Bachmann Sebastian, Schlichting Uwe, Geist Beate, Mutig Kerim, Petsch Thomas, Bacic Desa, Wagner Carsten A, Kaissling Brigitte, Biber Jürg, Murer Heini, Willnow Thomas E
Department of Anatomy, Charité, University Medical School of Berlin, Campus Mitte, Philippstrasse 12, D-10098 Berlin, Germany.
J Am Soc Nephrol. 2004 Apr;15(4):892-900. doi: 10.1097/01.asn.0000120389.09938.21.
Renal reabsorption of inorganic phosphate is mediated by the type IIa sodium phosphate cotransporter (NaPi-IIa) of the proximal tubule. Changes in renal phosphate handling are mainly attributable to altered NaPi-IIa brush border membrane (BBM) expression. Parathyroid hormone (PTH) induces inactivation of NaPi-IIa by endocytic membrane retrieval and degradation. The key elements triggering this process are not clear to date. Megalin serves as a receptor for the endocytosis of multiple ligands and is coexpressed with NaPi-IIa in the proximal tubule. Investigated was the role of megalin in the regulation of NaPi-IIa in steady state and during inactivation. Kidneys and tubular BBM fractions from mice with a renal-specific megalin gene defect and from controls were analyzed by light and electron microscopic histochemical techniques and Western blot test. Steady-state levels of NaPi-IIa in BBM were significantly enhanced, mRNA levels preserved, and phosphaturia reduced in the absence of megalin. Fluid-phase endocytosis was prevented and the apical endocytic apparatus markedly reduced. Systemic administration of PTH resulted in a defective retrieval and impaired degradation of NaPi-IIa. In vitro, the application of various stimuli of the PTH-induced signaling cascade had no effect either. Adequate steady-state expression of NaPi-IIa and the capacity of the proximal tubule cell to react on PTH-driven inactivation of NaPi-IIa by endocytosis and intracellular translocation require the presence of megalin.
近端肾小管的IIa型钠磷共转运体（NaPi-IIa）介导无机磷酸盐的肾重吸收。肾脏对磷酸盐处理的变化主要归因于NaPi-IIa刷状缘膜（BBM）表达的改变。甲状旁腺激素（PTH）通过内吞膜回收和降解诱导NaPi-IIa失活。迄今为止，触发这一过程的关键因素尚不清楚。巨膜蛋白作为多种配体内吞作用的受体，与NaPi-IIa在近端肾小管中共表达。研究了巨膜蛋白在稳态和失活过程中对NaPi-IIa调节的作用。通过光镜和电镜组织化学技术以及蛋白质印迹试验，分析了肾特异性巨膜蛋白基因缺陷小鼠和对照小鼠的肾脏及肾小管BBM组分。在缺乏巨膜蛋白的情况下，BBM中NaPi-IIa的稳态水平显著提高，mRNA水平保持不变，磷尿减少。液相内吞作用被阻止，顶端内吞装置明显减少。全身性给予PTH导致NaPi-IIa的回收缺陷和降解受损。在体外，应用PTH诱导的信号级联反应的各种刺激也没有效果。NaPi-IIa的充分稳态表达以及近端肾小管细胞通过内吞作用和细胞内转运对PTH驱动的NaPi-IIa失活作出反应的能力需要巨膜蛋白的存在。
