Molecular mechanisms of the electrogenic Na+ Bicarbonate Cotransporter (NBCe1)

描述（由申请人提供）：调节酸碱平衡（血液pH ~7.4）和Na+平衡对动物生命至关重要。在包括哺乳动物在内的所有脊椎动物中，体内平衡是通过严格调节NaHCO3水平来实现的。电致Na+碳酸氢盐共转运蛋白NBCel通过其在肾近端小管基底外膜的转运活性，是NaHCO3水平的主要调节剂。这种转运体的重要性通过人肾NBCel （hkNBCel）中自然发生的隐性点突变（R298S, R510H, S427L）得到证实，这种突变可导致肾近端小管酸中毒（pRTA）、青光眼和白内障。这些患者的血pH < 7.1, [HCO3-] < 11 mM提示hkNBCel是肾脏吸收HC03的主要途径。然而，NBCel失活导致pRTA和眼部病变的机制尚不清楚。我们对hkNBCel突变的初步实验表明，离子亲和力定位于NBCel蛋白的离散区域。生物物理分析和无代偿pRTA（突变型NBCel表型）表明，kNBCel在呼吸代偿和肾脏运输中起主要作用。由于具有kNBCel单氨基酸突变的患者具有引人注目的表型，我们建议使用kNBCel进行生物物理实验，旨在揭示负责其功能成分的蛋白质区域。来自人类突变的证据明确表明，hkNBCel中单个氨基酸的改变会显著改变其活性。我们假设，对携带额外序列修饰的hkNBCel的功能进行检查将识别出负责其功能的关键子结构域。这些信息可用于设计针对这些子域的治疗药物，以改变这一关键转运蛋白的活性，治疗代谢性酸中毒、青光眼和白内障。为了调查这一假设，我们将追求三个目标。首先，我们将通过评估n端突变的生物物理特性、新的人类NBCel突变和NBCel二聚体的作用，对我们的结构模型进行功能测试。其次，我们将确定NBCel同种异构体特异性n端的功能作用。第三，我们将使用人类kNBCel与fugu-NBCel的嵌合体，通过蛋白质的NBCel跨膜结构域划定结合和/或渗透途径。公共卫生声明：NBCel是肾脏中负责吸收碳酸氢钠（小苏打）的蛋白质。人类nbcell突变导致严重的肾脏疾病（代谢性酸中毒）和眼病（青光眼和白内障）。来自咸水河豚的NBCel在功能上有一些显著的差异，尽管蛋白质与人类的NBCel只有轻微的不同。我们将利用这些NBCel突变和人类/鱼类的差异来确定这种蛋白质是如何引起肾脏和眼部疾病的，以及如何改变其活性来预防或治疗疾病。

DESCRIPTION (provided by applicant): Regulation of acid-base homeostasis (blood pH ~7.4) and Na+ homeostasis is critical for animal life. In all vertebrates, including mammals, homeostasis is achieved through strict regulation of levels of NaHCO3. The electrogenic Na+ bicarbonate cotransporter, NBCel, is a major regulator of NaHCO3 levels through its transport activity at the basolateral membrane of the renal proximal tubule. The importance of this transporter is shown by naturally occurring, recessive, point mutations (R298S , R510H, S427L) in human kidney NBCel (hkNBCel), which cause profound proximal renal tubular acidosis (pRTA), glaucoma and cataracts. Blood pH < 7.1 and [HCO3-] < 11 mM in these patients indicate that hkNBCel is THE major HC03 absorption path of the kidney. However, the mechanism by which NBCel inactivation leads to pRTA and ocular pathologies is unclear. Our preliminary experiments with hkNBCel mutations show that ion affinities are localized to discrete areas of the NBCel protein. Biophysical analysis and uncompensated pRTA (the mutant NBCel phenotype), indicate that kNBCel has a major role in respiratory compensation as well as renal transport. Because of the compelling phenotype of patients with single amino acid mutations in kNBCel, we propose using kNBCel for biophysical experiments designed to reveal regions of the protein responsible for components of its function. Evidence from human mutations shows definitively that single amino acid changes in hkNBCel drastically alter its activity. We hypothesize that examination of the function of hkNBCel bearing additional sequence modifications will identify critical subdomains responsible for its function. This information can be used to design therapeutic agents targeted to those subdomains to modify the activity of this critical transporter to treat metabolic acidosis, glaucoma and cataracts. To investigate this hypothesis we will pursue 3 aims. First, we will functionally test our structural model by evaluating biophysical properties of mutations in the N-terminus, new human NBCel mutations and the role of NBCel dimers. Second, we will determine the functional roles of the isoform specific N-termini of NBCel. Third, we will use chimeras of human kNBCel with fugu-NBCel to delimit ion binding and/or permeation paths via the NBCel transmembrane domain of the protein. Lay Public Health statement: NBCel is the protein in the kidney responsible for absorbing sodium bicarbonate (baking soda). Human NBCel mutations cause severe kidney disease (metabolic acidosis) and eye disease (glaucoma and cataracts). NBCel from a salt-water puffer fish has some dramatic functional differences though the protein is only slightly different from human NBCel. We will use these NBCel mutations and human/fish differences to determine how this protein causes kidney and eye disease and how to modify its activity to prevent or treat disease.