial cell (BMEC)-like cells using standard methods. BMEC-like cells were cultured in Transwell filters, treated with ligands from a nuclear receptor agonist library, and assayed for leptin transport using an enzyme-linked immune sorbent assay. RNA sequencing was further used to identify differentially regulated genes and pathways. The role of a select hit in leptin transport was tested with the competitive substrate assay and after gene knockdown using CRISPR techniques.

RESULTS

Following a screen of 73 compounds, 17β-estradiol was identified as a compound that could significantly increase leptin transport. RNA sequencing revealed many differentially expressed transmembrane transporters after 17β-estradiol treatment. Of these, cationic amino acid transporter-1 (CAT-1, encoded by SLC7A1) was selected for follow-up analyses due to its high and selective expression in BMECs in vivo. Treatment of BMEC-like cells with CAT-1 substrates, as well as knockdown of CAT-1 expression via CRISPR-mediated epigenome editing, yielded significant increases in leptin transport.

CONCLUSIONS

A major female sex hormone, as well as an amino acid transporter, were revealed as regulators of leptin BBB transport in the iPSC-derived BBB model. Outcomes from this work provide insights into regulation of hormone transport across the BBB.

Lecithin coating as universal stabilization and functionalization strategy for nanosized drug carriers to overcome the blood-brain barrier

Lecithin coated cholesteryl oleate (ChOl) based nanoparticles (NPs) imitating natural lipoproteins represent a new and promising drug carrier strategy to cross the blood-brain barrier (BBB). In such systems lecithin serves as stabilizing as well as functionalizing agent and enables the adsorptive binding of apolipoprotein E₃ (ApoE) as potential drug targeting ligand. The present work is focused on the effect of size reduction on the lecithin coating and ApoE binding. Furthermore, the transferability of this lecithin coating strategy to other NP cores, namely polylactic-co-glycolic acid (PLGA) and polylactic acid (PLA), is investigated in order to provide a universal strategy for a wide range of cores to overcome the BBB. The ChOl NPs' size was successfully reduced from 100 nm to 70 nm. Varying the core size of ChOl NPs illustrated, that the at least needed lecithin amount for sufficient stabilization could be calculated surface area dependently. However, the size reduction led to reduced dye loading per NP and increased ApoE need per NP mass. These effects turned out as huge disadvantages of smaller NPs by weakening the observed ApoE mediated effects. Nevertheless, the extended understanding of the lecithin coating could be used to transfer the concept to other core materials. PLGA and PLA NPs were investigated as alternative core materials for lecithin coating. PLGA was found to be unsuitable, whereas in the case of PLA sufficient stabilization and 100% adsorptive binding efficiency to ApoE could be achieved. The ApoE mediated effects of transcytosis at an in vitro BBB model by bypassing lysosomes were reproduced in even stronger quantities than with a ChOl core, proving lecithin coating as transferable strategy to disguise various NPs with a certain lipophilicity as lipoproteins.

Effects of arsenolipids on in vitro blood-brain barrier model

Arsenic-containing hydrocarbons (AsHCs), a subgroup of arsenolipids (AsLs) occurring in fish and edible algae, possess a substantial neurotoxic potential in fully differentiated human brain cells. Previous in vivo studies indicating that AsHCs cross the blood-brain barrier of the fruit fly Drosophila melanogaster raised the question whether AsLs could also cross the vertebrate blood-brain barrier (BBB). In the present study, we investigated the impact of several representatives of AsLs (AsHC 332, AsHC 360, AsHC 444, and two arsenic-containing fatty acids, AsFA 362 and AsFA 388) as well as of their metabolites (thio/oxo-dimethylpropionic acid, dimethylarsinic acid) on porcine brain capillary endothelial cells (PBCECs, in vitro model for the blood-brain barrier). AsHCs exerted the strongest cytotoxic effects of all investigated arsenicals as they were up to fivefold more potent than the toxic reference species arsenite (iAs^III). In our in vitro BBB-model, we observed a slight transfer of AsHC 332 across the BBB after 6 h at concentrations that do not affect the barrier integrity. Furthermore, incubation with AsHCs for 72 h led to a disruption of the barrier at sub-cytotoxic concentrations. The subsequent immunocytochemical staining of three tight junction proteins revealed a significant impact on the cell membrane. Because AsHCs enhance the permeability of the in vitro blood-brain barrier, a similar behavior in an in vivo system cannot be excluded. Consequently, AsHCs might facilitate the transfer of accompanying foodborne toxicants into the brain.