Activation of RAR, RAR, or RXR Increases Barrier Tightness in Human Induced Pluripotent Stem Cell-Derived Brain Endothelial Cells

The blood-brain barrier (BBB) is critical to central nervous system (CNS) health. Brain microvascular endothelial cells (BMECs) are often used as in vitro BBB models for studying BBB dysfunction and therapeutic screening applications. Human pluripotent stem cells (hPSCs) can be differentiated to cells having key BMEC barrier and transporter properties, offering a renewable, scalable source of human BMECs. hPSC-derived BMECs have previously been shown to respond to all-trans retinoic acid (RA), and the goal of this study was to identify the stages at which differentiating human induced pluripotent stem cells (iPSCs) respond to activation of RA receptors (RARs) to impart BBB phenotypes. Here the authors identified that RA application to iPSC-derived BMECs at days 6-8 of differentiation led to a substantial elevation in transendothelial electrical resistance and induction of VE-cadherin expression. Specific RAR agonists identified RAR, RAR, and RXR as receptors capable of inducing barrier phenotypes. Moreover, RAR/RXR costimulation elevated VE-cadherin expression and improved barrier fidelity to levels that recapitulated the effects of RA. This study elucidates the roles of RA signaling in iPSC-derived BMEC differentiation, and identifies directed agonist approaches that can improve BMEC fidelity for drug screening studies while also distinguishing potential nuclear receptor targets to explore in BBB dysfunction and therapy.