Navigation of sperm cells in scalar turbulence: Theory of sperm chemotaxis in turbulent flow and its adaptation to dynamic concentration and velocity gradients

标量湍流中精子细胞的导航：湍流中精子趋化性理论及其对动态浓度和速度梯度的适应

• 批准号: 391963627
• 负责人: Professor Dr. Benjamin M. Friedrich
• 金额: --
• 依托单位: Exzellenzcluster Physics of Life (PoL)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/391963627?language=en
• 关键词: Navigation; sperm; cells; scalar; turbulence

Chemotaxis - the navigation of biological cells guided by chemical gradients - is crucial for bacterial foraging, immune responses, and guidance of sperm cells to the egg during fertilization. Cellular navigation represents a model system for the physics of autonomous motility at the microscale and its control by sensory cues. Previous work focused predominantly on idealized conditions of perfect chemical gradients. Yet, natural environments are characterized by perturbations, which distort extracellular chemical gradients. A prototypical example are turbulent flows of the ocean.In the model species of marine invertebrates, sperm and egg cells are directly spawned into open water, where sperm cells employ a dedicated gradient-sensing algorithm along helical paths to steer up concentration gradients of signaling molecules released by the egg. Small-scale turbulence distorts these concentration gradients and convects swimming cells. We propose to develop a theory of sperm chemotaxis in turbulent flow conditions, by combining an existing simulation framework of helical chemotaxis with hydrodynamic computations of turbulent advection, which is novel. Using this model system, we will target a gap in knowledge between (i) turbulent stirring of passive particles and (ii) chemotaxis of actively swimming cells studied previously in the absence of external flow. Thereby, we will address a fundamental and largely unexplored question: how biological cells navigate in dynamic and disordered environments. We will elucidate the competition between positive and negative effects of turbulence, i.e. faster establishment of concentration gradients and random distortion of these gradients. By this, we will confirm and explain the existence of an optimal turbulence strength that maximizes the probability of sperm-egg encounters. On a finer scale, our preliminary simulations suggest that small-scale turbulence creates extended filaments of high concentration along which sperm cell can 'surf' towards the egg. We will understand the mechanism of 'filament surfing' in terms of chemotactic steering and rotation by local shear flow. Next, we will study trapping of sperm cells in local concentration maxima and stochastic transitions between such maxima, and its dependence on the spatial and temporal resolution of gradient sensing. Thereby, we expect genuine physical insight into the exploration/exploitation trade-off in the context of cellular navigation. Previously, we pioneered the theory of helical chemotaxis, which represents one out of three basic gradient-sensing strategies of biological cells. Additionally, our group established a solid competence in hydrodynamic simulations. The proposed project will combine these two research fields into a new direction. Thereby, we will provide a concise understanding of cellular navigation in dynamic and disordered external fields in an important model system.

趋化性——生物细胞在化学梯度引导下的导航——对于细菌觅食、免疫反应和受精过程中精子细胞到卵子的引导至关重要。细胞导航代表了一个模型系统的自主运动的物理在微观尺度和其控制的感官线索。以前的工作主要集中在完美化学梯度的理想条件上。然而，自然环境的特点是扰动，扭曲细胞外的化学梯度。一个典型的例子是海洋的湍流。在海洋无脊椎动物的模式物种中，精子和卵细胞直接在开放水域中繁殖，精子细胞沿着螺旋路径使用专用的梯度传感算法来引导卵子释放的信号分子的浓度梯度。小规模的湍流扭曲了这些浓度梯度，使游动细胞对流。我们建议将现有的螺旋趋化模拟框架与湍流平流的流体动力学计算相结合，发展湍流条件下精子趋化理论，这是新颖的。使用该模型系统，我们将针对(i)被动颗粒的湍流搅拌和（ii）之前在没有外部流动的情况下研究的主动游动细胞的趋化性之间的知识差距。因此，我们将解决一个基本的和很大程度上未被探索的问题：生物细胞如何在动态和无序的环境中导航。我们将阐明湍流的积极和消极影响之间的竞争，即浓度梯度的更快建立和这些梯度的随机扭曲。通过这一点，我们将确认并解释存在一个最佳湍流强度，使精子-卵子相遇的概率最大化。在更精细的尺度上，我们的初步模拟表明，小规模的湍流产生了高浓度的延伸细丝，精子细胞可以沿着这些细丝“冲浪”走向卵子。我们将从趋化转向和局部剪切流旋转的角度来理解“纤维冲浪”的机制。接下来，我们将研究精子细胞在局部浓度最大值处的捕获和最大值之间的随机转换，以及它对梯度传感时空分辨率的依赖。因此，我们期望在蜂窝导航的背景下对探索/开发权衡进行真正的物理洞察。此前，我们开创了螺旋趋化理论，它代表了生物细胞的三种基本梯度传感策略之一。此外，我们的小组在流体动力学模拟方面建立了坚实的能力。本项目将把这两个研究领域结合起来，形成一个新的方向。因此，我们将提供一个简明的理解在一个重要的模型系统的动态和无序外场的细胞导航。