Impaired PTH-induced endocytotic down-regulation of the renal type IIa Na+/Pi-cotransporter in RAP-deficient mice with reduced megalin expression
Impaired PTH-induced endocytotic down-regulation of the renal type IIa Na+/Pi-cotransporter in RAP-deficient mice with reduced megalin expression
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DOI：
10.1007/s00424-003-1057-4
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发表时间：
2003-07-01
影响因子：
4.5
通讯作者：
Murer, H
中科院分区：
文献类型：
--
作者：
Bacic, D;Capuano, P;Murer, H
无机磷酸盐（P-i）在肾近端小管中的重吸收主要通过位于刷边膜（BBM）的Na+/P-i共转运蛋白IIa型（NaPi-IIa）发生，并受膳食P-i摄入量和甲状旁腺激素（PTH）等因素的调节。甲状旁腺素诱导的P-i重吸收抑制是由Na/P-i- iia从BBM内吞和随后的溶酶体降解介导的。meggalin参与受体介导的肾近端小管尿液蛋白的内吞作用。最近发现的受体相关蛋白（RAP）是一种新型的伴侣蛋白，负责内吞受体（如巨噬蛋白）的细胞内运输。RAP的基因破坏导致BBM中meggalin的减少和近端小管内吞机制的紊乱。在这里，我们研究了NaPi-IIa的分布和/或饮食P-i摄入和甲状旁腺激素对其的调节是否会影响rap缺陷小鼠近端小管，作为巨噬细胞蛋白丢失的模型。在rap缺陷小鼠中，megalin的表达强烈降低并局限于根尖下定位。肾内NaPi-IIa蛋白的分布和丰度没有改变。NaPi-IIa相互作用蛋白MAP17、PDZK-1、D-AKAP2和NHE-RF1的定位和丰度也正常。在BBM中表达的其他转运蛋白如Na+/H+交换器NHE-3和Na+/硫酸盐共转运蛋白NaSi正常表达。在整个动物和离体新鲜肾片中，甲状旁腺素诱导的NaPi-IIa内化在rap缺陷小鼠中被强烈延迟。近端小管PTH受体的表达不受RAP敲除的影响。cAMP、cGMP或PKC激活剂诱导rap缺陷小鼠的内化延迟。相比之下，野生型和rap缺陷小鼠都能够适当地适应高、正常和低P-i饮食，这可以通过尿P-i排泄和NaPi-IIa蛋白丰度来证明。
Inorganic phosphate (P-i) reabsorption in the renal proximal tubule occurs mostly via the Na+/P-i cotransporter type IIa (NaPi-IIa) located in the brush-border membrane (BBM) and is regulated, among other factors, by dietary P-i intake and parathyroid hormone (PTH). The PTH-induced inhibition of P-i reabsorption is mediated by endocytosis of Na/P-i-IIa from the BBM and subsequent lysosomal degradation. Megalin is involved in receptor-mediated endocytosis of proteins from the urine in the renal proximal tubule. The recently identified receptor-associated protein (RAP) is a novel type of chaperone responsible for the intracellular transport of endocytotic receptors such as megalin. Gene disruption of RAP leads to a decrease of megalin in the BBM and to a disturbed proximal tubular endocytotic machinery. Here we investigated whether the distribution of NaPi-IIa and/or its regulation by dietary P-i intake and PTH is affected in the proximal tubules of RAP-deficient mice as a model for megalin loss. In RAP-deficient mice megalin expression was strongly reduced and restricted to a subapical localization. NaPi-IIa protein distribution and abundance in the kidney was not altered. The localization and abundance of the NaPi-IIa interacting proteins MAP17, PDZK-1, D-AKAP2, and NHE-RF1 were also normal. Other transport proteins expressed in the BBM such as the Na+/H+ exchanger NHE-3 and the Na+/sulphate cotransporter NaSi were normally expressed. In whole animals and in isolated fresh kidney slices the PTH-induced internalization of NaPi-IIa was strongly delayed in RAP-deficient mice. PTH receptor expression in the proximal tubule was not affected by the RAP knock-out. cAMP, cGMP or PKC activators induced internalization which was delayed in RAP-deficient mice. In contrast, both wildtype and RAP-deficient mice were able to adapt to high-, normal, and low-P-i diets appropriately as indicated by urinary P-i excretion and NaPi-IIa protein abundance.