Spatial distributions of pericellular stiffness in natural extracellular matrices are dependent on cell-mediated proteolysis and contractility

Abstract

Bulk tissue stiffness has been correlated with regulation of cellular processes and conversely cells have been shown to remodel their pericellular tissue according to a complex feedback mechanism critical to development, homeostasis, and disease. However, bulk rheological methods mask the dynamics within a heterogeneous fibrous extracellular matrix (ECM) in the region proximal to a cell (pericellular region). Here, we use optical tweezers active microrheology (AMR) to probe the distribution of the complex material response function (α = α′ + α″, in units of μm/nN) within a type I collagen ECM, a biomaterial commonly used in tissue engineering. We discovered cells both elastically and plastically deformed the pericellular material. α′ is wildly heterogeneous, with 1/α′ values spanning three orders of magnitude around a single cell. This was observed in gels having a cell-free 1/α′ of approximately 0.5 nN/μm. We also found that inhibition of cell contractility instantaneously softens the pericellular space and reduces stiffness heterogeneity, suggesting the system was strain hardened and not only plastically remodeled. The remaining regions of high stiffness strongly suggest cellular remodeling of their surrounding matrix. To test this hypothesis, cells were incubated within the type I collagen gel for 24 hours in a media containing a broad-spectrum matrix metalloproteinase (MMP) inhibitor. While the pericellular material maintained stiffness asymmetry, stiffness magnitudes were reduced. Dual inhibition demonstrates that the combination of MMP activity and contractility is necessary to establish the pericellular stiffness landscape. This heterogeneity in stiffness suggests the distribution of pericellular stiffness, and not bulk stiffness alone, must be considered in the study of cell-ECM interactions and design of complex biomaterial scaffolds.

Degradation and Remodeling of Epitaxially Grown Collagen Fibrils

“…Cancer cells can also remodel collagen fibers during the process of migration. 4,8 This can either act to change mechanical properties close to the cell 23 or locally disorganize or organize collagen fibers, thus amplifying 2,14 or dampening the original aligned collagen fiber signal. This results in different degrees of contact guidance and different invasion or metastasis rates.…”

Early changes in the apparent diffusion coefficient and MMP‑9 expression of a cervical carcinoma U14 allograft model following irradiation

“…MMP-9 is a matrix protein involved in the degradation of ECM that is active within 24 h of the onset of stress-induced tissue injury, and may mediate collagen IV degradation in the BM and pericellular ECM, intercellular space dilatation and cellularity reduction (33,34). MMP-9 is activated by various stimuli, including irradiation and human papillomavirus (HPVs) in tumor tissues (15)(16).…”

Three-dimensional traction microscopy accounting for cell-induced matrix degradation

“…Most 3DTM approaches have assumed that the ECM has spatially uniform mechanical properties, which are typically measured by standard techniques such as bulk rheometry [18], indentation testing [19], and atomic force microscopy [20] prior to the encapsulation of the cell. However, since cells express enzymes (e.g., matrix metalloproteinase (MMP)) that can locally degrade the ECM [21,22,23,24,25], the assumption of uniform mechanical properties is questionable and is likely to introduce inaccuracies in estimating tractions. In particular, experimental studies have highlighted the importance of incorporating ECM heterogeneity in measuring stresses within the ECM [26,27].…”

“…In particular, experimental studies have highlighted the importance of incorporating ECM heterogeneity in measuring stresses within the ECM [26,27]. On the other hand, since the cell-induced changes are concentrated near the cells [21], it is challenging to directly measure the changes in mechanical properties using the techniques discussed above. Motivated by this, in this work we develop a computational approach that is able to accurately recover cellular tractions, as well as the mechanical heterogeneities in the ECM induced by cell remodeling.…”

“…In this manuscript we employ a compressible, hyperelastic Neo-Hookean model [40] to describe ECM behavior, while allowing the material parameters (i.e., the two Lamé parameters) to be nonuniform in the ECM. We treat the material parameters as unknowns in regions proximal to the cell surface, acknowledging that enzymes synthesized by cells may have degraded the ECM thereby changing its mechanical properties [21]. In addition, we assume that matrix properties are unaffected by cell remodeling far from the cell surface, where the material parameters are uniformly distributed and can be determined from standard mechanical tests.…”