The purpose of the present study was to clarify the expression, transport properties and regulation of ATP-binding cassette G2 (ABCG2) transporter at the rat blood-brain barrier (BBB). The rat homologue of ABCG2 (rABCG2) was cloned from rat brain capillary fraction. In rABCG2-transfected HEK293 cells, rABCG2 was detected as a glycoprotein complex bridged by disulfide bonds, possibly a homodimer. The protein transported mitoxantrone and BODIPY-prazosin. In rat brain capillary fraction, rABCG2 protein was also detected as a glycosylated and disulfide-linked complex. Immunohistochemical analysis revealed that rABCG2 was localized mainly on the luminal side of rat brain capillaries, suggesting that rABCG2 is involved in brain-to-blood efflux transport. For the regulation study, conditionally immortalized rat brain capillary endothelial (TR-BBB13), astrocyte (TR-AST4) and pericyte (TR-PCT1) cell lines were used as an in vitro BBB model. Following treatment of TR-BBB13 cells with conditioned medium of TR-AST4 cells, the Ko143 (an ABCG2-specific inhibitor)-sensitive transport activity and rABCG2 mRNA level were significantly increased, whereas conditioned medium of TR-PCT1 cells had no effect. These results suggest that rat brain capillaries express functional rABCG2 protein and that the transport activity of the protein is up-regulated by astrocyte-derived soluble factor(s) concomitantly with the induction of rABCG2 mRNA. Keywords: ABCG2, astrocyte, blood-brain barrier, in vitro BBB model.

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Bistable materials, which can be converted between two unique states by light irradiation, have attracted continuous research interest due to their potential applications as components of future molecular memory and switching devices. Photoswitchable magnetism, between a diamagnetic low spin (LS) and a paramagnetic metastable high spin (HS) state was first observed for iron(II) spin-crossover (SCO) complexes in the solution state by McGarvey et al. [1] and in the solid state by Hauser et al., [2] and this was called light-induced excited-spin-state trapping (LIESST). [3] Although many spin-crossover complexes have been shown to display the LIESST effect, the relaxation temperatures of the LIESST (metastable HS) states were usually lower than about 50 K. [3,4] Means were found, however, to realize systems with high-temperature photo-induced metastable states.Theshowed a photo-induced HS state at room temperature through irradiation with a high-powered pulsed laser. [5] Another approach to achieve photomagnetic effects at higher temperatures was proposed by Roux et al. in 1994: ligand-driven light-induced spin change (LD-LISC). [6] LD-LISC is based on the combination of a SCO complex with photochromic ligands. In photochromic molecules, structural and electronic state changes induced by light irradiation can alter the ligand field strength, which can lead to spin-state changes between HS and LS states. The advantage of the LD-LISC effect is that the light-induced spin conversions can operate in higher temperature ranges and can be stimulated by irradiation from relatively low-powered light sources. It should be noted that LD-LISC effects have been shown to occur in single molecules and even in solution; LD-LISC does not rely upon intermolecular cooperativity to enable spin conversion. Despite its benefits however, the number of materials exhibiting the LD-LISC effect is still limited. [7] Diarylethene, exhibiting photoisomerization between open and closed forms, is a useful molecule for LD-LISC

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