Modulation of Mrp1 (ABCc1) and Pgp (ABCb1) by Bilirubin at the Blood-CSF and Blood-Brain Barriers in the Gunn Rat

Silvia Gazzin,Francesco Fazzari,J Donald Ostrow,Claudio Tiribelli,Richard Wennberg,Andrea Lorena Berengeno,Alan Raseni,Nathalie Strazielle,Jean-François Ghersi-Egea,Rafael Linden

doi:10.1371/journal.pone.0016165

Abstract

Accumulation of unconjugated bilirubin (UCB) in the brain causes bilirubin encephalopathy. Pgp (ABCb1) and Mrp1 (ABCc1), highly expressed in the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) respectively, may modulate the accumulation of UCB in brain. We examined the effect of prolonged exposure to elevated concentrations of UCB on expression of the two transporters in homozygous, jaundiced (jj) Gunn rats compared to heterozygous, not jaundiced (Jj) littermates at different developmental stages (2, 9, 17 and 60 days after birth). BBB Pgp protein expression was low in both jj and Jj pups at 9 days (about 16–27% of adult values), despite the up-regulation in jj animals (2 and 1.3 fold higher than age matched Jj animals at P9 and P17–P60, respectively); Mrp1 protein expression was barely detectable. Conversely, at the BCSFB Mrp1 protein expression was rather high (60–70% of the adult values) in both jj and Jj at P2, but was markedly (50%) down-regulated in jj pups starting at P9, particularly in the 4th ventricle choroid plexuses: Pgp was almost undetectable. The Mrp1 protein down regulation was accompanied by a modest up-regulation of mRNA, suggesting a translational rather than a transcriptional inhibition. In vitro exposure of choroid plexus epithelial cells obtained from normal rats to UCB, also resulted in a down-regulation of Mrp1 protein. These data suggest that down-regulation of Mrp1 protein at the BSCFB, resulting from a direct effect of UCB on epithelial cells, may impact the Mrp1-mediated neuroprotective functions of the blood-cerebrospinal fluid barrier and actually potentiate UCB neurotoxicity.

Highlights

Severe jaundice from unconjugated bilirubin (UCB) can occur transiently in newborn infants with immature hepatic conjugating capacity [1,2,3,4,5] and lifelong in patients with Crigler-Najjar type I disease [6,7]
A second interface, the bloodcerebrospinal fluid barrier is provided by the epithelial cells of the choroid plexuses, and controls exchanges between plasma and CSF
In this study we investigated the effect of sustained unconjugated hyperbilirubinemia on the developmental protein expression of Mrp1 in the lateral and 4th ventricle choroid plexuses, and of Pgp in brain microvessels

Summary

Introduction

Severe jaundice from unconjugated bilirubin (UCB) can occur transiently in newborn infants with immature hepatic conjugating capacity [1,2,3,4,5] and lifelong in patients with Crigler-Najjar type I disease [6,7]. Over 99.9% of UCB in blood is bound to plasma proteins (primarily albumin) that do not enter the brain [8,9,10]. The serum binding sites approach saturation and unbound UCB (free bilirubin, Bf) will rise dramatically even at lower biliruibin/ albumin molar ratios [2,11]. Under these conditions, the accumulation of UCB in brain (kernicterus) can produce toxicity and result in permanent brain injury. A second interface, the bloodcerebrospinal fluid barrier is provided by the epithelial cells of the choroid plexuses, and controls exchanges between plasma and CSF

Methods

Results

Conclusion