Patterned hydrogels for simplified measurement of cell traction forces

Photoresponsive Hydrogels for Studying Mechanotransduction of Cells

Hydrogels are important platform materials for in vitro cellular studies. Mechanistic studies on durotaxis, the directional movement of a cell affected by a spatial gradient of stiffness of the underlying substrate, requires materials such as polyacrylamide, polyethylene glycol, or PDMS, in which the stiffness can be controlled in a spatiotemporal manner. Here, we describe the synthesis of an o-nitrobenzyl-based photocleavable cross-linker and its incorporation into a polyacrylamide hydrogel to render it photoresponsive. Precise control over the physical properties of the gel allows observation of glioblastoma durotaxis under surface stiffness conditions relevant to the actual brain environment.

Directed cell migration towards softer environments

How cells sense tissue stiffness to guide cell migration is a fundamental question in development, fibrosis and cancer. Although durotaxis-cell migration towards increasing substrate stiffness-is well established, it remains unknown whether individual cells can migrate towards softer environments. Here, using microfabricated stiffness gradients, we describe the directed migration of U-251MG glioma cells towards less stiff regions. This 'negative durotaxis' does not coincide with changes in canonical mechanosensitive signalling or actomyosin contractility. Instead, as predicted by the motor-clutch-based model, migration occurs towards areas of 'optimal stiffness', where cells can generate maximal traction. In agreement with this model, negative durotaxis is selectively disrupted and even reversed by the partial inhibition of actomyosin contractility. Conversely, positive durotaxis can be switched to negative by lowering the optimal stiffness by the downregulation of talin-a key clutch component. Our results identify the molecular mechanism driving context-dependent positive or negative durotaxis, determined by a cell's contractile and adhesive machinery.

pH Modification of High-Concentrated Collagen Bioinks as a Factor Affecting Cell Viability, Mechanical Properties, and Printability

The 3D bioprinting of cell-incorporated gels is a promising direction in tissue engineering applications. Collagen-based hydrogels, due to their similarity to extracellular matrix tissue, can be a good candidate for bioink and 3D bioprinting applications. However, low hydrogel concentrations of hydrogel (<10 mg/mL) provide insufficient structural support and, in highly concentrated gels, cell proliferation is reduced. In this study, we showed that it is possible to print highly concentrated collagen hydrogels with incorporated cells, where the viability of the cells in the gel remains very good. This can be achieved simply by optimizing the properties of the bioink, particularly the gel composition and pH modification, as well as by optimizing the printing parameters. The bioink composed of porcine collagen hydrogel with a collagen concentration of 20 mg/mL was tested, while the final bioink collagen concentration was 10 mg/mL. This bioink was modified with 0, 5, 9, 13, 17 and 20 μL/mL of 1M NaOH solution, which affected the resulting pH and gelling time. Cylindrical samples based on the given bioink, with the incorporation of porcine adipose-derived stromal cells, were printed with a custom 3D bioprinter. These constructs were cultivated in static conditions for 6 h, and 3 and 5 days. Cell viability and morphology were evaluated. Mechanical properties were evaluated by means of a compression test. Our results showed that optimal composition and the addition of 13 μL NaOH per mL of bioink adjusted the pH of the bioink enough to allow cells to grow and divide. This modification also contributed to a higher elastic modulus, making it possible to print structures up to several millimeters with sufficient mechanical resistance. We optimized the bioprinter parameters for printing low-viscosity bioinks. With this experiment, we showed that a high concentration of collagen gels may not be a limiting factor for cell proliferation.

Focal adhesion displacement magnitude is a unifying feature of tensional homeostasis

Tensional homeostasis is widely recognized to exist at the length scales of organs and tissues, but the cellular length scale mechanism for tension regulation is not known. In this study, we explored whether tensional homeostasis emerges from the behavior of the individual focal adhesion (FA), which is the subcellular structure that transmits cell stress to the surrounding extracellular matrix. Past studies have suggested that cell contractility builds up until a certain displacement is achieved, and we thus hypothesized that tensional homeostasis may require a threshold level of substrate displacement. Micropattern traction microscopy was used to study a wide range of FA traction forces generated by bovine vascular smooth muscle cells and bovine aortic endothelial cells cultured on substrates of stiffness of 3.6, 6.7, 13.6, and 30 kPa. The most striking feature of FA dynamics observed here is that the substrate displacement resulting from FA traction forces is a unifying feature that determines FA tensional stability. Beyond approximately 1 μm of substrate displacement, FAs, regardless of cell type or substrate stiffness, exhibit a precipitous drop in temporal fluctuations of traction forces. These findings lead us to the conclusion that traction force dynamics collectively determine whether cells or cell ensembles develop tensional homeostasis, and this insight is necessary to fully understand how matrix stiffness impacts cellular behavior in healthy conditions and, more important, in pathological conditions such as cancer or vascular aging, where environmental stiffness is altered. STATEMENT OF SIGNIFICANCE: Tensional homeostasis is widely recognized to exist at the length scales of organs and tissues, but the cellular length scale mechanism for tension regulation is not known. In this study, we explored whether tensional homeostasis emerges from the behavior of the individual focal adhesion (FA), which is the subcellular structure that transmits cell stress to the extracellular matrix. We utilized micropattern traction microscopy to measure time-lapses of FA forces in vascular smooth muscle cells and in endothelial cells. We discovered that the magnitude of the substrate displacement determines whether the FA has low temporal variability of traction forces. This finding is significant since it is the first known feature of tensional homeostasis that is broadly unifying across a range of environmental conditions and cell types.

Epithelial tissue geometry directs emergence of bioelectric field and pattern of proliferation

Patterns of proliferation are templated by both gradients of mechanical stress as well as by gradients in membrane voltage (Vm), which is defined as the electric potential difference between the cytoplasm and the extracellular medium. Either gradient could regulate the emergence of the other, or they could arise independently and synergistically affect proliferation within a tissue. Here, we examined the relationship between endogenous patterns of mechanical stress and the generation of bioelectric gradients in mammary epithelial tissues. We observed that the mechanical stress gradients in the tissues presaged gradients in both proliferation and depolarization, consistent with previous reports correlating depolarization with proliferation. Furthermore, disrupting the Vm gradient blocked the emergence of patterned proliferation. We found that the bioelectric gradient formed downstream of mechanical stresses within the tissues and depended on connexin-43 (Cx43) hemichannels, which opened preferentially in cells located in regions of high mechanical stress. Activation of Cx43 hemichannels was necessary for nuclear localization of Yap/Taz and induction of proliferation. Together, these results suggest that mechanotransduction triggers the formation of bioelectric gradients across a tissue, which are further translated into transcriptional changes that template patterns of growth.

Microtubule-Actomyosin Mechanical Cooperation during Contact Guidance Sensing

Cancer cell migration through and away from tumors is driven in part by migration along aligned extracellular matrix, a process known as contact guidance (CG). To concurrently study the influence of architectural and mechanical regulators of CG sensing, we developed a set of CG platforms. Using flat and nanotextured substrates with variable architectures and stiffness, we show that CG sensing is regulated by substrate stiffness and define a mechanical role for microtubules and actomyosin-microtubule interactions during CG sensing. Furthermore, we show that Arp2/3-dependent lamellipodia dynamics can compete with aligned protrusions to diminish the CG response and define Arp2/3- and Formins-dependent actin architectures that regulate microtu-bule-dependent protrusions, which promote the CG response. Thus, our work represents a comprehen-sive examination of the physical mechanisms influ-encing CG sensing.

Aligned extracellular matrix architectures in tumors direct migration of invasive cancer cells. Tabdanov et al. show that the mechanical properties of aligned extracellular matrix environments influence invasive cell behavior and define a mechanical role for microtubules and actomyosin-microtubule interactions during sensing of contact guidance cues that arise from aligned extracellular matrix.

A two-layer skin construct consisting of a collagen hydrogel reinforced by a fibrin-coated polylactide nanofibrous membrane

Background: Repairs to deep skin wounds continue to be a difficult issue in clinical practice. A promising approach is to fabricate full-thickness skin substitutes with functions closely similar to those of the natural tissue. For many years, a three-dimensional (3D) collagen hydrogel has been considered to provide a physiological 3D environment for co-cultivation of skin fibroblasts and keratinocytes. This collagen hydrogel is frequently used for fabricating tissue-engineered skin analogues with fibroblasts embedded inside the hydrogel and keratinocytes cultivated on its surface. Despite its unique biological properties, the collagen hydrogel has insufficient stiffness, with a tendency to collapse under the traction forces generated by the embedded cells. Methods: The aim of our study was to develop a two-layer skin construct consisting of a collagen hydrogel reinforced by a nanofibrous poly-L-lactide (PLLA) membrane pre-seeded with fibroblasts. The attractiveness of the membrane for dermal fibroblasts was enhanced by coating it with a thin nanofibrous fibrin mesh. Results: The fibrin mesh promoted the adhesion, proliferation and migration of the fibroblasts upwards into the collagen hydrogel. Moreover, the fibroblasts spontaneously migrating into the collagen hydrogel showed a lower tendency to contract and shrink the hydrogel by their traction forces. The surface of the collagen was seeded with human dermal keratinocytes. The keratinocytes were able to form a basal layer of highly mitotically-active cells, and a suprabasal layer. Conclusion: The two-layer skin construct based on collagen hydrogel with spontaneously immigrated fibroblasts and reinforced by a fibrin-coated nanofibrous membrane seems to be promising for the construction of full-thickness skin substitute.

A novel method for quantifying traction forces on hexagonal micropatterned protein features on deformable poly-dimethyl siloxane sheets

Many methods exist for quantifying cellular traction forces, including traction force microscopy and microfabricated post arrays. However, these methodologies have limitations, including a requirement to remove cells to determine undeflected particle locations and the inability to quantify forces of cells with low cytoskeletal stiffness, respectively. Here we present a novel method of traction force quantification that eliminates both of these limitations. Through the use of a hexagonal pattern of microcontact-printed protein spots, a novel computational algorithm, and thin surfaces of polydimethyl siloxane (PDMS) blends, we demonstrate a system that: •quantifies cellular forces on a homogeneous surface that is stable and easily manufactured.•utilizes hexagonal patterns of protein spots and computational geometry to quantify cellular forces without need for cell removal.•quantifies cellular forces in cells with low cytoskeletal rigidity.

Methods of Micropatterning and Manipulation of Cells for Biomedical Applications

Micropatterning and manipulation of mammalian and bacterial cells are important in biomedical studies to perform in vitro assays and to evaluate biochemical processes accurately, establishing the basis for implementing biomedical microelectromechanical systems (bioMEMS), point-of-care (POC) devices, or organs-on-chips (OOC), which impact on neurological, oncological, dermatologic, or tissue engineering issues as part of personalized medicine. Cell patterning represents a crucial step in fundamental and applied biological studies in vitro, hence today there are a myriad of materials and techniques that allow one to immobilize and manipulate cells, imitating the 3D in vivo milieu. This review focuses on current physical cell patterning, plus chemical and a combination of them both that utilizes different materials and cutting-edge micro-nanofabrication methodologies.

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

Cell-generated forces drive an array of biological processes ranging from wound healing to tumor metastasis. Whereas experimental techniques such as traction force microscopy are capable of quantifying traction forces in multidimensional systems, the physical mechanisms by which these forces induce changes in tissue form remain to be elucidated. Understanding these mechanisms will ultimately require techniques that are capable of quantifying traction forces with high precision and accuracy in vivo or in systems that recapitulate in vivo conditions, such as microfabricated tissues and engineered substrata. To that end, here we review the fundamentals of traction forces, their quantification, and the use of microfabricated tissues designed to study these forces during cell migration and tissue morphogenesis. We emphasize the differences between traction forces in two- and three-dimensional systems, and highlight recently developed techniques for quantifying traction forces.

Anisotropic forces from spatially constrained focal adhesions mediate contact guidance directed cell migration

Directed migration by contact guidance is a poorly understood yet vital phenomenon, particularly for carcinoma cell invasion on aligned collagen fibres. We demonstrate that for single cells, aligned architectures providing contact guidance cues induce constrained focal adhesion maturation and associated F-actin alignment, consequently orchestrating anisotropic traction stresses that drive cell orientation and directional migration. Consistent with this understanding, relaxing spatial constraints to adhesion maturation either through reduction in substrate alignment density or reduction in adhesion size diminishes the contact guidance response. While such interactions allow single mesenchymal-like cells to spontaneously 'sense' and follow topographic alignment, intercellular interactions within epithelial clusters temper anisotropic cell-substratum forces, resulting in substantially lower directional response. Overall, these results point to the control of contact guidance by a balance of cell-substratum and cell-cell interactions, modulated by cell phenotype-specific cytoskeletal arrangements. Thus, our findings elucidate how phenotypically diverse cells perceive ECM alignment at the molecular level.

Tensional homeostasis in endothelial cells is a multicellular phenomenon

Mammalian cells of various types exhibit the remarkable ability to adapt to externally applied mechanical stresses and strains. Because of this adaptation, cells can maintain their endogenous mechanical tension at a preferred (homeostatic) level, which is essential for normal physiological functions of cells and tissues and provides protection against various diseases, including atherosclerosis and cancer. Conventional wisdom is that the cell possesses the ability to maintain tensional homeostasis on its own. Recent findings showed, however, that isolated cells cannot maintain tensional homeostasis. Here we studied the effect of multicellular interactions on tensional homeostasis by measuring traction forces in isolated bovine aortic endothelial cells and in confluent and nonconfluent cell clusters of different sizes. We found that, in isolated cells, the traction field exhibited a highly dynamic and erratic behavior. However, in cell clusters, dynamic fluctuations of the traction field became attenuated with increasing cluster size, at a rate that was faster in nonconfluent than confluent clusters. The driving mechanism of attenuation of traction field fluctuations was statistical averaging of the noise, and the impeding mechanism was nonuniform stress distribution in the clusters, which resulted from intercellular force transmission, known as a "global tug-of-war." These results show that isolated cells could not maintain tensional homeostasis, which confirms previous findings, and that tensional homeostasis is a multicellular phenomenon, which is a novel finding.