Bile acids permeabilize the blood brain barrier after bile duct ligation in rats via Rac1-dependent mechanisms

Abstract

Background

The blood brain barrier tightly regulates the passage of molecules into the brain and becomes leaky following obstructive cholestasis. The aim of this study was to determine if increased serum bile acids observed during cholestasis permeabilize the blood brain barrier.

Methods

Rats underwent bile duct ligation or deoxycholic or chenodeoxycholic acid injections and blood brain barrier permeability assessed. In vitro, the permeability of rat brain microvessel endothelial cell monolayers, the expression and phosphorylation of occludin, ZO-1 and ZO-2 as well as the activity of Rac1 was assessed after treatment with plasma from cholestatic rats, or bile acid treatment, in the presence of a Rac1 inhibitor.

Results

Blood brain barrier permeability was increased in vivo and in vitro following bile duct ligation or treatment with bile acids. Associated with the bile acid-stimulated increase in endothelial cell monolayer permeability was elevated Rac1 activity and increased phosphorylation of occludin. Pretreatment of endothelial cell monolayers with a Rac1 inhibitor prevented the effects of bile acid treatment on occludin phosphorylation and monolayer permeability.

Conclusions

These data suggest that increased circulating serum bile acids may contribute to the increased permeability of the blood brain barrier seen during obstructive cholestasis via disruption of tight junctions.

Introduction

The blood brain barrier (BBB) is a protective barrier that protects the brain from the passage of molecules from the circulation [1]. Cerebral endothelial cells form tight junctions with adjacent cells via interactions of the tight junction proteins occludin and claudin-5 and intracellular docking proteins ZO-1 and ZO-2 [2]. When these tight junctions are compromised, molecules are able to passively diffuse into the brain. Opening of these tight junctions can be mediated by a number of signal transduction pathways including Rac1 [3], Protein kinase C (PKC) [4], or Jun-N-terminal kinase (JNK) [5], that may result in either a rearrangement of the cytoskeleton [6], [7] or the phosphorylation of the tight junction proteins occludin [8] or claudin-5 [9].

The BBB becomes compromised during the course of various liver diseases [10], [11], [12], which may contribute to the various neurological changes associated with these diseases [13], [14], [15]. During liver damage, serum bile acid concentrations are increased. Under normal physiological conditions, ∼95% of bile is recycled and stored in the gallbladder with the remaining 5% being excreted [16]. These bile acids exert their effects through the nuclear receptor farnesoid X receptor (FXR) [17] or the membrane bound receptor, TGR5 [18]. In addition to signalling through FXR, bile acids have been shown to activate an array of cell signalling pathways such as ERK1/2 and JNK [19]. However, when hepatocytes are damaged and cannot reabsorb bile acids, the serum levels are increased.

Bile acids have been shown previously to induce permeability in epithelial cells [20], [21], however, the role of bile acids in the permeabilization of the BBB during cholestasis remains unknown. Thus the aims of our study were to assess BBB integrity during cholestasis and determine if bile acids can permeabilize the barrier formed by the endothelial cells that make up the BBB in vitro as well as in vivo.