Glutamate cycling may drive organic anion transport on the basal membrane of human placental syncytiotrophoblast.
Glutamate cycling may drive organic anion transport on the basal membrane of human placental syncytiotrophoblast.
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谷氨酸循环可能驱动有机阴离子在人胎盘合成细胞基底膜上的转运。
DOI：
10.1113/jp270743
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发表时间：
2015-10-15
期刊：
影响因子：
--
通讯作者：
Lewis RM
中科院分区：
文献类型：
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作者：
Lofthouse EM;Brooks S;Cleal JK;Hanson MA;Poore KR;O'Kelly IM;Lewis RM
胎盘从胎儿循环中清除废物、药物和环境毒素，其中两种转运蛋白是定位于胎盘合胞体滋养层基底膜的OAT 4和OATP 2B 1。我们提供的证据表明，OAT 4和OATP 2B 1介导谷氨酸外流时，表达在非洲爪蟾卵母细胞和灌注胎盘，溴磺酞（OAT 4和OATP 2B 1底物）刺激谷氨酸外流。此外，只有在存在天冬氨酸的情况下才能看到谷氨酸的流出，这将阻止胎盘对谷氨酸的再吸收，这与谷氨酸穿过基底膜的循环一致。我们认为谷氨酸的跨膜梯度流出通过OAT 4和OATP 2B 1驱动胎盘从胎儿循环中摄取，谷氨酸的再摄取维持了这一驱动梯度。有机阴离子转运蛋白OAT 4（SLC 22 A11）和有机阴离子转运多肽OATP 2B 1（SLCO 2B 1）在胎盘合体滋养层的基底膜中表达。这些转运蛋白介导交换，其中一种有机阴离子的摄取与反离子的流出偶联。在胎盘中，这些交换剂介导胎盘摄取雌激素合成的底物以及清除胎儿循环中的废物和外源性物质。然而，在胎盘和其他组织中驱动这种转运的反离子的身份尚不清楚。虽然谷氨酸不是已知的OAT 4或OATP 2B 1底物，但我们认为其高细胞内浓度有可能驱动胎儿循环中底物的蓄积。在离体灌注胎盘中，观察到胎盘和胎儿循环之间的谷氨酸交换。这种交换不能用已知的谷氨酸交换剂来解释。然而，谷氨酸外排受到OAT 4和OATP 2B 1底物（溴磺酞）的反式刺激。只有在谷氨酸再摄取受到抑制（通过添加天冬氨酸）时，才观察到谷氨酸与溴磺酞的交换。为了确定OAT 4和/或OATP 2B 1是否介导谷氨酸交换，在非洲爪蟾卵母细胞中研究了谷氨酸的吸收和流出。我们的数据表明，在表达OAT 4或OATP 2B 1的非洲爪蟾卵母细胞中，细胞内[14 C]谷氨酸的流出可通过细胞外谷氨酸（OAT 4）、硫酸雌酮和溴磺酞（OAT 4和OATP 2B 1）或普伐他汀（OATP 2B 1）等条件刺激。谷氨酸通过胎盘的循环，包括通过OAT 4和OATP 2B 1的外排和随后的再摄取，将驱动胎盘从胎儿循环中摄取有机阴离子。
The placenta removes waste products, drugs and environmental toxins from the fetal circulation and two of the transport proteins responsible for this are OAT4 and OATP2B1 localised to the basal membrane of placental syncytiotrophoblast. We provide evidence that OAT4 and OATP2B1 mediate glutamate efflux when expressed in Xenopus oocytes and that in the perfused placenta, bromosulphothalein (an OAT4 and OATP2B1 substrate) stimulates glutamate efflux. Furthermore the efflux of glutamate can only be seen in the presence of aspartate, which will block glutamate reuptake by the placenta, consistent with cycling of glutamate across the basal membrane. We propose that glutamate efflux down its transmembrane gradient drives placental uptake via OAT4 and OATP2B1 from the fetal circulation and that reuptake of glutamate maintains this driving gradient. The organic anion transporter OAT4 (SLC22A11) and organic anion transporting polypeptide OATP2B1 (SLCO2B1) are expressed in the basal membrane of the placental syncytiotrophoblast. These transporters mediate exchange whereby uptake of one organic anion is coupled to efflux of a counter‐ion. In placenta, these exchangers mediate placental uptake of substrates for oestrogen synthesis as well as clearing waste products and xenobiotics from the fetal circulation. However, the identity of the counter‐ion driving this transport in the placenta, and in other tissues, is unclear. While glutamate is not a known OAT4 or OATP2B1 substrate, we propose that its high intracellular concentration has the potential to drive accumulation of substrates from the fetal circulation. In the isolated perfused placenta, glutamate exchange was observed between the placenta and the fetal circulation. This exchange could not be explained by known glutamate exchangers. However, glutamate efflux was trans‐stimulated by an OAT4 and OATP2B1 substrate (bromosulphothalein). Exchange of glutamate for bromosulphothalein was only observed when glutamate reuptake was inhibited (by addition of aspartate). To determine if OAT4 and/or OATP2B1 mediate glutamate exchange, uptake and efflux of glutamate were investigated in Xenopus laevis oocytes. Our data demonstrate that in Xenopus oocytes expressing either OAT4 or OATP2B1 efflux of intracellular [14C]glutamate could be stimulated by conditions including extracellular glutamate (OAT4), estrone‐sulphate and bromosulphothalein (both OAT4 and OATP2B1) or pravastatin (OATP2B1). Cycling of glutamate across the placenta involving efflux via OAT4 and OATP2B1 and subsequent reuptake will drive placental uptake of organic anions from the fetal circulation.