Micromechanics of cellularized biopolymer networks

Abstract

Collagen gels are widely used in experiments on cell mechanics because they mimic the extracellular matrix in physiological conditions. Collagen gels are often characterized by their bulk rheology; however, variations in the collagen fiber microstructure and cell adhesion forces cause the mechanical properties to be inhomogeneous at the cellular scale. We study the mechanics of type I collagen on the scale of tens to hundreds of microns by using holographic optical tweezers to apply pN forces to microparticles embedded in the collagen fiber network. We find that in response to optical forces, particle displacements are inhomogeneous, anisotropic, and asymmetric. Gels prepared at 21°C and 37°C show qualitative difference in their micromechanical characteristics. We also demonstrate that contracting cells remodel the micromechanics of their surrounding extracellular matrix in a strainand distance-dependent manner. To further understand the micromechanics of cellularized extracellular matrix, we have constructed a computational model which reproduces the main experiment findings.micromechanics | collagen | fiber network T he mechanical properties of the extracellular matrix (ECM) play a central role in developmental biology (1), tissue homeostasis, and remodeling (2). Alteration of the ECM elasticity is a signature of many diseases such as pulmonary and atrial fibrosis, Ehlers-Danlos syndrome, and infantile cortical hyperostosis (3). The mechanical cues from the ECM also strongly correlate with the clinical prognosis of various types of cancers (4).In recent years, many studies have shown that to mimic the physiological conditions in vitro, mechanical cues from a truly 3D ECM are necessary (5). Type I collagen gel has gained popularity as arguably the most used in vitro model of a 3D ECM (2). As the most abundant protein in animal tissue and accounting for 25% of the human whole-body protein content (6), type I collagen is the major component of the ECM in skin, tendon, and organs. Despite its lack of biochemical complexity compared with live tissue, reconstituted type I collagen gel has been successfully used to provide mechanistic insights into processes such as morphogenesis (7), wound repair (8), and cell migration (9). In particular, the rheology and especially the rigidity of collagen gel have been shown to tune the growth and migratory phenotypes of cancer cells in vitro (10, 11).Structurally, collagen gels are formed by fibrous networks and typically have pore sizes of a few to tens of microns (12)(13)(14). The typical size of these structural discontinuities is comparable to the size of cells and is much larger than cell-ECM adhesion complexes (15,16). It is therefore expected that a cell senses the micromechanical properties of its surrounding matrix, rather than the macroscopic rheology of the ECM (16,17). Although many studies have focused on the (nonlinear) bulk rheology of empty and cellularized collagen ECM (18-22), the micromechanics of the porous biopolymer network is largely unexplored, presumably...

On the mechanism of long-range orientational order of fibroblasts

“…For fibroblasts, this idea is supported by the fact that cells secrete materials for ECM, such as collagen (16,17). Furthermore, as fibroblasts move, they contract and align ECM fibers depending on their direction of motion (18,19). Recent experimental results also show that a fiber network under strain over a threshold time can change its structural and A B properties in a nonreversible fashion (21).…”

“…Therefore, we further test models in which nonlocal interactions between fibroblasts are introduced. Biologically, these models are motivated by the observations (16)(17)(18)(19) that fibroblasts are able to deposit and reorganize fibril materials around them. Using this model, we investigated whether coupling between cells and their mechanical environment can recapitulate patterns of fibroblasts both in vitro and in vivo.…”

Microfabricated tissues for investigating traction forces involved in cell migration and tissue morphogenesis

“…Limitations also arise from the difficulty associated with creating systems that recapitulate the in vivo microenvironment and that have well-defined mechanical properties. For example, a combination of spatial heterogeneities and large pore sizes in collagen gels can result in a breakdown of the continuous medium assumption [140]. Thus, the relevant mechanical properties of collagen are difficult to determine because of discrepancies between bulk rheology and micromechanics of the collagen network [140].…”

“…For example, a combination of spatial heterogeneities and large pore sizes in collagen gels can result in a breakdown of the continuous medium assumption [140]. Thus, the relevant mechanical properties of collagen are difficult to determine because of discrepancies between bulk rheology and micromechanics of the collagen network [140]. Furthermore, discrete fiducial markers cannot fully capture the deformation of a continuous substratum, and any deformations of the matrix in between markers remain uncertain [64,65].…”

Fibrin network adaptation to cell-generated forces

“…Microrheology has advantages over macrorheology such as much smaller sample volume, a high-frequency bandwidth (0 to 100 kHz), in situ measurement, and higher sensitivity to intracellular dynamics (Mizuno et al 2008;Tassieri et al 2012). Microrheology can be classified as passive microrheology, measuring spontaneous thermally driven fluctuations of beads, or active microrheology, driving the beads by an external force (oscillatory optical tweezers or magnetic tweezers (Jones et al 2015;Wessel et al 2015)). Passive microrheology by using optical tweezers is non-invasive and can access the high-frequency domain, and can be used to characterize polymer network behavior (Morse 1998).…”