POSTTRANSCRIPTIONAL REGULATION OF ANGIOTENSIN RECEPTORS

在大鼠中，从单个Atlar合成了两个不同的血管紧张素LA受体(AT]Ar)mRNAs 吉恩。这些转录本由外显子1和3(EL，3)和外显子1，2和3(EL，2，3)组成。这两份成绩单 编码相同的受体蛋白，仅在其5‘前导序列(5’LS)的长度上不同。在我们的 在之前的资助期间，我们确定EL，2，3转录本在外显子2中具有RNA顺式元件， 抑制翻译的效率。这些剪接变体在翻译效率上的差异导致了更高的 表达EL，3转录本和表达EL，3转录本的细胞的图谱密度和信号活性 El，2，3。我们的新研究表明，Atlar剪接变体的翻译效率随着年龄的增长而降低 老鼠。我们还发现，反应时间--即上下调节肾上腺皮质所需的时间 与年轻大鼠相比，老年大鼠对钠摄入量改变的ATjR密度显著延长 动物。老年动物的这种迟缓的反应时间也与肾上腺反应减弱有关；即， 钠摄入量改变引起的老年大鼠血浆醛固酮的变化幅度明显较小 与年幼的动物相比。这些观察结果支持了临床和实验研究，表明衰老是 与组织对血管紧张素II的反应性降低有关，并使我们得出了以下一般性假设： 与衰老相关的组织对Ang 11的反应性降低是由于AT^R的翻译调节受损 转录本通过rna顺式作用元件在外显子2内；这种失调导致速度、幅度 和AT2R反应对血管紧张素转换酶11的阈值敏感性，从而有助于众所周知的年龄相关 内流和电解质动态平衡受损。我们计划在四岁以下的幼鼠和老年鼠身上测试这一假说。 肾素血管紧张素系统的调控，包括目标1：对低钠饮食和血管紧张素转换酶的反应 在维持生理盐水的大鼠中，以模拟限钠所达到的降压剂量进行输注 目标2：在限钠后重新平衡到正常钠(NS)饮食期间和在重新平衡期间 血管紧张素转换酶抑制剂刺激大鼠达到降低血管紧张素Ⅱ水平的平衡 维持LS饮食2周。在目标3中，我们将研究rna顺式元件的作用机制。 在外显子2内参与Ang II介导的肾上腺小球和血管平滑的ATjR调节 肌肉细胞。钠摄入量改变对ATjR表达和活性的调节在不同组织中是不同的。 钠限制上调肾上腺肾小球细胞ATjRs密度，增加肾上腺ATjR- 介导的醛固酮分泌。相反，限钠可下调血管AT：RS并减少 血管对血管紧张素Ⅱ的收缩能力。ATxRs在肾脏和肾脏的特定区域也有不同的调节 通过改变钠的摄入量来影响大脑。在这项提案中，我们计划重点关注肾上腺和肠系膜阻力。 这是因为在这些组织中，ATjR受到钠摄入量变化的相互调节。这些研究将 确定AT：R在这些相互调控的组织中的转录后调节机制 这些机制是如何在老年大鼠对钠摄入量改变作出反应的。

In the rat,two distinct angiotensin type la receptor (AT]aR) mRNAs are synthesized from a single ATlaR gene. These transcripts are comprised of exons 1 and 3 (El,3) and exons 1, 2 and 3 (El,2,3). These 2 transcripts code for identical receptor proteins and differ only in the lengths of their 5' leader sequence (5'LS). During our previous funding period, we established that the El,2,3 transcript possesses RNA ciselements within exon 2 that inhibit the efficiency of translation. These splice variant differences in translational efficiency result in higher ATlaR densities and signaling activity in cells expressing the El,3 transcript compared to those expressing the El,2,3. Our new studies suggest that the translational efficiency of ATlaR splice variants is diminished in aged rats. We have also found that the response time - i.e., the time it takes to up and down regulate adrenal cortical ATjR densities in response to altered sodium intake is significantly longer in aged rats compared to young animals. This sluggish response time in aged animals is also associated with diminished adrenal responses; i.e., the magnitude ofchange in plasma aldosterone induced by altered sodium intake is significantly less in aged rats compared to young animals. These observations support clinical and experimental studies that indicate aging is associated with reduced tissue responsiveness to Ang II and have led us to the following general hypothesis: Reduced tissue responsiveness to Ang 11that is associated with aging is due to impaired translational regulation of AT^R transcripts via RNA cis acting elements within exon 2; this dysregulation leads to attenuation in the rapidity, magnitude and threshold sensitivity of the AT2R response to Ang 11 and thereby contributes to the well known age-associated impairments influid and electrolyte homeostasis. We plan to test this hypothesis in young and aged rats under four manipulations of the renin angiotensin system including in Aim 1: response to a low sodium (LS)diet and Ang II infusion at a subpressor dose that mimics the levels achieved by sodium restriction in rats maintained on a NS diet; and in Aim 2: during re-equilibration to a normal sodium (NS) diet after sodium restriction and during re- equilibration to reduced levels of Ang II by challenge with an angiotensin converting enzyme inhibitor in rats maintained on a LS diet for 2 weeks. In Aim 3, we will investigate the mechanisms of RNA cis acting elements within exon 2 that contribute to Ang II-mediated ATjR regulation in adrenal glomerulosa and vascular smooth muscle cells. The regulation of ATjR expression and activityby altered sodium intake varies in different tissues. Sodium restriction up-regulates the density of ATjRs on adrenal glomerulosa cells and increases adrenal ATjR- mediated aldosterone secretion. In contrast, sodium restriction down-regulates vascular AT:Rs and reduces vascular contractility to Ang II. ATxRs are also differentially regulated in specific regions of the kidney and brain by altered sodium intake. In this proposal, we plan to focus on the adrenal and mesenteric resistance arteries because the ATjR is reciprocally regulated in these tissues by altered sodium intake. These studies will determine the post-transcriptional mechanisms of AT:R regulation in these reciprocally regulated tissues and how these mechanisms are impaired in aged rats in response to altered sodium intake.