Efflux mechanisms at the developing brain barriers: ABC-transporters in the fetal and postnatal rat

Proteins of the ATP-binding cassette (ABC) family, present at the blood-brain barrier interfaces, have been shown to reduce the entry of compounds from blood into the brain by active efflux. Their substrates are diverse including many drugs and toxins and therefore provide an important mechanism for brain neuroprotection. However, knowledge of their presence and function in the developing brain is very limited. We have used qPCR and immunocytochemistry to determine gene expression and localisation of four main barrier ABC-transporters (pgp/ABCB1, MRP1/ABCC1, MRP4/ABCC4 and BCRP/ABCG2) in the fetal and neonatal rat brain cerebral blood vessels (site of blood-brain barrier) and choroid plexus (site of blood-CSF barrier). The study shows that ABC-transporters localise to the brain barriers even at early fetal stages and although pgp expression was lower in the fetus, the other transporters were expressed at comparable levels in fetal and adult brains suggesting direct neuroprotection of the brain in addition to that provided by the placenta. BCRP was expressed at higher levels in developing choroid plexus and was only detected at fetal stages on the blood-facing side of epithelial cells indicating a particular role of this transporter for early brain efflux mechanisms.