Molecular mechanisms of hyphophosphatemia and phosphate regulatory protein (Phosphatonin and PEX)

X-连锁低磷血症是一种遗传性的PI稳态紊乱，其特征是脊柱性骨病、生长速度减慢和身材矮小、低磷血症和肾脏PI重吸收障碍。最近，人类的XLH是由PEX基因突变引起的，PEX基因编码一种与中性内肽酶同源的蛋白质。然而，PEX基因产物功能异常引发XLH型病理生理学级联反应的机制尚不清楚。在本研究中，我们研究了XLH的小鼠X-连锁Hyp同源物中Na/PI共转运缺陷的细胞和分子机制。小鼠Pex基因预测编码一个749个氨基酸的蛋白质，与人的PEX序列有95%的同源性。在Hyp小鼠中，Pex基因3‘端缺失。对Hyp小鼠体内Na/PI共转运蛋白的分析表明，II型Na/PI共转运蛋白的mRNA和蛋白均下降了50%。核连续分析表明，信息的减少是由于II型Na/PI共转运蛋白基因转录减少所致。对II型Na/PI共转运体基因启动子的功能分析表明，维生素D反应元件和磷酸反应元件对肾脏的转录起重要作用。在表达荧光素酶基因的OK细胞中，在II型启动子的控制下，Hyp小鼠血清抑制了荧光素酶的活性，这表明磷酸因子直接抑制了II型Na/PI共转运蛋白基因的转录。基于这些发现，我们认为PEX参与了一个影响肾脏PI处理的磷酸因子的处理/失活，PEX的丢失导致了II型Na/PI共转运体基因的异常转录控制。

X-linked hypophosphatemia (XLH) is a genetic disorder of Pi homeostasis characterized by rachitic bone disease, decreased growth rate and short stature, hypophosphatemia, and impaired renal Pi reabsorption. Recently, XLH in humans is caused by mutations in the PEX gene which codes for a protein homologous to neutral endopeptidases. However, the mechanism by which aberrant function of the PEX gene product initiates the pathophysiological cascade underlyning XLH remains unknown. In the present study, we investigated the cellular and molecular mechanisms for the defect of Na/Pi cotransport in murine X-linked Hyp homologs of XLH.Mouse Pex cDNA is predicted to encode a protein of 749 amino acids with 95% identity to the human PEX sequence. The 3'end of Pex cDNA was deleted in the Hyp mouse. Analysis of Na/Pi cotransport in the Hyp mouse showed that there is a 50% decrease both in the type II Na/Pi cotransporter mRNA and in protein. Nuclear run-on assays demonstrated that the decrease in message is caused by decreased transcription of the type II Na/Pi cotransporter gene. Functional analysis of the type II Na/Pi cotransporter gene promoter showed that vitamin D responsive elements and a phosphate responsive element are important for the transcripton in the kidney. In OK cells expressing the luciferase gene under the control of the type II promoter, Hyp mouse serum suppressed the luciferase activity, suggesting that a phosphaturic factor causes directly depressing the transcription of the type II Na/Pi cotransporter gene. Based on these findings, we suggest that PEX is involved in the processing/inactivation of a phosphaturic factor that influences renal Pi handling and that loss of PEX causes abnormal transcriptional control of the type II Na/Pi cotransporter gene.