Search

The A-subclass of ATP-binding cassette (ABC) transporters is a highly conserved superfamily of potent lipid transporters. Although the ABCA1-like subgroup of ABCA1-4, and A7 have been shown to mediate the transport of endogenous lipids, the roles of the ABCA6-like subgroup transporters, which have been identified as a unique gene cluster on human chromosome 17q24 (ABCA5, A6, A8, A9, and A10) and mouse chromosome 11 (Abca5, a6, a8a, a8b, and a9), remains largely unknown. The purpose of the present study was to clarify the spatial and temporal expression profiles of Abca6-like subgroup transporters in embryonic and postnatal mouse brains by a combination of in situ hybridization and quantitative polymerase chain reaction (PCR) using magnetically isolated brain vascular endothelial cells. In embryonic brains, the transcripts of Abca5, a8a and a8b were detected predominantly in the mantle zone, where postmitotic neurons differentiate. At the postnatal stages, they were expressed in various nuclei and neuronal layers. Abca9 mRNA was detected diffusely in the embryonic and postnatal brains and sequential and/or strong spotted signals were detected in the leptomeninges on the brain surface. PCR detected expression of Abca8a and Abca9 mRNAs in isolated vascular endothelial cells. Expression signals for Abca6 mRNA were hardly observed at any stages examined. These distinct spatio-temporal expression patterns of Abca6-like subgroup transporters may reflect their functional significance and diversity to regulate lipid transport, particularly in neurons, leptomeningeal cells, and vascular endothelial cells.

Guanidinoacetate (GAA) is a biosynthetic precursor of creatine, which plays a critical role in homeostasis of high-energy phosphates in the brain, but cerebral accumulation of GAA leads to neurological complications, such as epilepsy and seizures. The purpose of the present study was to clarify the contribution of the γ-aminobutyric acid (GABA) transport systems to GAA transport in astrocytes by means of uptake studies in rat brain slices, primary astrocyte cultures and Chinese hamster ovary (CHO) cells expressing human GABA transporters (GATs). GAA uptake by rat brain slices was Na + - and Cl - -dependent, and GABA-sensitive. The inhibitory effect of GABA, a common substrate of GATs, on GAA uptake by the brain slices was similar to that of β-alanine, a selective substrate of GAT2/Slc6a13, GAT3/Slc6a11, and taurine transporter (TauT)/Slc6a6. Taurine, a high-affinity substrate of TauT/Slc6a6, exhibited a lesser inhibitory effect. In contrast, betaine, a substrate of betaine-GABA transporter 1 (BGT1)/Slc6a12, and creatine, a substrate of creatine transporter (CRT)/Slc6a8, had little inhibitory effect. A similar inhibition profile was observed in primary-cultured astrocytes. CHO cells expressing human GAT2/SLC6A13, GAT3/SLC6A11 and BGT1/SLC6A12 exhibited GAA transport, whereas CHO cells expressing GAT1/SLC6A1 did not. The Michaelis-Menten values in CHO cells expressing GAT2/SLC6A13 and GAT3/SLC6A11 were similar to those in primary-cultured astrocytes. Overall, our results suggest that astrocytic GAT2/Slc6a13 and GAT3/Slc6a11 play major roles in GAA uptake as regulatory mechanisms of GAA in rat brain, while TauT/Slc6a6, BGT1/Slc6a12, and CRT/Slc6a8 make relatively small contributions., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Spermine is the end-product in the polyamine biosynthetic pathway, and its excess accumulation induces neuroexcitatory responses and neurotoxicity. The purpose of this study was to elucidate the involvement of transport systems at the brain barriers in the clearance of spermine. In vivo rat spermine elimination from brain parenchyma across the blood-brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier (BCSFB) was assessed by intracerebral and intracerebroventricular administration techniques, respectively. To characterize spermine transport at the BCSFB, a transport study using rat choroid plexus was performed. After the intracerebral microinjection of [ 3 H]spermine, no time-dependent decrease in [ 3 H]spermine in the ipsilateral cerebrum was observed, suggesting the low contribution of the BBB to spermine clearance from the brain. In contrast, the [ 3 H]spermine concentration in the CSF after intracerebroventricular administration was time-dependently decreased with an elimination rate constant of 0.352 min -1 , and the elimination clearance of [ 3 H]spermine was 6.6-fold greater than that of [ 14 C]D-mannitol, reflecting bulk flow of the CSF. This [ 3 H]spermine elimination was attenuated by co-administration of unlabeled excess spermine, indicating carrier-mediated elimination of spermine from the CSF. [ 3 H]Spermine transport into the choroid plexus was strongly inhibited by unlabeled spermine, other polyamines (spermidine and putrescine), and organic cation transporter substrates such as corticosterone and 1-methyl-4-phenylpyridinium. However, other substrates/inhibitors for organic cation transporters (decynium-22 and tetraethylammonium) had little effect. Consequently, our study indicates that transporting molecules at the BCSFB, distinct from typical organic cation transporters, are involved in spermine clearance from the CSF.