Department of Biomedical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts, USA.
Biophys J. 2013 Jul 2;105(1):11-20. doi: 10.1016/j.bpj.2013.05.032.
Recent observations suggest that cells on fibrous extracellular matrix materials sense mechanical signals over much larger distances than they do on linearly elastic synthetic materials. In this work, we systematically investigate the distance fibroblasts can sense a rigid boundary through fibrous gels by quantifying the spread areas of human lung fibroblasts and 3T3 fibroblasts cultured on sloped collagen and fibrin gels. The cell areas gradually decrease as gel thickness increases from 0 to 150 μm, with characteristic sensing distances of >65 μm below fibrin and collagen gels, and spreading affected on gels as thick as 150 μm. These results demonstrate that fibroblasts sense deeper into collagen and fibrin gels than they do into polyacrylamide gels, with the latter exhibiting characteristic sensing distances of <5 μm. We apply finite-element analysis to explore the role of strain stiffening, a characteristic mechanical property of collagen and fibrin that is not observed in polyacrylamide, in facilitating mechanosensing over long distances. Our analysis shows that the effective stiffness of both linear and nonlinear materials sharply increases once the thickness is reduced below 5 μm, with only a slight enhancement in sensitivity to depth for the nonlinear material at very low thickness and high applied traction. Multiscale simulations with a simplified geometry predict changes in fiber alignment deep into the gel and a large increase in effective stiffness with a decrease in substrate thickness that is not predicted by nonlinear elasticity. These results suggest that the observed cell-spreading response to gel thickness is not explained by the nonlinear strain-stiffening behavior of the material alone and is likely due to the fibrous nature of the proteins.
最近的观察表明，细胞在纤维状细胞外基质材料上感知机械信号的距离比在具有线性弹性的合成材料上感知的距离要大得多。在这项工作中，我们通过定量研究在倾斜的胶原蛋白和纤维蛋白凝胶上培养的人肺成纤维细胞和 3T3 成纤维细胞的扩展面积，系统地研究了成纤维细胞通过纤维凝胶感知刚性边界的距离。随着凝胶厚度从 0 增加到 150μm，细胞面积逐渐减小，在纤维蛋白和胶原蛋白凝胶下的特征感知距离大于 65μm，而在厚度为 150μm 的凝胶上扩散受到影响。这些结果表明，成纤维细胞在胶原蛋白和纤维蛋白凝胶中感知的深度比在聚丙烯酰胺凝胶中感知的深度要深，后者在特征感知距离上的<5μm。我们应用有限元分析来探索应变硬化的作用，应变硬化是胶原蛋白和纤维蛋白的一种力学特性，在聚丙烯酰胺中观察不到，它有助于远距离的机械感知。我们的分析表明，一旦厚度降低到 5μm 以下，线性和非线性材料的有效刚度都会急剧增加，只有在非常低的厚度和高施加牵引力下，非线性材料对深度的灵敏度才有轻微增强。简化几何形状的多尺度模拟预测纤维在凝胶中的排列会发生变化，并且随着基质厚度的减小，有效刚度会大幅增加，而这种变化无法用非线性弹性来预测。这些结果表明，观察到的细胞对凝胶厚度的扩展反应不能仅用材料的非线性应变硬化行为来解释，很可能是由于蛋白质的纤维性质。
