Decreased expression of multidrug efflux transporters in the brains of GSK-3beta transgenic mice.

Multidrug efflux transporters protect cells in the brain from potentially harmful substances but also from therapeutically useful drugs. Thus any condition that causes changes in their expression is of some importance with regard to drug access. In this study, changes in efflux transporter expression are investigated in mice containing a mutant constitutively active glycogen synthase kinase-3 (GSK-3beta) transgene, driven by the Thy-1 promoter so limiting its localization predominantly to neurons and some glial cells. As expected, decreases in beta-catenin were evident via Western blot analyses of cortical homogenates prepared from brains of these transgenic mice. As assessed by real time qRT-PCR, decreased transcript levels of the mdr1b isoform of P-glycoprotein, Mrp1 and Mrp4, (transporters associated with neurons and/or glial cells) were observed in the cortex but not the subventricular zone or hippocampus of the transgenic compared to wild type mouse brains. By contrast, no such decreases were evident with the mdr1a isoform of P-glycoprotein and Bcrp, transporters predominantly found in brain endothelium. Such transporter expression changes could not be accounted for by alterations in blood vessel density or neuronal to glial cell ratios as analyzed both from immunocytochemical staining and from RT-PCR. These observations support previous in vitro data showing that manipulations to GSK-3beta activity that alter signaling via beta-catenin can influence the expression of efflux transporters. Implications from this are that drug distribution into cells within the brain of these transgenic mice could be enhanced, hence warranting further investigation.

Activation of beta-catenin signalling by GSK-3 inhibition increases p-glycoprotein expression in brain endothelial cells.

This study investigates involvement of beta-catenin signalling in regulation of p-glycoprotein (p-gp) expression in endothelial cells derived from brain vasculature. Pharmacological interventions that enhance or that block beta-catenin signalling were applied to primary rat brain endothelial cells and to immortalized human brain endothelial cells, hCMEC/D3, nuclear translocation of beta-catenin being determined by immunocytochemistry and by western blot analysis to confirm effectiveness of the manipulations. Using the specific glycogen synthase kinase-3 (GSK-3) inhibitor 6-bromoindirubin-3'-oxime enhanced beta-catenin and increased p-gp expression including activating the MDR1 promoter. These increases were accompanied by increases in p-gp-mediated efflux capability as observed from alterations in intracellular fluorescent calcein accumulation detected by flow cytometry. Similar increases in p-gp expression were noted with other GSK-3 inhibitors, i.e. 1-azakenpaullone or LiCl. Application of Wnt agonist [2-amino-4-(3,4-(methylenedioxy) benzylamino)-6-(3-methoxyphenyl)pyrimidine] also enhanced beta-catenin and increased transcript and protein levels of p-gp. By contrast, down-regulating the pathway using Dickkopf-1 or quercetin decreased p-gp expression. Similar changes were observed with multidrug resistance protein 4 and breast cancer resistance protein, both known to be present at the blood-brain barrier. These results suggest that regulation of p-gp and other multidrug efflux transporters in brain vasculature can be influenced by beta-catenin signalling.

Neural precursor cell influences on blood-brain barrier characteristics in rat brain endothelial cells.

This study explores the effects of neural precursor cells (NPCs) on barrier characteristics in brain vasculature. Primary rat brain endothelial cells were exposed to conditioned medium from NPCs isolated from day 14 embryonic rat brains and maintained as free-floating undifferentiated neurospheres. Such exposure increased brain endothelial transcript levels of the mdr1a but not mdr1b gene encoding P-glycoprotein (Pgp) and reduced proliferation but did not alter transendothelial resistance (TER). These effects were compared to those seen following co-culture with differentiating NPCs or with primary astrocytes. NPCs, if grown adherent, differentiate into glial and neuronal cells as assessed by immunocytochemical and mRNA analysis. Brain endothelial cells when co-cultured with these cells also showed reduced proliferation and enhanced mdr1a expression, but in addition increased TER. Similar increases were observed in co-culture with astrocytes. These results suggest that undifferentiated NPCs produce factors that influence Pgp expression whereas their progeny also affect tight junction integrity.