Mapping Spatial Distributions of Pericellular Stiffness in a Naturally Derived Extracellular Matrix

Abstract

Many cell types in vivo are surrounded by an extracellular matrix (ECM), the molecular and mechanical scaffolding of natural tissues. Quantifying the mechanical interactions between the cell and its ECM both spatially and temporally, at a scale relevant to the interaction, is imperative to study how cells are regulated in physiological and pathological processes. Here we use optical tweezers based active microrheology (AMR) to measure the distribution of pericellular stiffness surrounding isolated dermal fibroblasts embedded within collagen gels and observe important new insights into how cells modulate their mechanical microenvironment in a contractility and matrix metalloproteinase-dependent manner. The pericellular stiffness surrounding isolated DFs was measured in four conditions: normal media (control), ROCK inhibitor (Y27632) for 1 hour, broad spectrum MMP inhibitor (BB94) for 24 hours, or a combination of Y27632 and BB94 for 1 and 24 hours respectively. With respect to these results, cooperation may be necessary between both MMP activity and cellular contractility in order to create a normal pericellular mechanical topography within the complex material of a type I collagen system. There is a growing body of correlations between bulk ECM stiffness and cell phenotype in tissue models, which tend to be performed with a set of ECMs, each with unique but homogenous bulk stiffness, with shear moduli spanning 30 to ∼1000 Pa. If we estimate shear moduli values from observed α values (via GSER, which inherently introduces error), then remarkably this same range was observed by AMR around single cells in our study. This begs the question: which stiffness value is important? We speculate that no single value of stiffness guides cells, rather it is the evolution and distribution of stiffness that must be accounted for within mechanics focused cell-ECM hypothesis testing.