Signalling role(s) for the unconventional RdgB proteins: are they lipid sensors for phosphatidic acid ?

高血压导致心脏体积增大（心脏肥厚），是心力衰竭发展的主要危险因素。五分之一的人死于这种疾病。血管紧张素II是一种刺激心脏肥厚的激素，它通过与血管紧张素II I型（ATI）受体结合起作用。一个复杂的细胞内信号传导程序被启动来刺激肥大，最近发现了一种名为ATRAP的新蛋白，它可以防止血管紧张素II的影响。ATRAP被发现是因为它与ATI受体结合，但ATRAP如何抑制心脏肥厚尚不清楚。我们在ATRAP和脂质结合蛋白rdgb - β之间发现了意想不到的联系。我们建议在脂质信号传导的背景下，通过产生“信号脂质”磷脂酸（PA）的磷脂酶来定义ATRAP和rdgb - β这两种蛋白质之间的联系。我们将建立这种蛋白质-脂质网络在血管紧张素II信号传导过程中如何运作。当血管紧张素II与受体结合时，磷脂酶的活性受到刺激。rdgb - β是未表征的，我们发现它具有不寻常的脂质结合特性-它与PA结合。我们的概念是rdgb - β隔离“PA”信号，从而抑制信号级联，从而抑制心脏肥厚。我们将研究rdgb - β如何结合‘PA’并处理它。因为ATRAP结合了rdgb - β，我们认为在两个膜之间形成了一个“桥梁”。这使得“PA”从信号发生的质膜上移除，并被送到脂质被重新用于制造新脂质的隔室。为了形成桥，rdgb - β必须与一个膜上的ATRAP和另一个膜上的其他蛋白质相互作用。因此，我们将通过rdgb - β作为诱饵来识别这些蛋白质，以寻找新的蛋白质。我们还将通过增加或减少细胞中的蛋白质水平来研究rdgb - β和ATRAP的重要性。这将告诉我们血管紧张素II信号是如何受到影响的。如果rdgb - β加强了ATRAP对血管紧张素II信号传导的抑制，这将提供强有力的证据，证明ATRAP使用的分子机制是参与信号脂质PA的去除。为了进一步测试该模型，我们将删除模型生物（果蝇）中的rdgb - β基因，并与我们在印度班加罗尔的项目合作伙伴合作检查表型。为了确定PA与rdgb - β结合的重要性，我们将制造不能结合PA的突变蛋白。这些突变体将被检查以挽救苍蝇缺陷。rdgb - β和ATRAP之间的相互作用以及PA与rdgb - β的结合可以提供ATRAP如何抑制血管紧张素II信号功能的分子解释，因此可以为心血管疾病的干预提供新的治疗靶点。在临床上，通过抑制血管紧张素II信号传导的ACE抑制剂阻止血管紧张素II的产生或阻止血管紧张素II与其受体结合的药物被用于治疗高血压。由于大多数药物都有副作用，因此经常使用针对不同系统的药物组合。因此，提出的研究很可能导致不同的分子靶点，可以提供更有效的治疗。了解内源性血管紧张素II信号抑制剂ATRAP的功能，可能为药物靶向治疗提供新的策略。由于ATRAP与rdgb - β相互作用，靶向rdgb - β的可能性可能提供一个独特的机会，产生一类新的药物，这些药物可能基于结合rdgb - β脂质结合口袋中的小疏水分子。由于高血压是折磨人类的最常见疾病之一，这些药物带来的好处是巨大的。

High blood pressure causes an increase in the size of the heart (cardiac hypertrophy) and is a major risk factor for the development of heart failure. One in five people die from this condition. Angiotensin II is a hormone that stimulates cardiac hypertrophy and it functions by binding to the Angiotensin II type I (ATI) receptor. A complex programme of intracellular signalling is initiated to stimulate hypertrophy and a new protein called ATRAP has been recently identified that protects against the effects of Angiotensin II. ATRAP was discovered because it binds to the ATI receptor but how ATRAP suppresses cardiac hypertrophy is not known. We have made an unexpected connection between ATRAP and a lipid binding protein, RdgB-beta. We propose to define the connection between the two proteins, ATRAP and RdgB-beta in the context of lipid signalling via enzymes called phospholipases that produce the 'signalling lipid', phosphatidic acid (PA). We will establish how this protein-lipid network operates during Angiotensin II signalling. The activity of phospholipases is stimulated when Angiotensin II binds to the receptor. RdgB-beta is uncharacterised and we have discovered that it has unusual lipid binding properties - it binds PA. Our concept is that RdgB-beta sequesters the 'PA' signal and therefore restrains the signalling cascade resulting in inhibition of cardiac hypertrophy. We will examine how RdgB-beta binds 'PA' and disposes of it. Because ATRAP binds RdgB-beta we think that a 'bridge' between two membranes is formed. This allows the 'PA' to be removed from the plasma membrane where signalling occurs and sent to the compartment where lipids are re-used for making new lipids. To form the bridge, RdgB-beta has to interact with ATRAP on one membrane and other proteins on the opposite membrane. We will therefore identify these proteins by using RdgB-beta as bait to fish for new proteins.We will also study the importance of RdgB-beta and ATRAP by increasing or decreasing the protein levels in the cells. This will inform us on how Angiotensin II signalling is affected. If RdgB-beta reinforces the restraint put by ATRAP on Angiotensin II signalling, this will provide strong evidence that the molecular mechanism used by ATRAP is to participate in the removal of the signalling lipid, PA. To further test the model, we will delete the gene for RdgB-beta in a model organism (Drosophila) and examine the phenotype in collaboration with our project partner in Bangalore, India. To determine the importance of PA binding to RdgB-beta, we will make mutant proteins that cannot bind PA. These mutants will be examined for rescue of the fly defect. The interaction between RdgB-beta and ATRAP together with the binding of PA to RdgB-beta could provide the molecular explanation of how ATRAP is able to suppress the function of Angiotensin II signalling and could therefore offer a novel therapeutic target for intervention in cardiovascular diseases. In the clinic, inhibition of Angiotensin II signalling by ACE inhibitors that prevents the production of Angiotensin II or drugs that prevent binding of Angiotensin II to its receptor are used for treatment for hypertension. Since most drugs have side-effects, drug combination that targets different systems are often used. Therefore the proposed research could well lead to a different molecular target which could provide a more effective treatment. Understanding how the endogenous inhibitor of Angiotensin II signalling, ATRAP, functions, may provide new strategies for drug targeting. Because ATRAP interacts with RdgB-beta, the possibility that targeting RdgB-beta may provide a unique opportunity to generate a new class of drugs that could be based on binding small hydrophobic molecules in the lipid binding pocket of RdgB-beta. The benefit derived from such drugs is huge as high blood pressure is one of the most common diseases that afflict humans.