Hypophosphatemia : cDNA cloning of a Na^+-dependent phosphate co-transporter from human kidney

低磷酸盐血症性维生素D抵抗性佝偻病是人类最常见的低磷酸盐血症性佝偻病。其特征在于磷酸盐的肾小管重吸收低，血浆磷酸盐低，尽管存在低磷酸盐血症和骨软化性骨病，但缺乏升高的1，25-二羟基维生素D。在本研究中，一个cDNA（NPT-1）编码的蛋白质69%的氨基酸序列相同的Na/Pi协同转运蛋白NaPi-1从人肾文库中分离。将从该克隆NPT-1转录的RNA注射到爪蟾卵细胞中，导致表达具有高转运亲和力的Na/Pi共转运活性（Km=0.29mM）。动力学特性（[Pi]，[Na]）表明，NPT-1 mRNA表达的转运活性与注射巨肾poly（A）^+RNA的卵母细胞的转运活性相似，北方印迹显示，NPT-1 mRNA在肾皮质、肝和脑中表达，而在其它组织中不表达。NaPi-3和NPT-1反义寡核苷酸的杂交去除完全抑制了poly（A）^+ RNA诱导的卵母细胞Na^+依赖性磷酸盐摄取。这些结果表明，两个高亲和力的Na/Pi协同转运蛋白（NaPi-3和NPT-1）存在于人肾皮质，NPT-1定位于人6号染色体。NaPi-3将该位置映射到人类5号染色体。高亲和力Na/Pi共转运蛋白在遗传性低磷血症佝偻病伴高钙尿症（HHRH）中的作用正在研究中，HHRH是一种常染色体遗传性疾病，由近端肾小管对Pi的重吸收缺陷引起。

Hypophosphatemic vitamin D-resistant rickets is the most common form of hypophosphatemic rickets in humans. It is characterized by low renal tubular reabsorption of phosphate, low plasma phosphate, absence of elevated 1,25-dihydroxy-vitamine D despite the presence of hypophosphatemia and osteomalacic bone disease. In the present study, a cDNA (NPT-1) encoding a protein 69% identical in amino acid sequence to that of the Na/Pi cotransporter NaPi-1 was isolated from a human kidney library. Injected of RNA transcribed from this clone, NPT-1, into Xenopus oocuytes results in expression of a Na/Pi cotransport activity with high affinity for transport (Km=0.29mM). Kinetic characterization ([Pi], [Na]) showed that expressed transport activity has properties similar to that of oocytes injected juman kidney poly (A)^+RNA.Northern blotting showes that NPT-1 mRNA is expressed in kidney cortex, liver and brain but not in other tissues. Hybrid depletion with antisense oligonucleotide of NaPi-3 and NPT-1 completely inhibited poly(A)^+RNA-induced Na^+-dependent phosphate uptake in oocytes. These finding indicate that two high affinity Na/Pi cotransporter (NaPi-3 and NPT-1) are present in human kidney cortex.NPT-1 maps the location to human chromosome 6. NaPi-3 maps the location to human chromosome 5. The role of high affinity Na/Pi cotransporter in Hereditary Hypophosphatemic Rickets with Hypercarciuria (HHRH) which is autosomally inherited disorder and caused by defective reabsorption of Pi in the proximal tubule of the kidney is under study.