Structure Of Beta Amyloid Fibrils

阿尔茨海默氏病与淀粉样β蛋白（Ab）的异常沉积有关，该蛋白在脑神经元内以原纤维形式存在。为了表征这些原纤维在不同pH条件下组装的方式，我们对合成的全长Ab肽以及具有截短序列的各种Ab肽进行了透射电子显微镜（TEM）和扫描透射电子显微镜（STEM）。负染色制备物的TEM图像提供了关于原纤维宽度和形态的信息，未染色制备物的STEM分析提供了每长度质量（MPL）的定量测定，从而提供了原纤维内β-折叠的数量。总之，结果揭示了从阿尔茨海默病相关的Ab肽生长的原纤维的结构如何取决于生长条件，包括蛋白质浓度和pH。MPL测量揭示了基本的flipzing单位或“原丝”的存在，其由明确定义的数量的交叉β片层组成。NIDDK的科学家正在使用EM水平上的原纤维形态变化与从NMR光谱学获得的肽骨架构象的原子水平信息相关联。STEM测量显示，在静止条件下通过接种单体的几代生长的Ab（1-40）原纤维由三层β折叠组成，而在搅拌介质中生长的原纤维含有双β折叠结构。

Alzheimer's disease is associated with the abnormal deposits of amyloid beta protein (Ab) that occurs as fibrils within the cerebral neuropil. To characterize the way in which these fibrils assemble under different pH conditions, we have performed transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) on synthetic full-length Ab peptides as well as various Ab peptides with truncated sequences. TEM images of negatively stained preparations provide information about the fibril width and morphology, and STEM analysis of unstained preparations provides a quantitative determination of the mass-per-length (MPL) and thus the numbers of beta-sheets within fibrils. Taken together the results reveal how the structure of fibrils grown from Alzheimer-related Ab peptides depend on the growth conditions, including protein concentration and pH. MPL measurements reveal the existence of fundamental fibrillizing units, or "protofilaments," consisting of well-defined numbers of cross-beta sheets. Variations in fibril morphology at the EM level are being used by scientists in NIDDK to correlate with atomic-level information about the peptide backbone conformation obtained from NMR spectroscopy. STEM measurements have shown that Ab(1-40) fibrils grown by several generations of seeding the monomer under quiescent conditions are composed of triple beta sheets whereas fibrils grown in agitated medium contain double beta sheet structures.