Structure and Function of Serum Amyloid A in Health and Disease

摘要 这项研究的重点是血清淀粉样蛋白A（SAA）的正常和病理作用，SAA是一种神秘的生物标志物， 炎症和AA淀粉样变性的蛋白质前体，慢性炎症的危及生命的并发症。 SAA是一种小的可溶性内在无序蛋白质，其在细胞中迅速且显著地增加高达1,000倍。 炎症、感染或损伤后的血浆。这种急剧但短暂的增长对生存的好处是 目前还不清楚，SAA在免疫应答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承诺方面 对SAA生物合成的影响强烈支持SAA-脂质相互作用的重要性。我们的新结构模型表明 SAA如何结合脂质和细胞受体，从而改变脂质代谢。初步生化研究迫使我们 提出SAA在清除损伤部位细胞膜碎片中的重要新作用。我们强大 生物物理和生物化学方法将确定SAA隔离磷脂的机制 和其他脂质，以促进它们的分解和安全地从损伤部位清除， 这是组织愈合的先决条件。我们还将确定各种脂质和其他因素 通过SAA调节淀粉样蛋白的形成，从而帮助减缓或阻断这种致病过程。 三个互补的具体目标发展了SAA在免疫应答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，结构和细胞为基础的方法来建立 SAA与其功能配体、脂质和CD 36细胞受体相互作用的分子基础。我们将 测试我们的新结构模型，表明SAA通过一个独特的非极性面结合脂质， 蛋白质优先结合高度弯曲的脂蛋白或从头形成它们以隔离脂质。我们将 确定脂质上的SAA构象并定量其与CD 36的结合。目标2将探索一种新的协同作用 SAA和分泌型磷脂酶A2（sPLA 2）之间， 与SAA同时存在。初步研究迫使我们提出SAA的一种新的有益功能 在溶解磷脂及其水解产物以产生sPLA 2的底物并除去其 产品.结果将确立SAA迄今为止未知的重要原始作用。目标3将决定效果 各种脂质、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，血浆脂质如三酰甘油调节SAA从其主要血浆载体的释放， 高密度脂蛋白，并最终形成淀粉样蛋白。结果将有助于确定是否降脂 治疗AA淀粉样变性有希望。这项研究的完成将为 这种神秘蛋白质为其在免疫应答和脂质稳态中的作用提供了分子基础， 帮助找到急需的新疗法或重新利用现有疗法来治疗危及生命的人类疾病。

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.