An Active Gel based on DNA and DNA-Associated Motor Proteins

We describe an active polymer network in which processive molecular motors control network elasticity. This system consists of actin filaments cross-linked by filamin A (FLNa) and contracted by bipolar filaments of muscle myosin II. The myosin motors stiffen the network by more than two orders of magnitude by pulling on actin filaments anchored in the network by FLNa cross-links, thereby generating internal stress. The stiffening response closely mimics the effects of external stress applied by mechanical shear. Both internal and external stresses can drive the network into a highly nonlinear, stiffened regime. The active stress reaches values that are equivalent to an external stress of 14 Pa, consistent with a 1-pN force per myosin head. This active network mimics many mechanical properties of cells and suggests that adherent cells exert mechanical control by operating in a nonlinear regime where cell stiffness is sensitive to changes in motor activity. This design principle may be applicable to engineering novel biologically inspired, active materials that adjust their own stiffness by internal catalytic control.

Biomaterials, 2010

The microenvironment of cells is dynamic and undergoes remodeling with time. This is evident in development, aging, pathological processes, and at tissue-biomaterial interfaces. But in contrast, the majority of the biomimetic materials have static properties. Here, we show that a previously developed DNA crosslinked hydrogel circumvents the need of environmental factors and undergoes controlled stiffness change via DNA delivery, a feasible approach to initiate property changes in vivo, different from previous attempts. Two types of fibroblasts, L929 and GFP, were subject to the alterations in substrate rigidity presented in the hydrogels. Our results show that exogenous DNA does not cause appreciable cell shape change. Cells do respond to mechanical alterations as demonstrated in the cell projection area and polarity (e.g., Soft vs. Soft / Medium), and the responses vary depending on magnitude (e.g., Soft / Medium vs. Soft / Stiff) and range of stiffness changes (e.g., Soft / Medium vs. Medium / Stiff). The two types of fibroblasts share specific responses in common (e.g., Soft / Medium), while differ in others (e.g., Medium / Stiff). For each cell type, the projection area and polarity respond differently. This approach provides insight into pathology (e.g., cancer) and tissue functioning, and assists in designing biomaterials with controlled dynamic stiffness by choosing the range and magnitude of stiffness change.

Physics Reports-review Section of Physics Letters - PHYS REP-REV SECT PHYS LETT, 2007

This review presents some of our recent results on active polar gels. Active polar gels are viscoelastic materials formed by polar filaments which are maintained in a non-equilibrium state by constant consumption of energy. This non-equilibrium state is characterized by the existence of internal stresses and spontaneous flows. A defining example of an active polar gel is provided by the acto-myosin cytoskeleton of eukaryotic cells. It is formed by actin filaments interacting with myosin molecular motors which are driven by the hydrolysis of adenosine-tri-phosphate (ATP).We first present a hydrodynamic theory of active polar gels. The hydrodynamic equations are generic as they only rely on symmetry arguments. We then use the hydrodynamic approach to study the spontaneous generation of flow in an active polar film and the formation of vortex defects. The last part of this review is devoted to an analysis of the active gel theory in situations which are reminiscent of structures formed...

Soft Matter, 2019

Actin–myosin networks exhibit macroscopic contraction due to the activity of myosin motors. Contraction is preceded by thousands of seconds by changes of the microscopic dynamics, in analogy to dynamic precursors in passive gels under external loads.

Nature Reviews Materials, 2018

Epithelial tissue is one of the four basic types of animal tissue and contains more than 60% of the cells in a vertebrate's body 1. Together with connective, muscle and nervous tissue, epithelial tissue forms the human body and plays crucial roles in many biological processes, including wound healing 2 , embryonic development 3 , morphogenesis 4 , homeostasis 4 and cancer metastasis 5. Epithelia have dynamic physical properties and can behave like a fluid 6,7 or like a solid 6. They exhibit various complicated behaviours, such as collective migration 8 , oscillation 9,10 , turbulent motion 11 , active cell rearrangement 12 , cell division and extrusion 13,14 , guided by biochemical and biophysical cues in the microenvironment, which can be sensed by epithelial cells. Apart from genes and biochemical signalling cascades, the mechanical properties of both the tissue and its microenvironment substantially impact epithelial behaviours 8. Epithelial cells sense the mechanical environment by exerting forces to the substrate mainly through actomyosin-dependent contraction, provided by the active walking of myosin motor proteins along actin filaments. These forces can also propagate through the tissue through cell-cell junctions, in particular E-cadherin-mediated adherens junctions, which form dynamic cellular contacts 15. Cell-substrate and cell-cell junctions are under constant mechanical stress 16 , which causes remodelling of the junctions 17 and triggers cell signalling events within the tissue 8. Therefore, the rheological behaviour of biological materials, from cytoskeleton networks to large cell assemblies, is a complicated compromise between competing forces, cellular events and exogenous stimuli. For example, motor protein activity changes the viscoelasticity of the cytoskeleton, causing the network to be substantially stiffened in a stress-dependent manner 18 and allowing cells to respond to substrate stiffness changes through remodelling and rheological adaptation 19. At the tissue level, epithelia sense their mechanical environment through cell-substrate adhesion complexes, and they exhibit durotaxis in response to changes in rigidity; for example, during neural crest migration in Xenopus laevis in vivo 20 and on surfaces with stiffness gradients in vitro 21. To relieve high intercellular mechanical stress, cells can be delaminated from the tissue at sites of highest crowding to mitigate epithelial hyperplasia, for example, during homeostasis in the Drosophila germ band 14 , zebrafish fins 13 and colon epithelia 13. Epithelia and their microenvironment show reciprocal mechanical effects. During Drosophila oogenesis, disorganized extracellular matrix (ECM) can be remodelled into a global polarized, restrictive ECM, which aligns with the actin bundles in the follicular epithelium

Nanoscale, 2019

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The European Physical Journal E, 2005

We develop a general theory for active viscoelastic materials made of polar filaments. This theory is motivated by the dynamics of the cytoskeleton. The continuous consumption of a fuel leads to a non equilibrium state characterized by the generation of flows and stresses. Our theory can be applied to experiments in which cytoskeletal patterns are set in motion by active processes such as those which are at work in cells.

Small, 2019

Atomic force microscopy rheological measurements (Rheo-AFM) of the linear viscoelastic properties of single, charged colloids having a star-like architecture with a hard core and an extended, deformable double-stranded DNA (dsDNA) corona dispersed in aqueous saline solutions are reported. This is achieved by analyzing indentation and relaxation experiments performed on individual colloidal particles by means of a novel model-free Fourier transform method that allows a direct evaluation of the frequency-dependent linear viscoelastic moduli of the system under investigation. The method provides results that are consistent with those obtained via a conventional fitting procedure of the force-relaxation curves based on a modified Maxwell model. The outcomes show a pronounced softening of the dsDNA colloids, which is described by an exponential decay of both the Young's and the storage modulus as a function of the salt concentration within the dispersing medium. The strong softening is related to a critical reduction of the size of the dsDNA corona, down to ≈70% of its size in a salt-free solution. This can be correlated to significant topological changes of the dense star-like polyelectrolyte forming the corona, which are induced by variations in the density profile of the counterions. Similarly, a significant reduction of the stiffness is obtained by increasing the length of the dsDNA chains, which we attribute to a reduction of the DNA density in the outer region of the corona.

Annals of Biomedical Engineering, 2012

Despite cellular environments having dynamic characteristics, many laboratories utilized static polyacrylamide hydrogels to study the ECM-cell relationship. To attain a more in vivo like environment, we have developed a dynamic, DNA-crosslinked hydrogel (DNA gel). Through the controlled delivery of DNA, we can temporally decrease or increase gel stiffness while expanding or contracting the gel, respectively. These dual mechanical changes make DNA gels a cell-ECM model for studying dynamic mechanoregulated processes, such as wound healing. Here, we characterized DNA gels on a mechanical and cellular level. In contrast to our previous publication, in which we examined the increasing stiffness effects on fibroblast morphology, we examined the effects of decreased matrix stiffness on fibroblast morphology. In addition, we quantified the bulk and/or local stress and strain properties of dynamic gels. Gels generated about 0.5 Pa stress and about 6-11% strain upon softening to generate larger and more circular fibroblasts. These results complemented our previous study, where dynamic gels contracted upon stiffening to generate smaller and longer fibroblasts. In conclusion, we developed a biomaterial that increases and decreases in stiffness while contracting and expanding, respectively. We found that the dynamic deformation directionality of the matrix determined the fibroblast morphology and possibly influences function.

2005