Comparative gene expression profiles of ABC transporters in brain microvessel endothelial cells and brain in five species including human.

While P-glycoprotein (PGP, ABCB1) is known to play an important role in drug exclusion at the blood brain barrier (BBB), less is known about the contribution of other members in the ATP-binding cassette (ABC) transporter family to BBB drug efflux, or whether these transporters are expressed differently in humans and in mammalian species of pharmacological interest. We used quantitative real-time PCR to determine mRNA expression levels for the majority of ABC family members in brain and in isolated brain microvessel endothelial capillary cells (BMEC) from human, rat, mouse, pig and cow. We confirmed BBB expression of several well-characterized ABC family members that are implicated in xenobiotic exclusion from the brain, including ABCB1 (PGP), ABCG2 (BCRP), ABCC1 (MRP1), ABCC4 (MRP4), and ABCC5 (MRP5). In addition, we detected high expression and enrichment in BMEC of several less well-characterized ABC transporters in one or more species, including ABCA2-4, ABCB4, ABCB6-8, ABCB10, ABCC3, ABCC6, ABCC10, and ABCE1. We also uncovered species differences in the expression of a number of transporters, including ABCG2 and ABCC4. This study identifies several additional ABC family members that may contribute to xenobiotic efflux at the human BBB, and compares the expression of a broad array of efflux transporters between human and four other species relevant to pharmacological research.

Expression of the thyroid hormone transporters monocarboxylate transporter-8 (SLC16A2) and organic ion transporter-14 (SLCO1C1) at the blood-brain barrier.

Thyroid hormones require transport across cell membranes to carry out their biological functions. The importance of transport for thyroid hormone signaling was highlighted by the discovery that inactivating mutations in the human monocarboxylate transporter-8 (MCT8) (SLC16A2) cause severe psychomotor retardation due to thyroid hormone deficiency in the central nervous system. It has been reported that Mct8 expression in the mouse brain is restricted to neurons, leading to the model that organic ion transporter polypeptide-14 (OATP14, also known as OATP1C1/SLCO1C1) is the primary thyroid hormone transporter at the blood-brain barrier, whereas MCT8 mediates thyroid hormone uptake into neurons. In contrast to these reports, we report here that in addition to neuronal expression, MCT8 mRNA and protein are expressed in cerebral microvessels in human, mouse, and rat. In addition, OATP14 mRNA and protein are strongly enriched in mouse and rat cerebral microvessels but not in human microvessels. In rat, Mct8 and Oatp14 proteins localize to both the luminal and abluminal microvessel membranes. In human and rodent choroid plexus epithelial cells, MCT8 is concentrated on the epithelial cell apical surface and OATP14 localizes primarily to the basal-lateral surface. Mct8 and Oatp14 expression was also observed in mouse and rat tanycytes, which are thought to form a barrier between hypothalamic blood vessels and brain. These results raise the possibility that reduced thyroid hormone transport across the blood-brain barrier contributes to the neurological deficits observed in affected patients with MCT8 mutations. The high microvessel expression of OATP14 in rodent compared with human brain may contribute to the relatively mild phenotype observed in Mct8-null mice, in contrast to humans lacking functional MCT8.

Involvement of a matrix metalloproteinase in MIS-induced cell death during urogenital development.

Programmed cell death of the Müllerian duct eliminates the primitive female reproductive tract during normal male sexual differentiation. Müllerian inhibiting substance (MIS or AMH) triggers regression by propagating a BMP-like signaling pathway in the Müllerian mesenchyme that culminates in apoptosis of the Müllerian duct epithelium. Presently, the paracrine signal(s) used in this developmental event are undefined. We have identified a member of the matrix metalloproteinase gene family, Mmp2, as one of the first candidate target genes downstream of the MIS cascade to function as a paracrine death factor in Müllerian duct regression. Consistent with a role in regression, Mmp2 expression was significantly elevated in male but not female Müllerian duct mesenchyme. Furthermore, this sexually dimorphic expression of Mmp2 was extinguished in mice lacking the MIS ligand, suggesting strongly that Mmp2 expression is regulated by MIS signaling. Using rat organ genital ridge organ cultures, we found that inhibition of MMP2 activity prevented MIS-induced regression, whereas activation of MMP2 promoted ligand-independent Müllerian duct regression. Finally, MMP2 antisense experiments resulted in partial blockage of Müllerian duct regression. Based on our findings, we propose that similar to other developmental programs where selective elimination or remodeling of tissues occurs, localized induction of extracellular proteinases is critical for normal male urogenital development.