Macrophages use filopodia to withdraw particles toward the cell body for phagocytosis. This can require substantial forces, which the cell generates after bio-mechanical stimuli are transmitted to the filopodium. Adaptation mechanisms to mechanical stimuli are essential for cells, but can a cell iteratively improve filopodia pulling? If so, the underlying mechanic adaptation principles organized on the protein level are unclear. Here, we tackle this problem using optically trapped 1 μm beads, which we tracked interferometrically at 1 MHz during connection to the tips of dorsal filopodia of macrophages. We observe repetitive failures while the filopodium tries to pull the bead out of the optical trap. Analyses of mean bead motions and position fluctuations on the nano-meter and microsecond scale indicate mechanical ruptures caused by a force-dependent actin-membrane connection. We found that beads are retracted three times slower under any load between 5 and 40 pN relative to the no-load transport, which has the same speed as the actin retrograde flow obtained from fluorescent speckle tracking. From this duty ratio of pulling velocities, we estimated a continuous on/off binding with τ_off = 2⋅τ_on, with measured off times τ_off = 0.1–0.5 s. Remarkably, we see a gradual increase of filopodia pulling forces from 10 to 30 pN over time and after failures, which points toward an unknown adaptation mechanism. Additionally, we see that the attachment strength and friction between the bead and filopodium tip increases under load and over time. All observations are typical for catch-bond proteins such as integrin-talin complexes. We present a mechanistic picture of adaptive mechanotransduction, which formed by the help of mathematical models for repetitive tip ruptures and reconnections. The analytic mathematical model and the stochastic computer simulations, both based on catch-bond lifetimes, confirmed our measurements. Such catch-bond characteristics could also be important for other immune cells taking up counteracting pathogens.

巨噬细胞利用丝状伪足将颗粒拉向细胞体进行吞噬作用。这可能需要大量的力，这是细胞在生物力学刺激传递到丝足后产生的。对机械刺激的适应机制对于细胞来说至关重要，但是细胞可以迭代地改善丝状伪足的牵拉吗？如果是这样，那么在蛋白质水平上组织的潜在机械适应原理尚不清楚。在这里，我们使用光学捕获的 1 μm 珠子解决了这个问题，在连接到巨噬细胞背侧丝状伪足的尖端期间，我们以 1 MHz 干涉跟踪了该珠子。当丝状足试图将珠子从光阱中拉出时，我们观察到重复的失败。对纳米和微秒尺度上的平均珠运动和位置波动的分析表明，由力依赖性肌动蛋白-膜连接引起的机械破裂。我们发现，相对于空载运输，在 5 至 40 pN 之间的任何负载下，珠子的缩回速度要慢三倍，空载运输的速度与从荧光斑点跟踪获得的肌动蛋白逆行流的速度相同。根据拉速度的占空比，我们估计了连续的开/关结合，τ _off  = 2⋅τ _on，测量的关闭时间 τ _off  = 0.1–0.5 s。值得注意的是，我们看到丝状伪足拉力随着时间的推移和失败后从 10 pN 逐渐增加到 30 pN，这表明存在未知的适应机制。此外，我们发现珠子和丝状足尖端之间的附着强度和摩擦力在负载下和随着时间的推移而增加。所有观察结果都是捕获结合蛋白（例如整联蛋白-talin 复合物）的典型观察结果。我们提出了自适应机械传导的机制图，它是在重复尖端破裂和重新连接的数学模型的帮助下形成的。分析数学模型和随机计算机模拟均基于捕获键寿命，证实了我们的测量结果。这种捕捉结合的特性对于其他抵抗病原体的免疫细胞也很重要。