Neonatal Proximal Tubule Acidification

描述（由申请人提供）：新生儿比成人更容易发生体液和电解质紊乱，部分原因是肾脏不成熟。我们已经证明，新生儿近端小管NaCl和NaHCO3运输的速率和特征与成人有显著差异。近端小管对NaCl和NaHCO3的重吸收在很大程度上依赖于腔内Na+/H+交换剂，在成人中是NHE3。尽管新生儿具有大量的Na+/H+交换活性，但其顶膜几乎没有NHE3。我们提供的证据表明，新生儿有不同的Na+/H+异构体比成人。我们发现，与主要表达NHE3和少量NHE8的成人相比，新生儿近端小管的刷状边界膜表达丰富的NHE8和少量NHE3。然而，我们对NHE8的功能特征、调控以及在出生后发育过程中导致NHE3亚型转换的原因知之甚少。本提案的目的是表征NHE8在体外和体内的功能。我们将在全细胞贴片配置上使用高度敏感的pH选择电极技术，我们可以完全控制细胞外和细胞内环境来表征NHE8的功能。我们将使用一种新的动物模型来研究肾脏发育过程中NHE8异构体转换的因素，这是一种去肾上腺甲状腺功能减退的大鼠，其中近端小管Na+/H+交换和NHE3的成熟几乎在新生儿水平上被阻止。我们已经表明，NHE3水平增加糖皮质激素和甲状腺激素。去肾上腺甲状腺功能减退的大鼠将允许我们验证这样的假设，即新生儿的相对甲状腺功能减退、糖皮质激素缺乏状态会阻止NHE3的表达，使其产生另一种对新生儿近端小管酸化具有不同调节特性的异构体。我们将研究NHE8是否受到糖皮质激素和甲状腺激素的反向调节，从而导致成人的下调。最后，我们现在有了一只NHE8缺失小鼠，这将使我们能够检查这种发育性NHE异构体在肾脏酸化中的相对功能重要性，以及它在新生儿和成人酸中毒反应中的相对重要性。

DESCRIPTION (provided by applicant): Neonates are more prone to fluid and electrolyte disorders than adults due, in part, to the immature kidney. We have shown that there are marked differences between the rate and characteristics of neonatal proximal tubule NaCl and NaHCO3 transport than that in the adult. Proximal tubule reabsorption of NaCl and NaHCO3 is dependent in large part on the luminal Na+/H+ exchanger, which in adults is NHE3. Neonates have almost no apical membrane NHE3 despite having a significant amount of Na+/H+ exchanger activity. We provide evidence that the neonate has a different Na+/H+ isoform than the adult. We show that the brush border membrane of the neonatal proximal tubule expresses abundant NHE8 and little NHE3, compared to the adult that primarily expresses NHE3 and little NHE8. However, we know very little about the functional characteristics of NHE8, its regulation and what causes the isoform switch during postnatal development to NHE3. The aims of this proposal are to characterize the function of NHE8 in vitro and in vivo. We will use a highly sensitive technique of pH selective electrodes on the whole-cell patch configuration where we have total control of the extra and intracellular milieu to characterize the function of NHE8. We will examine the factors responsible for the isoform switch from NHE8 during postnatal development using a novel animal model to study renal development, the adrenalectomized-hypothyroid rat, where the maturation of proximal tubule Na+/H+ exchange and NHE3 is nearly arrested at the neonatal level. We have shown that NHE3 levels are increased by the glucocorticoids and thyroid hormone. The adrenalectomized-hypothyroid rat will allow us to test the hypothesis that the relative hypothyroid, glucocorticoid deficient state of the neonate prevents the expression of NHE3 making another isoform with different regulatory properties necessary for neonatal proximal tubule acidification. We will examine if NHE8 is regulated in the opposite direction by glucocorticoids and thyroid hormone resulting in down regulation in the adult. Finally, we now have a NHE8 null mouse that will allow us to examine the relative functional importance of this developmental NHE isoform in renal acidification and its relative importance in response to acidosis in both neonates and adults. PUBLIC HEALTH RELEVANCE: Neonates are more prone to develop acidosis and volume depletion from diseases such as diarrhea that can be life threatening. This susceptibility to acidosis is in large part due to the fact that the newborn kidney is less well developed and in some cases uses different transport proteins for electrolyte transport than adults. This proposal will characterize a neonatal transporter that likely plays an important role in neonatal kidney acidification and salt transport.