Soft Substrates Containing Hyaluronan Mimic the Effects of Increased Stiffness on Morphology, Motility, and Proliferation of Glioma Cells

Particle diffusion in extracellular hydrogels

Hyaluronic acid is an abundant polyelectrolyte in the human body that forms extracellular hydrogels in connective tissues. It is essential for regulating tissue biomechanics and cell-cell communication, yet hyaluronan overexpression is associated with pathological situations such as cancer and multiple sclerosis. Due to its enormous molecular weight (in the range of millions of Daltons), accumulation of hyaluronan hinders transport of macromolecules including nutrients and growth factors through tissues and also hampers drug delivery. However, the exact contribution of hyaluronan to tissue penetrability is poorly understood due to the complex structure and molecular composition of tissues. Here we reconstitute biomimetic hyaluronan gels and systematically investigate the effects of gel composition and crosslinking on the diffusion of microscopic tracer particles. We combine ensemble-averaged measurements via differential dynamic microscopy with single-particle tracking. We show that the particle diffusivity depends on the particle size relative to the network pore size and also on the stress relaxation dynamics of the network. We furthermore show that addition of collagen, the other major biopolymer in tissues, causes the emergence of caged particle dynamics. Our findings are useful for understanding macromolecular transport in tissues and for designing biomimetic extracellular matrix hydrogels for drug delivery and tissue regeneration.

Stiffness Sensing by Cells

Physical stimuli are essential for the function of eukaryotic cells, and changes in physical signals are important elements in normal tissue development as well as in disease initiation and progression. The complexity of physical stimuli and the cellular signals they initiate are as complex as those triggered by chemical signals. One of the most important, and the focus of this review, is the effect of substrate mechanical properties on cell structure and function. The past decade has produced a nearly exponentially increasing number of mechanobiological studies to define how substrate stiffness alters cell biology using both purified systems and intact tissues. Here we attempt to identify common features of mechanosensing in different systems while also highlighting the numerous informative exceptions to what in early studies appeared to be simple rules by which cells respond to mechanical stresses.

Tumor Cell Mechanosensing During Incorporation into the Brain Microvascular Endothelium

Introduction

Tumor metastasis to the brain occurs in approximately 20% of all cancer cases and often occurs due to tumor cells crossing the blood-brain barrier (BBB). The brain microenvironment is comprised of a soft hyaluronic acid (HA)-rich extracellular matrix with an elastic modulus of 0.1-1 kPa, whose crosslinking is often altered in disease states.

Methods

To explore the effects of HA crosslinking on breast tumor cell migration, we developed a biomimetic model of the human brain endothelium, consisting of brain microvascular endothelial cell (HBMEC) monolayers on HA and gelatin (HA/gelatin) films with different degrees of crosslinking, as established by varying the concentration of the crosslinker Extralink.

Results and Discussion

Metastatic breast tumor cell migration speed, diffusion coefficient, spreading area, and aspect ratio increased with decreasing HA crosslinking, a mechanosensing trend that correlated with tumor cell actin organization but not CD44 expression. Meanwhile, breast tumor cell incorporation into endothelial monolayers was independent of HA crosslinking density, suggesting that alterations in HA crosslinking density affect tumor cells only after they exit the vasculature. Tumor cells appeared to exploit both the paracellular and transcellular routes of trans-endothelial migration. Quantitative phenotyping of HBMEC junctions via a novel Python software revealed a VEGF-dependent decrease in punctate VE-cadherin junctions and an increase in continuous and perpendicular junctions when HBMECs were treated with tumor cell-secreted factors.

Conclusions

Overall, our quantitative results suggest that a combination of biochemical and physical factors promote tumor cell migration through the BBB.

Mechanical Durotactic Environment Enhances Specific Glioblastoma Cell Responses

Background: A hallmark of glioblastoma is represented by their ability to widely disperse throughout the brain parenchyma. The importance of developing new anti-migratory targets is critical to reduce recurrence and improve therapeutic efficacy. Methods: Polydimethylsiloxane substrates, either mechanically uniform or presenting durotactic cues, were fabricated to assess GBM cell morphological and dynamical response with and without pharmacological inhibition of NNMII contractility, of its upstream regulator ROCK and actin polymerization. Results: Glioma cells mechanotactic efficiency varied depending on the rigidity compliance of substrates. Morphologically, glioma cells on highly rigid and soft bulk substrates displayed bigger and elongated aggregates whereas on durotactic substrates the same cells were homogeneously dispersed with a less elongated morphology. The durotactic cues also induced a motility change, cell phenotype dependent, and with cells being more invasive on stiffer substrates. Pharmacological inhibition of myosin or ROCK revealed a rigidity-insensitivity, unlike inhibition of microfilament contraction and polymerization of F-actin, suggesting that alternative signalling is used to respond to durotactic cues. Conclusions: The presence of a distinct mechanical cue is an important factor in cell migration. Together, our results provide support for a durotactic role of glioma cells that acts through actomyosin contractility to regulate the aggressive properties of GBM cells.

Soft Hyaluronic Gels Promote Cell Spreading, Stress Fibers, Focal Adhesion, and Membrane Tension by Phosphoinositide Signaling, Not Traction Force

Cells respond to both physical and chemical aspects of their substrate. Whether intracellular signals initiated by physical stimuli are fundamentally different from those elicited by chemical stimuli is an open question. Here, we show that the requirement for a stiff substrate (and, therefore, high cellular tension) for cells to produce large focal adhesions and stress fibers is obviated when a soft substrate contains both hyaluronic acid (HA) and an integrin ligand (collagen I). HA is a major extracellular matrix component that is often up-regulated during wound healing and tumor growth. HA, together with collagen I, promotes hepatocellular carcinoma cell (Huh7) spreading on very soft substrates (300 Pa), resulting in morphology and motility similar to what these cells develop only on stiff substrates (>30 kPa) formed by polyacrylamide that contains collagen but not HA. The effect of HA requires turnover of polyphosphoinositides and leads to the activation of Akt. The inhibition of polyphosphoinositide turnover causes Huh7 cells and fibroblasts to decrease spreading and detach, whereas cells on stiffer substrates show almost no response. Traction force microscopy shows that the cell maintains a low strain energy and net contractile moment on HA substrates compared to stiff polyacrylamide substrates. Membrane tension measured by tether pulling is similar on soft HA and stiff polyacrylamide substrates. These results suggest that simultaneous signaling stimulated by HA and an integrin ligand can generate phosphoinositide-mediated signals to the cytoskeleton that reproduce those generated by high cellular tension.

Fabrication and characterization of silk microfiber-reinforced methacrylated gelatin hydrogel with turnable properties

Despite considerable research effort, the natural hydrogels presently available for tissue engineering suffer from several major drawbacks, one of the significant issue is their poor mechanical strength which are unable to satisfy some mechanical requirements for successful outcomes. Herein, to mimic the composition and structure of the natural extracellular matrix, the micron-sized silk fibers obtained by alkaline hydrolysis were used as a reinforcement phase in a GelMA hydrogel, resulting in a material with significantly greater stiffness than pure GelMA hydrogel alone. In addition, the hydrogel demonstrated tunable compressive strength, swelling capacity, and degradation properties based on the silk fiber length. Experiments with cells indicated that MC3T3-E1 pre-osteoblasts quickly adhered to and proliferated on the surface of the composite hydrogels, as revealed by FDA/PI staining and CCK-8 assays. In addition, various cellular responses, including cell adhesion, changes in cellular morphology and cell proliferation behavior, occurred on the composite hydrogel and varied with fiber length. Overall, this study introduces a series of fiber-reinforced, tunable composite hydrogels that could be useful for various tissue engineering applications.

Regulation of actin assembly by PI(4,5)P2 and other inositol phospholipids: An update on possible mechanisms

Actin cytoskeleton dynamics depend on a tight regulation of actin filament formation from an intracellular pool of monomers, followed by their linkage to each other or to cell membranes, followed by their depolymerization into a fresh pool of actin monomers. The ubiquitous requirement for continuous actin remodeling that is necessary for many cellular functions is orchestrated in large part by actin binding proteins whose affinity for actin is altered by inositol phospholipids, most prominently PI(4,5)P2 (phosphatidylinositol 4,5-bisphosphate). The kinetics of PI(4,5)P2 synthesis and hydrolysis, its lateral distribution within the lipid bilayer, and coincident detection of PI(4,5)P2 and another signal, all play a role in determining when and where a particular PI(4,5)P2-regulated protein is inactivated or activated to exert its effect on the actin cytoskeleton. This review summarizes a range of models that have been developed to explain how PI(4,5)P2 might function in the complex chemical and structural environment of the cell based on a combination of experiment and computational studies.

Glial Tissue Mechanics and Mechanosensing by Glial Cells

Understanding the mechanical behavior of human brain is critical to interpret the role of physical stimuli in both normal and pathological processes that occur in CNS tissue, such as development, inflammation, neurodegeneration, aging, and most common brain tumors. Despite clear evidence that mechanical cues influence both normal and transformed brain tissue activity as well as normal and transformed brain cell behavior, little is known about the links between mechanical signals and their biochemical and medical consequences. A multi-level approach from whole organ rheology to single cell mechanics is needed to understand the physical aspects of human brain function and its pathologies. This review summarizes the latest achievements in the field.

Manipulating cell fate: dynamic control of cell behaviors on functional platforms

We review the recent advances and new horizons in the dynamic control of cell behaviors on functional platforms and their applications.