Zebrafish model of blood-brain barrier to improve drug delivery to the brain

Division Of Basic Sciences - Nci

Investigators

Linked publications & trials

Abstract

Not only are ABC transporters responsible for drug resistance in cancer, but they are a major component of the blood-brain barrier (BBB) and blood-placental barrier. The three most prominent transporters at the blood-brain barrier are ABCB1, ABCC1, and ABCG2. We previously developed a murine model for analysis of ABCG2 expression at the blood-brain barrier based on the fact that luciferin is an ABCG2 substrate and its entry into the brain is prevented by transporter expression. In this model, firefly luciferase is under the expression of the GFAP promoter, leading to its expression in the astrocytes. When mice are injected with luciferin, no light signal from the brain is detected due to ABCG2 preventing luciferin from crossing the blood-brain barrier. However, when luciferin is coadministered with an ABCG2 inhibitor, it can cross the blood-brain barrier and react with luciferase expressed in the astrocytes to produce light which can be quantitatively measured. Because studies of the BBB in mice are time-consuming and expensive, we are developing homologous models in the zebrafish, as components of the zebrafish BBB appear to be very similar to those of the mammalian BBB. We have developed and characterized a transgenic zebrafish line with NanoLuciferase, derived by Promega from a deep sea shrimp, under the control of the GFAP promoter. In this model, NanoLuc is expressed in the developing zebrafish brain and spinal cord. Coelenterazine is one of the substrates for NanoLuc and is transported by both ABCB1 and ABCG2. Furimazine, a coelenterazine derivative with very high yield of light, is an ABCG2 substrate. Thus, this model can be used to study the role of transporters at the blood-brain barrier, but could also be used to screen compounds that might increase permeability of the barrier irregardless of the mechanism. We have shown that addition of a substrate, such as furimazine, and an Abcg2 inhibitor to the water containing larval zebrafish result in light generation consistent with penetration of furimazine across the BBB. If zebrafish are to be considered an appropriate model for study of transporters at the blood-brain barrier, the zebrafish homologs of human transporters must be carefully characterized. Zebrafish do not have a direct homolog of human ABCB1 but instead have 2 similar variants-Abcb4 and Abcb5. Expression of these transporters in heterologous systems has enabled their detailed characterization and inhibition properties. In collaboration with Matthew Hall at NCATS, we have found that zebrafish Abcb4 is nearly identical to human ABCB1 in conferring resistance to 90 known ABCB1 substrates. Abcb5 is also a functional transporter and confers resistance to many ABCB1 substrates but has a slightly narrower substrate specificity. While zebrafish Abcb4 is the only homolog that localizes to the BBB, Abcb4 and Abcb5 are expressed at other barrier and excretory sites in zebrafish, such as the gut, liver and kidneys. Zebrafish also have 4 homologs of human ABCG2-Abcg2a, Abcg2b, Abcg2c and Abcg2d. We have functionally categorized these zebrafish ABCG2 orthologs and determined the brain tissue distribution of zebrafish ABCG2 homologs. To determine substrates of the transporters, we stably expressed each in HEK-293 cells and performed cytotoxicity and fluorescent efflux assays with known ABCG2 substrates. We found Abcg2a had the greatest substrate overlap with ABCG2, and Abcg2d appeared to be the least functionally similar. Using RNAscope in situ hybridization we identified abcg2a as the only homolog expressed at the adult and larval zebrafish BBB, based on its localization to claudin-5 positive brain vasculature. These results demonstrate the conserved function of zebrafish Abcg2a and suggest that zebrafish may be an appropriate model organism for the studying the role of ABCG2 at the BBB. Having identified zebrafish Abcb4 and Abcg2a as the homologous transporters at the zebrafish BBB, we characterized the ability of NanoLuc substrates to be transported by zebrafish and human transporters at the BBB. We examined several coelenterazine derivatives as well as some furimazine derivatives and found that furimazine was the brightest NanoLuc substrate tested and was transported by human ABCG2 and zebrafish Abcg2a. Coelenterazine h was the brightest coelenterazine derivative and was also transported by human ABCG2 and zebrafish Abcg2a. Thus, these compounds could be used to study the role of ABCG2 at the BBB. We also received more furimazine derivatives from Promega Corporation that were not found to penetrate the BBB in their studies in hopes of finding other NanoLuc substrates that might be transported by Abcb4 or Abcg2a in the zebrafish.