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

蛋白质分子执行生命的许多基本功能，如果不是大部分的话。然而，在非常偶然的情况下，只有在异常情况下，许多蛋白质聚集成线状细丝，其中包含数千个相同蛋白质分子的副本，但基本上是无用的形式。这些绳状结构，被称为原纤维，通常是人类头发大小的万分之一。纤维的形成过程一直是深入研究的焦点，因为它在许多健康疾病中都有发生，如阿尔茨海默病、帕金森病和最著名的疯牛病。特别是，有人认为原纤维的机械特性可能在这些病理中起作用。当然，对长丝生长过程的完整了解是缺乏的，这将是很有价值的。在这一领域之前的工作已经用一系列能够达到纳米长度尺度的技术，包括电子显微镜和原子力显微镜（AFM），探索了蛋白质细丝的特性，这导致了对蛋白质单元结构排列的很好的理解。用原子力显微镜测量了一些细丝与固体支架结合后的弹性性能。然而，现有的方法受到许多限制，挑战是实现一个可能更接近地模拟人体条件的模型实验。特别是，我们想要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微小力。因此，我们建议使用光学镊子装置来研究蛋白质原纤维的生长和力学特性。如果物体比其悬浮的液体更能弯曲光线（例如，悬浮在水中的一个非常小的聚苯乙烯颗粒），则可以被激光束中存在的光子压力所捕获。典型的粒子捕获力非常小——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束，我们就可以移动这些被困住的物体。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，纤维会推动自己向前移动。此外，通过将生长中的纤维指向墙壁并用激光束推动其自由端，我们将测试外部结构是否会阻止纤维的生长。从生物物理学的角度来看，这项研究的新颖之处在于研究了一个医学上重要的系统。从物理的角度来看，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。这些将更广泛地用于研究半柔性细丝的普遍行为。

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.