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

低磷性维生素d抗性佝偻病是人类最常见的低磷性佝偻病。其特点是肾小管对磷酸盐的重吸收低，血浆磷酸盐低，尽管存在低磷血症和骨疏松性骨病，但缺乏升高的1,25-二羟基维生素D。在本研究中，从人肾文库中分离到一个cDNA (NPT-1)，其编码的蛋白质与Na/Pi共转运蛋白NaPi-1的氨基酸序列相同69%。将该克隆转录的RNA NPT-1注入爪蟾卵母细胞，可表达Na/Pi共转运活性，具有高转运亲和力（Km=0.29mM）。动力学表征（[Pi], [Na]）表明，表达的转运活性与注射人肾poly (A)^+RNA的卵母细胞具有相似的性质。Northern blotting显示NPT-1 mRNA在肾皮质、肝和脑中表达，而在其他组织中不表达。NaPi-3和NPT-1的反义寡核苷酸杂交耗竭完全抑制poly(A)^+ rna诱导的Na^+依赖性磷酸盐摄取。这些发现表明两种高亲和力的Na/Pi共转运蛋白（NaPi-3和NPT-1）存在于人肾皮质中。NPT-1将其定位在人类6号染色体上。NaPi-3将其定位到人类5号染色体上。高亲和力Na/Pi共转运蛋白在遗传性低磷佝偻病伴高钙血症（HHRH）中的作用正在研究中。HHRH是一种常遗传性疾病，由肾近端小管对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.