Structure and Function of Serum Amyloid A in Health and Disease

摘要 本研究的重点是血清淀粉样蛋白A(SAA)的正常和病理作用，这是一种神秘的生物标志物。 炎症和AA淀粉样变性的蛋白前体，这是慢性炎症的一种威胁生命的并发症。 SAA是一种小的、可溶的、天生无序的蛋白质，在 炎症、感染或受伤后的血浆。这种急剧但短暂的增长对生存的好处 目前还不清楚，SAA在免疫反应中的有益作用也不清楚。然而，进化上保守的 SAA的结构、其亲脂性以及肝脏和局部组织的快速和主要摄取 SAA的生物合成有力地支持了SAA-脂质相互作用的重要性。我们的新结构模型表明 SAA如何结合脂类和细胞受体，从而改变脂类代谢。初步的生物化学研究迫使我们 提出SAA在清除损伤部位的细胞膜碎片方面的重要新作用。我们强大的 生物物理和生化方法将确定SAA隔离磷脂的机制 和其他来自细胞膜的脂类，以促进它们的分解和从损伤部位安全地清除 是组织愈合的先决条件的过程。我们还将确定各种血脂和其他因素如何 通过SAA调节淀粉样蛋白的形成，从而帮助减速或阻止这一致病过程。 三个互补的特定靶向发展了SAA在免疫反应和免疫反应中作用的基本新概念 淀粉样蛋白形成。目标1将使用一系列生物物理、结构和基于细胞的方法来建立 SAA与其功能配体、脂质和CD36细胞受体相互作用的分子基础。我们会 测试我们的新结构模型，表明SAA通过一个独特的非极面结合脂类，其形状定义 蛋白质倾向于与高度弯曲的脂蛋白结合或形成新的脂蛋白以隔离脂类。我们会 测定脂质表面SAA构象，并定量测定其与CD36的结合。目标2将探索一种新的协同效应 在SAA和分泌型磷脂酶A2(SPLA2)之间，一种急性期血浆蛋白上调 与SAA同时发生炎症反应。初步研究迫使我们提出了SAA的一种新的有益功能 在溶解磷脂及其水解物以生成sPLA2底物并去除其 产品。这一结果将确立迄今为止未知的SAA的重要原始作用。目标3将决定效果 各种脂质及其降解产物、pH和硫酸肝素对SAA淀粉样蛋白形成的影响。我们还将测试 我们的新想法是，三酰甘油等血浆脂类调节其主要血浆载体的SAA释放， 高密度脂蛋白，并最终形成淀粉样蛋白。这一结果将有助于确定降脂 治疗再生障碍性淀粉样变性的方法前景看好。这项研究的完成将为 这种神秘的蛋白质，为其在免疫反应和脂质平衡中的作用提供了分子基础，以及 帮助寻找急需的新疗法或改变现有疗法的用途，以治疗一种威胁生命的人类疾病。

Abstract This research is focused on normal and pathologic actions of serum amyloid A (SAA), an enigmatic biomarker of inflammation and a protein precursor of AA amyloidosis, a life-threatening complication of chronic inflammation. SAA is a small soluble intrinsically disordered protein that increases rapidly and dramatically up to 1,000-fold in plasma following inflammation, infection or injury. The advantage for survival of this steep but transient increase is unclear, and the beneficial role of SAA in immune response is obscure. However, evolutionarily conserved aspects of the SAA structure, its lipophilic character, and the rapid and major commitment of liver and local tissues to SAA biosynthesis strongly support the importance of SAA-lipid interactions. Our new structural model suggests how SAA binds lipids and cell receptors, thereby rerouting lipid metabolism. Pilot biochemical studies compel us to propose a vital new role for SAA in clearing cellular membrane debris from the injured sites. Our powerful biophysical and biochemical approach will determine the mechanism via which SAA sequesters phospholipids and other lipids from cell membranes to facilitate their breakdown and safe removal from the injured sites in a process that is prerequisite for tissue healing. We will also determine how various lipids and other factors modulate amyloid formation by SAA, and thereby help decelerate or block this pathogenic process. Three complementary specific aims develop new concepts fundamental to SAA action in immune response and amyloid formation. Aim 1 will use an array of biophysical, structural and cell-based methods to establish molecular underpinnings for SAA interactions with its functional ligands, lipids and CD36 cell receptor. We will test our new structural model suggesting that SAA binds lipids via a unique apolar face whose shape defines protein's preference for binding to highly curved lipoproteins or forming them de novo to sequester lipids. We will determine the SAA conformation on the lipid and quantify its binding to CD36. Aim 2 will probe a novel synergy between SAA and secretory phospholipase A2 (sPLA2), an acute-phase plasma protein upregulated simultaneously with SAA in inflammation. Pilot studies compel us to propose a novel beneficial function of SAA in solubilizing phospholipids and their hydrolytic products to generate substrates for sPLA2 and to remove its products. The results will establish the hitherto unknown vital primordial role of SAA. Aim 3 will determine effects of various lipids, their degradation products, pH and heparin sulfate on SAA amyloid formation. We will also test our new idea that plasma lipids such as triacylglycerol modulate the SAA release from its major plasma carrier, high-density lipoprotein, and ultimately form amyloid. The results will help determine whether lipid-lowering therapies hold promise for treating AA amyloidosis. Completion of this research will establish raison d'etre for this enigmatic protein, provide a molecular basis for its action in immune response and lipid homeostasis, and help find much-needed new therapies or repurpose existing ones to treat a life-threatening human disease.