Fibrous networks in biological materials from the nucleus to the tissue: Effects of particle inclusions, filament remodeling enzymes, and molecular motors.

Filamentous networks are ubiquitous in biology. Collagen fibers form much of the extracellular matrix, the cytoskeleton controls cell mechanics, and chromatin fibers span the volume of the nucleus. The mechanical properties of biopolymer fibers, the way in which the fibers link into networks, the reorganization of network structure by enzymes and molecular motors, and the types of cells within the network all affect the way in which these materials respond to mechanical stress. When sparsely connected fibrous networks such as the cytoskeleton or the extracellular matrix are deformed in shear, only a small fraction of the fibers carries most of the load, and remodeling enzymes can preferentially react with either the tensed or the slack fibers to remodel the network to optimize its response to mechanical stresses. When particles larger than the network mesh size are present, several of the nonlinear responses of biopolymer networks are qualitatively altered: fibrous networks switch from compression-softening to compression-stiffening but lose their strain stiffening in response to shear or extension. In the context of the nucleus, chromatin remodeling enzymes produce active materials that can be fluidized or solidified by the opposing actions of ATPases such as BRG1 or cohesin. These actities affect both the mechanical properties of the nucleus and the ability of cells to move through constricted spaces.