Active transmembrane water cycling kinetics: A Cellular Metabolic 1H MR Biomarker

描述（由申请人提供）：跨质膜的水转运是基本的细胞功能。响应渗透梯度的净水运动改变细胞体积。水分子的稳态交换（水循环），没有净流量或体积变化，通过磷脂双层和膜蛋白的被动扩散发生。在一个非常令人兴奋的发现中，我们发现稳态水交换与酵母细胞膜主要离子转运蛋白P型的活性相关。 H+-ATP酶。我们还从心脏获得了初步数据，即水交换与Na+，K+-ATP酶活性相关。Na ~+，K ~+-ATP酶存在于几乎所有的哺乳动物细胞中。这一发现是使用具有稳态细胞外弛豫试剂（RRe）的MR弛豫成像（MRR）通过其纵向时间常数（T1）值（1H 2 O T1 MRR/RRe）来区分细胞内和细胞外水信号。双位点交换分析确定跨膜水交换动力学的平均细胞内水寿命（i）和内部和细胞外的水分数（Vi和Ve）。逆<$i（<$i-1）是一阶速率 水流出的常数。我们的研究结果表明，i-1或水循环揭示了膜蛋白介导的活动。 本申请的长期目标是开发1H 2 O T1 MRR测量水交换作为代谢转运活性的高分辨率分子成像方法。首先，我们追求基础科学，揭示因果机制，连接水交换动力学和膜运输活动。模型系统将是具有稳态RRe浓度（[RRe]）的离体灌注心脏。这些RRe是目前用于人体MRI研究的造影剂。将实施稳态[RRe]的1H 2 O T1 MRR测量的成像版本，并将其用作<$i-1、Vi和Ve的金标准测量，以确定从已经使用的推注[RRe]动态对比增强MRI（非稳态[RRe]）测量中获得的相同参数的准确度/精密度。最后，我们将在体内大鼠心脏中测量和比较稳态和推注[RRe]的i-1和Vi。 该项目将开始阐明影响i-1量级的因素，并评估其作为一种新型细胞代谢转运活性1H MRI生物标志物的潜在用途。该生物标志物将受益于1H MRI的高信号和空间分辨率，从而允许高分辨率功能成像。现有的快门速度DCE-MRI研究已经报道了人类骨肉瘤/骨骼肌和恶性乳腺肿瘤的解剖学上准确的参数图。其他病理生理状态，例如，缺血、心力衰竭和急性肾衰竭也可能改变了i-1。例如，急性肾衰竭涉及肾转运活性的丧失（以及可能的水通量）。我们的方法可能会定义代谢损伤的严重程度，这可能与预后相关。

DESCRIPTION (provided by applicant): Water transport across plasma membranes is a fundamental cellular function. Net water movement in response to an osmotic gradient changes cell volume. Steady-state exchange of water molecules (water cycling), with no net flux or volume change, occurs by passive diffusion through the phospholipid bilayer and via membrane proteins. In a very exciting discovery, we have found that steady-state water exchange correlates with the activity of the yeast cellular membrane major ion transport protein, the P-type H+-ATPase. We also have preliminary data from the heart that water exchange correlates with Na+,K+-ATPase activity. Na+,K+-ATPase is present in almost all mammalian cells. This discovery was made using MR relaxography (MRR) with steady-state extracellular relaxation reagent (RRe) to distinguish intra- and extracellular water signals by their longitudinal time constant (T1) values (1H2O T1 MRR/RRe). Two-site exchange analysis determines trans-membrane water exchange kinetics in terms of the mean intracellular water lifetime (¿i) and intra- and extracellular water fractions (Vi and Ve). The inverse ¿i (¿i-1) is the first order rate constant for water efflux. Our results show that ¿i-1 or water cycling reveals membrane protein mediated activities. The long-term objective of this application is to develop 1H2O T1 MRR measured water exchange as a high-resolution molecular imaging method for metabolic transport activity. First, we pursue the basic science to reveal the causal mechanisms that link water exchange kinetics and membrane transport activity. The model system will be the isolated perfused heart with steady-state RRe concentration ([RRe]). These RRe are contrast agents currently used in human MRI studies. Imaging versions of 1H2O T1 MRR measurements with steady-state [RRe] will be implemented and used as gold standard measures of ¿i-1, Vi and Ve to establish the accuracy/precision of the same parameters obtained from bolus [RRe] Dynamic-Contrast-Enhanced -MRI (non-steady state [RRe]) measurements, which are already in use. Finally, we will measure and compare ¿i-1 and Vi with steady state and bolus [RRe] in the in vivo rat heart. This project will begin to elucidate the factors affecting ¿i-1 magnitude and assess its potential use as a novel cellular metabolic transport activity 1H MRI biomarker. This biomarker would benefit from the high signal and spatial resolution of 1H MRI, thus allowing high resolution functional imaging. Existing shutter-speed DCE-MRI studies have reported anatomically accurate parametric ¿i maps of human osteosarcoma/skeletal muscle and malignant breast tumors. Other patho-physiological states, e.g., ischemia, heart failure and acute renal failure may also have altered ¿i-1. For example, acute renal failure involves loss of renal transport activity (and likely, water fluxes). Potentially our method will define the severity of metabolic damage, which will likely correlate with prognosis.