Random peptide sequences binding amyloid monoclonal antibodies

Antibodies against Aß amyloid are indispensable research tools and
potential therapeutics for Alzheimer’s Disease, but display several
unusual properties, such as specificity for aggregated forms of the
peptide, ability to distinguish polymorphic aggregate structures and
ability to recognize generic aggregation-related epitopes formed by
unrelated amyloid sequences. Understanding the mechanisms underlying these
unusual properties of anti-amyloid antibodies and the structures of their
corresponding epitopes is crucial for the understanding why antibodies
display different therapeutic activities and for the development of more
effective therapeutic agents. Here we employed a novel “epitomic” approach
to map the fine structure of the epitopes of 28 monoclonal antibodies
against amyloid-beta using immunoselection of random sequences from a
phage display library, deep sequencing and pattern analysis to define the
critical sequence elements recognized by the antibodies. Although most of
the antibodies map to major linear epitopes in the amino terminal 1-14
residues of Aß, the antibodies display differences in the target sequence
residues that are critical for binding and in their individual preferences
for non-target residues, indicating that the antibodies bind to
alternative conformations of the sequence by different mechanisms.
Epitomic analysis also identifies more discontinuous, non-overlapping
sequence Aß segments than peptide array approaches that may constitute the
conformational epitopes that underlie the aggregation specificity of
antibodies. Aggregation specific antibodies recognize sequences that
display a significantly higher predicted propensity for forming amyloid
than antibodies that recognize monomer, indicating that the ability of
random sequences to aggregate into amyloid is a critical element of their
binding mechanism.

抗Aβ淀粉样蛋白（Aß）抗体是阿尔茨海默病（Alzheimer’s Disease）研究中不可或缺的工具，同时也是潜在的治疗候选药物，但这类抗体展现出若干特殊性质：对Aβ肽的聚集形式具有特异性、能够区分多态性聚集结构，以及可识别由无关淀粉样蛋白序列形成的通用聚集相关表位。阐明抗淀粉样蛋白抗体的这些特殊特性的作用机制，以及其对应表位的结构，对于理解为何不同抗体展现出各异的治疗活性，以及开发更高效的治疗制剂至关重要。本研究采用一种新型的“表位组学（epitomic）”方法，通过对噬菌体展示文库中的随机序列开展免疫筛选、深度测序与模式分析，明确28株抗β淀粉样蛋白单克隆抗体所识别的关键序列元件，进而绘制其表位的精细结构。尽管多数抗体结合于Aβ氨基末端1-14位残基的主要线性表位，但这些抗体在结合所必需的靶序列残基以及对非靶序列残基的偏好性上均存在差异，这表明不同抗体通过不同机制结合该序列的不同构象。与肽阵列技术相比，表位组学分析鉴定出了更多不连续、非重叠的Aβ序列片段，这些片段或可构成支撑抗体聚集特异性的构象型表位。聚集特异性抗体所识别的序列，其预测的淀粉样形成倾向显著高于识别单体的抗体，这表明随机序列聚集为淀粉样蛋白的能力是其结合机制中的关键要素。