Contribution of glial cells and pericytes to the mRNA profiles of P-glycoprotein and multidrug resistance-associated proteins in an in vitro model of the blood-brain barrier.

P-glycoprotein (P-gp) and the multidrug resistance-associated proteins (MRP), whose expression is associated with multidrug resistance, have been recently located in the brain capillary endothelial cells (BCEC) forming the blood-brain barrier (BBB), without taking into account a possible influence or contribution of glial cells and pericytes. Using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR), the present study analysed the transcriptional expression of P-gp and the seven homologues of MRP transporters in BCECs in solo culture or in an in vitro model of the BBB consisting of a co-culture of BCECs and glial cells. Pericytes, glial cells, isolated brain capillaries and bovine grey matter extracts were also tested. P-gp mRNA, absent in glial cells, was found in brain capillaries and in co-cultured BCECs with an increased signal compared to the in solo culture. No amplification was observed in pericytes or grey matter. While MRP2, MRP3 and MRP7 remained undetected, MRP1, absent in capillaries or grey matter, was amplified in BCECs, glial cells and pericytes. MRP4 gave a low signal in most cultures. MRP5 was ubiquitously expressed, displaying a potent signal in all conditions. In spite of its presence in cultured glial cells, MRP6 mRNA expression appeared to be restricted to BCECs, with the same upregulation in the co-cultured condition as observed with P-gp. Moreover, MRP6 was the only transporter whose endothelial mRNA expression was influenced by the presence of pericytes. The tissue distribution of the expression of these transporters and the contribution of the different cell populations are discussed.