Investigation of the mechanical properties of filamentous protein aggregates using optical tweezers.

蛋白质分子执行许多生命的基本功能，如果不是大部分的话。然而，在非常偶然的情况下，也只有在不正常的情况下，许多蛋白质才会聚集成含有数千个相同蛋白质分子拷贝的线状细丝，但实际上是一种无用的形式。这些绳状结构被称为纤维，通常是人类头发大小的一万分之一。纤维的形成过程一直是深入研究的焦点，因为它出现在许多健康疾病中，如阿尔茨海默病、帕金森病，最著名的是疯牛病。特别是，有人提出，纤维的机械性能可能在其中一些病理中发挥作用。可以肯定的是，对细丝生长过程缺乏完整的了解，这将是非常有价值的。以前在这一领域的工作已经用一系列能够达到纳米长度尺度的技术来探索蛋白质细丝的性质，包括电子显微镜和原子力显微镜(AFM)，这导致对蛋白质单元的结构安排有了很好的了解。还用原子力显微镜测量了一些纤维与固体载体结合后的弹性性质。然而，现有的方法受到一些限制，挑战是实现一个可能更接近人体条件的模型实验。特别是，我们考虑减少与合成表面的相互作用，并直接测量在生长过程中作用在单丝水平上的惊人的微小作用力。因此，我们建议使用光镊仪来研究蛋白质原纤维的生长和力学性质。弯曲光的程度超过悬浮液体的物体(例如，悬浮在水中的非常小的聚苯乙烯颗粒)可能会被激光束中存在的光子的压力所捕获。粒子上的典型捕获力非常小--在0.1到100皮牛顿之间，不到踢足球所需力的百万分之一--粒子的位置可以精确到几纳米。通过简单地控制激光光束，我们就可以移动这些被困物体。这就是光学镊子背后的原理。我们打算使用的设备将使我们能够控制生长的蛋白质原纤维的位置和方向，使其远离任何干扰表面。通过使纤维以特定的方式摆动，我们希望观察到一种被称为单臂游泳的现象，在这种现象中，纤维自己向前推进。此外，通过将不断生长的纤维指向墙壁，并用我们的激光推动自由端，我们将测试外部结构是否会阻止(停止)纤维生长。从生物物理学的角度来看，这项研究的新颖性在于研究了一个具有重要医学意义的系统。从物理角度来看，我们希望组装一系列不同尺寸和蛋白质组成的长丝将使我们能够开发出具有更广泛机械性能的长丝调色板。这些将被更普遍地用于研究半柔性长丝的普遍行为。

Protein molecules carry out many if not most of the essential functions of life. However very occasionally, and only under abnormal conditions, many proteins aggregate into linear filaments containing thousands of copies of the same protein molecule but in an essentially useless form. These rope-like structures, known as fibrils , are typically one-10,000th the size of a human hair. The process of fibril formation has been the focus of intensive research because of its occurrence in many health disorders such as Alzheimer's Disease, Parkinson's Disease and, most famously, Mad Cow Disease. In particular it has been suggested that the mechanical properties of fibrils might play a role in some of these pathologies. For certain, a complete understanding of the filament growth process is lacking and would be of great value. Previous work in this area has explored the properties of protein filaments with a range of techniques able to access nanometre length-scales, including electron microscopy and atomic force microscopy (AFM), and this has led to a good understanding of the structural arrangement of the protein units. The elastic properties of some filaments have also been measured with AFM after binding to a solid support. However, the existing approaches suffer from a number of limitations and the challenge is to achieve a model experiment that might mimic conditions in the human body more closely. In particular we have in mind to reduce the interaction with synthetic surfaces and to measure directly the phenomenally tiny forces that act on the single filament level during growth. We therefore propose to study the growth and mechanical properties of protein fibrils using an optical tweezer apparatus. Objects that bend light more than the liquid in which they are suspended (for instance, a very small polystyrene particle suspended in water) can be trapped by the pressure of the photons present in a laser beam. The typical trapping forces on a particle are exceptionally small - between 0.1 and 100 piconewtons, less than one hundredth of a millionth of a millionth of the force required to kick a football - and the position of the particle can be determined down to a few nanometers. By simply steering the laser beam we can then move these trapped objects. This is the principle behind optical tweezers. The equipment we intend to use will enable us to control the position and direction of a growing protein fibril, keeping it away from any interfering surface. By causing a fibril to wiggle in a defined way, we hope to observe a phenomenon known as one-armed swimming , where the filament propels itself along. Furthermore, by pointing a growing fibril into a wall and pushing the free end with our laser beam, we will test whether an external structure will stop ( stall ) the fibril growth. From the biophysical point of view the novelty of this study lies in investigating a medically important system. From the physical point of view, we hope that assembling a range of filaments with different sizes and protein composition will enable us to develop a palette of filaments with a wider range of mechanical properties. These will be used more generally to investigate the universal behaviour of semi-flexible filaments.