Opposing Rigidity-Protein Gradients Reverse Fibroblast Durotaxis

Positive and negative durotaxis - mechanisms and emerging concepts

Cell migration is controlled by the coordinated action of cell adhesion, cytoskeletal dynamics, contractility and cell extrinsic cues. Integrins are the main adhesion receptors to ligands of the extracellular matrix (ECM), linking the actin cytoskeleton to the ECM and enabling cells to sense matrix rigidity and mount a directional cell migration response to stiffness gradients. Most models studied show preferred migration of single cells or cell clusters towards increasing rigidity. This is referred to as durotaxis, and since its initial discovery in 2000, technical advances and elegant computational models have provided molecular level details of stiffness sensing in cell migration. However, modeling has long predicted that, depending on cell intrinsic factors, such as the balance of cell adhesion molecules (clutches) and the motor proteins pulling on them, cells might also prefer adhesion to intermediate rigidity. Recently, experimental evidence has supported this notion and demonstrated the ability of cells to migrate towards lower rigidity, in a process called negative durotaxis. In this Review, we discuss the significant conceptual advances that have been made in our appreciation of cell plasticity and context dependency in stiffness-guided directional cell migration.

Mg-Fe layered double hydroxides modified titanium enhanced the adhesion of human gingival fibroblasts through regulation of local pH level

The durability of dental implants is closely related to osseointegration and surrounding soft tissue sealing. Appropriate local pH favors fibroblasts adhesion and contributes to soft tissue sealing. Layered double hydroxides (LDHs) are characterized by adjustable alkalinity, offering a possibility to investigate the influence of pH on cellular behaviors. Herein, we fabricated MgFe LDHs modified titanium. During calcination, the local pH value of LDHs increase, without altering other physics and chemical properties via OH^- exchange mechanism. In vitro studies showed that LDHs films calcined at 250 °C for 2 h provide a local pH of 10.17, which promote early adhesion, proliferation, and type I collagen expression of human gingival fibroblasts (hGFs) through the formation of focal adhesion complex and activation of focal adhesion kinase related signaling pathways. In conclusion, endowing the titanium surface with appropriate alkalinity by MgFe LDHs films enhances the adhesion of hGFs, providing a new strategy of designing multifunctional biomaterials for soft tissue sealing around dental implants.

Surface Creasing-Induced Micropatterned GelMA Using Heating-Hydration Fabrication for Effective Vascularization

BACKGROUND

Surface modification is used to modify the biomaterials for the regulation of cell culture using different approaches, such as chemical graft and mechanical treatment. However, those conventional methodologies often require precise fabrication in a high resolution involving either high cost or laborious steps to remove chemical residues that are toxic to the cells.

METHODS

A novel and simple method was proposed and evaluated to rapidly generate surface ceases on the gelatin methacrylate (gelMA) surface using the heating-hydration process. Human umbilical vein endothelial cells (HUVECs) were cultured on the gelMA surface. The surface binding was characterized using the RGD (Arg-Gly-Asp) antibodies and cell adhesion pattern captured by scanning electron microscopy. The effect of the heating-hydration parameters on the creasing formation was investigated. The morphology of HUVECs cultured on such micropatterned gelMA was characterized and compared.

RESULTS

It is found that the hydration solution, gelMA mixture, and hydration rate are the major factors that influence the cracking sizes in the range from 20 to 120 µm which resulted in capillary-like patterns on the gelMA surface. Low concentration of gelMA, high water concentration of cooling agent, and slow hydration rate result in the long creases, and heating of at least 60 min is required for complete dehydration. Strong fluorescence was around the creases with RGD-staining. Consequently, micropatterned gelMA demonstrated good biocompatibility with endothelial cells with more than 95% cell viability and continuous cell proliferation throughout 2 weeks as well as a good trace of neovascular formation. In comparison, normal gelMA surface did not exhibit RGD-fluorescent signals, and the cultured HUVECs on it were rounded with no spreading for network formation.

CONCLUSION

The heating-hydration approach can successfully and easily produce the micropatterned gelMA that allows rapid and effective vascularization to potentially improve the functionalities of the tissue-engineered construct.

Microfluidic devices fitted with "flowver" paper pumps generate steady, tunable gradients for extended observation of chemotactic cell migration

Microfluidics approaches have gained popularity in the field of directed cell migration, enabling control of the extracellular environment and integration with live-cell microscopy; however, technical hurdles remain. Among the challenges are the stability and predictability of the environment, which are especially critical for the observation of fibroblasts and other slow-moving cells. Such experiments require several hours and are typically plagued by the introduction of bubbles and other disturbances that naturally arise in standard microfluidics protocols. Here, we report on the development of a passive pumping strategy, driven by the high capillary pressure and evaporative capacity of paper, and its application to study fibroblast chemotaxis. The paper pumps-flowvers (flow + clover)-are inexpensive, compact, and scalable, and they allow nearly bubble-free operation, with a predictable volumetric flow rate on the order of μl/min, for several hours. To demonstrate the utility of this approach, we combined the flowver pumping strategy with a Y-junction microfluidic device to generate a chemoattractant gradient landscape that is both stable (6+ h) and predictable (by finite-element modeling calculations). Integrated with fluorescence microscopy, we were able to recapitulate previous, live-cell imaging studies of fibroblast chemotaxis to platelet derived growth factor (PDGF), with an order-of-magnitude gain in throughput. The increased throughput of single-cell analysis allowed us to more precisely define PDGF gradient conditions conducive for chemotaxis; we were also able to interpret how the orientation of signaling through the phosphoinositide 3-kinase pathway affects the cells' sensing of and response to conducive gradients.

Augmenting Tendon-to-Bone Repair with Functionally Graded Scaffolds

Tendon-to-bone repair often fails because the functionally graded attachment is not regenerated during the healing process. Biomimetic scaffolds that recapitulate the unique features of the native tendon-to-bone attachment hold great promise for enhancing the healing process. Among various types of scaffolds that are developed and evaluated for tendon-to-bone repair, those with gradations (in either a stratified or a continuous fashion) in composition, structure, mechanical properties, and cell phenotype have gained the most attention. In this progress report, the recent efforts in the rational design and fabrication of functionally graded scaffolds based upon electrospun nanofiber mats and inverse opal structures, as well as the evaluation of their applications in augmenting tendon-to-bone repair, are reviewed. This report concludes with perspectives on the necessary future steps for clinical translation of the scaffolds.

Photoregulated Gradient Structure and Programmable Mechanical Performances of Tough Hydrogels with a Hydrogen-Bond Network

Gradient materials exist widely in natural living organisms, affording fascinating biological and mechanical properties. However, the synthetic gradient hydrogels are usually mechanically weak or only have relatively simple gradient structures. Here, we report on tough nanocomposite hydrogels with designable gradient network structure and mechanical properties by a facile post-photoregulation strategy. Poly(1-vinylimidazole-co-methacrylic acid) hydrogels containing gold nanorods (AuNRs) are in a glassy state and show typical yielding and forced elastic deformation at room temperature. The gel slightly contracts its volume when the temperature is above the glass-transition temperature that results in a collapse of the chain segments and formation of denser intra- and interchain hydrogen bonds. Consequently, the mechanical properties of the gels are enhanced, when the temperature returns to room temperature. The mechanical performances of hydrogels can also be locally tuned by near-infrared light irradiation due to the photothermal effect of AuNRs. Hydrogels with arbitrary two-dimensional gradients can be facilely developed by site-specific photoirradiation. The treated and untreated regions with different stiffness and yielding stress possess construct behaviors in stretching or twisting deformations. A locally reinforced hydrogel with the kirigami structure becomes notch-insensitive and exhibits improved strength and stretchability because the treated regions ahead the cuts have better resistance to crack advancement. These tough hydrogels with programmable gradient structure and mechanics should find applications as structural elements, biological devices, etc.

Functionally graded biomaterials for use as model systems and replacement tissues

The heterogeneity of native tissues requires complex materials to provide suitable substitutes for model systems and replacement tissues. Functionally graded materials have the potential to address this challenge by mimicking the gradients in heterogeneous tissues such as porosity, mineralization, and fiber alignment to influence strength, ductility, and cell signaling. Advancements in microfluidics, electrospinning, and 3D printing enable the creation of increasingly complex gradient materials that further our understanding of physiological gradients. The combination of these methods enables rapid prototyping of constructs with high spatial resolution. However, successful translation of these gradients requires both spatial and temporal presentation of cues to model the complexity of native tissues that few materials have demonstrated. This review highlights recent strategies to engineer functionally graded materials for the modeling and repair of heterogeneous tissues, together with a description of how cells interact with various gradients.

Multifunctional Biomaterials: Combining Material Modification Strategies for Engineering of Cell-Contacting Surfaces

In the human body, cells in a tissue are exposed to signals derived from their specific extracellular matrix (ECM), such as architectural structure, mechanical properties, and chemical composition (proteins, growth factors). Research on biomaterials in tissue engineering and regenerative medicine aims to recreate such stimuli using engineered materials to induce a specific response of cells at the interface. Although traditional biomaterials design has been mostly limited to varying individual signals, increasing interest has arisen on combining several features in recent years to improve the mimicry of extracellular matrix properties. Tremendous progress in combinatorial surface modification exploiting, for example, topographical features or variations in mechanics combined with biochemical cues has enabled the identification of their key regulatory characteristics on various cell fate decisions. Gradients especially facilitated such research by enabling the investigation of combined continuous changes of different signals. Despite unravelling important synergies for cellular responses, challenges arise in terms of fabrication and characterization of multifunctional engineered materials. This review summarizes recent work on combinatorial surface modifications that aim to control biological responses. Modification and characterization methods for enhanced control over multifunctional material properties are highlighted and discussed. Thereby, this review deepens the understanding and knowledge of biomimetic combinatorial material modification, their challenges but especially their potential.

A subtle relationship between substrate stiffness and collective migration of cell clusters

The physical cues from the extracellular environment mediates cell signaling spatially and temporally. Cells respond to physical cues from their environment in a non-monotonic fashion. Despite our understanding of the role of substrate rigidity on single cell migration, how cells respond collectively to increasing extracellular matrix stiffness is not well established. Here we patterned multicellular epithelial Madin-Darby canine kidney (MDCK) islands on polyacrylamide gels of varying stiffness and studied their expansion. Our findings show that the MDCK islands expanded faster with increasing stiffness only up to an optimum stiffness, over which the expansion plateaued. We then focused on the expansion of the front of the assemblies and the formation of leader cells. We observed cell front destabilization only above substrate stiffness of a few kPa. The extension of multicellular finger-like structures at the edges of the colonies for intermediate and high stiffnesses from 6 to 60 kPa responded to higher substrate stiffness by increasing focal adhesion areas and actin cable assembly. Additionally, the number of leader cells at the finger-like protrusions increased with stiffness in correlation with an increase of the area of these multicellular protrusions. Consequently, the force profile along the epithelial fingers in the parallel and transverse directions of migration showed an unexpected relationship leading to a global force decrease with the increase of stiffness. Taken together, our findings show that epithelial cell colonies respond to substrate stiffness but in a non-trivial manner that may be of importance to understand morphogenesis and collective cell invasion during tumour progression.

Fibroblasts stimulate macrophage migration in interconnected extracellular matrices through tunnel formation and fiber alignment

In vivo, macrophages and fibroblasts navigate through and remodel the three-dimensional (3D) extra-cellular matrix (ECM). The orientation of fibers, the porosity, and degree of cross-linking can change the interconnectivity of the ECM and affect cell migration. In turn, migrating cells can alter their microenvironment. To study the relationships between ECM interconnectivity and migration of cells, we assembled collagen hydrogels with dense (DCN) or with loosely interconnected networks (LCN). We find that in DCNs, RAW 264.7 macrophages in monocultures were virtually stationary. In DCN co-cultures, Balb/c 3T3 fibroblasts created tunnels that provided conduits for macrophage migration. In LCNs, fibroblasts aligned fibers up to a distance of 100 μm, which provided tracks for macrophages. Intra-cellular and extra-cellular fluorescent fragments of internalized and degraded collagen were detected inside both cell types as well as around their cell peripheries. Macrophages expressed higher levels of urokinase-type plasminogen activator receptor associated protein (uPARAP)/mannose receptor 1 (CD206) compared to α₂β₁ indicating that collagen internalization in these cells occurred primarily via integrin-independent mechanisms. Network remodeling indicated by higher Young's modulus was observed in fibroblast monocultures as a result of TGF-β secretion. This work unveils new roles for fibroblasts in forming tunnels in networked ECM to modulate macrophage migration.

Label-free cell sorting strategies via biophysical and biochemical gradients

Isolating active mesenchymal stem cells from a heterogeneous population is an essential step that determines the efficacy of stem cell therapy such as for osteoarthritis. Nowadays, the gold standard of cell sorting, fluorescence-activated cell sorting, relies on labelling surface markers via antibody-antigen reaction. However, sorting stem cells with high stemness usually requires the labelling of multiple biomarkers. Moreover, the labelling process is costly, and the high operating pressure is harmful to cell functionality and viability. Although label-free cell sorting, based on physical characteristics, has gained increasing interest in the past decades, it has not shown the ability to eliminate stem cells with low stemness. Cell motility, as a novel sorting marker, is hence proposed for label-free sorting active stem cells. Accumulating evidence has demonstrated the feasibility in manipulating directional cell migration through patterning the biophysical, biochemical or both gradients of the extracellular matrix. However, applying those findings to label-free cell sorting has not been well discussed and studied. This review thus first provides a brief overview about the effect of biophysical and biochemical gradients of the extracellular matrix on cell migration. State-of-the-art fabrication techniques for generating such gradients of hydrogels are then introduced. Among current research, the authors suggest that hydrogels with dual-gradients of biochemistry and biophysics are potential tools for accurate label-free cell sorting with satisfactory selectivity and efficiency.

TRANSLATIONAL POTENTIAL OF THIS ARTICLE

The reviewed label-free cell sorting approaches enable us to isolate active cell for cytotherapy. The proposed system can be further modified for single-cell analysis and drug screening.

Deconstructing Immune Microenvironments of Lymphoid Tissues for Reverse Engineering

The immune microenvironment presents a diverse panel of cues that impacts immune cell migration, organization, differentiation, and the immune response. Uniquely, both the liquid and solid phases of every specific immune niche within the body play an important role in defining cellular functions in immunity at that particular location. The in vivo immune microenvironment consists of biomechanical and biochemical signals including their gradients, surface topography, dimensionality, modes of ligand presentation, and cell-cell interactions, and the ability to recreate these immune biointerfaces in vitro can provide valuable insights into the immune system. This manuscript reviews the critical roles played by different immune cells and surveys the current progress of model systems for reverse engineering of immune microenvironments with a focus on lymphoid tissues.

Multi-cellular transitional organotypic models to investigate liver fibrosis

Hepatic fibrosis is the result of wound healing and inflammation resulting in organ dysfunction. Hepatocytes, liver sinusoidal endothelial cells (LSECs), Kupffer cells (KCs), and hepatic stellate cells (HSCs) play critical roles in fibrogenesis. As the liver undergoes fibrosis, there are populations of cells that are healthy, fibrotic as well as those undergoing fibrosis. We investigated how a varying mechanical environment could induce changes in hepatic cells. In this study, a gradient in the mechanical properties of the microenvironment resulted in transitioning phenotypes in hepatic cells. We have designed detachable polyelectrolyte multilayers (PEMs) whose elastic moduli ranged from 21 to 43 kPa to serve as Space of Disse mimics. We assembled novel 3D organotypic liver models comprised of hepatocytes, LSECs, HSCs, KCs, and the Space of Disse mimic. We demonstrate how cells in contact with a mechanical gradient exhibit different properties compared to cells cultured using non-gradient PEMs. Significant differences were observed in HSC and KC proliferation between 3D cultures assembled with gradient and non-gradient PEMs. While HSCs on the stiffer regions of the gradient PEMs expressed both GFAP and α-SMA, cells in cultures assembled with homogeneous 43 kPa multilayers primarily expressed α-SMA. Over an 8-day culture, the elastic modulus in the 21 and 43 kPa regions of the gradient PEMs increased by 1.6 and 3.7-fold, respectively. This was accompanied by a 4-fold increase in hydroxyproline. Such in vitro tissues can be used to investigate the effects of liver fibrosis. STATEMENT OF SIGNIFICANCE: We have assembled a liver model assembled with four major primary hepatic cell types to investigate how a varying mechanical environment induces changes in hepatic cells. In this study, a gradient in the mechanical properties of the microenvironment results in transitioning phenotypes in hepatic cells. Our goal was to investigate the interplay between mechanical properties and a multi-cellular engineered liver tissue. In these models, Kupffer cell proliferation and hepatic stellate cell activation occurred due to mechanical cues and inter-cellular signaling across a distance of 2000 μm. These models are unique, in that, fibrosis was initiated purely through changes to the microenvironment. These models were not exposed to fibrogenic factors nor were the models assembled with cells from fibrotic rats. To the best of our knowledge, these are the first liver models that capture how a gradient microenvironment can result in transitioning cellular phenotypes.

Photo Processing for Biomedical Hydrogels Design and Functionality: A Review

A large number of opportunities for biomedical hydrogel design and functionality through photo-processing have stretched the limits of innovation. As both photochemical understanding and engineering technologies continue to develop, more complicated geometries and spatiotemporal manipulations can be realized through photo-exposure, producing multifunctional hydrogels with specific chemical, biological and physical characteristics for the achievement of biomedical goals. This report describes the role that light has recently played in the synthesis and functionalization of biomedical hydrogels and primarily the design of photoresponsive hydrogels via different chemical reactions (photo crosslinking and photo degradation) and conventional light curing processes (micropatterning, stereolithography and two/multiphoton techniques) as well as typical biomedical applications of the hydrogels (cell culture, differentiation and in vivo vascularization) and their promising future.

Extracellular matrix remodeling in 3D: implications in tissue homeostasis and disease progression

The extracellular matrix (ECM) plays a critical role in regulating cell behavior during tissue homeostasis and in disease progression. Through a combination of adhesion, contraction, alignment of ECM proteins and subsequent degradation, cells change the chemical, mechanical, and physical properties of their surrounding matrix. Other contributing factors to matrix remodeling are the de novo synthesis of ECM proteins, post-translational modifications and receptor-mediated internalization. In this review, we highlight how each of these processes contributes to the maintenance of homeostasis and in disease conditions such as cancer and liver fibrosis. This article is categorized under: Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement.

Studying Electrotaxis in Microfluidic Devices

Collective cell migration is important in various physiological processes such as morphogenesis, cancer metastasis and cell regeneration. Such migration can be induced and guided by different chemical and physical cues. Electrotaxis, referring to the directional migration of adherent cells under stimulus of electric fields, is believed to be highly involved in the wound-healing process. Electrotactic experiments are conventionally conducted in Petri dishes or cover glasses wherein cells are cultured and electric fields are applied. However, these devices suffer from evaporation of the culture medium, non-uniformity of electric fields and low throughput. To overcome these drawbacks, micro-fabricated devices composed of micro-channels and fluidic components have lately been applied to electrotactic studies. Microfluidic devices are capable of providing cells with a precise micro-environment including pH, nutrition, temperature and various stimuli. Therefore, with the advantages of reduced cell/reagent consumption, reduced Joule heating and uniform and precise electric fields, microfluidic chips are perfect platforms for observing cell migration under applied electric fields. In this paper, I review recent developments in designing and fabricating microfluidic devices for studying electrotaxis, aiming to provide critical updates in this rapidly-growing, interdisciplinary field.

Direct reprogramming and biomaterials for controlling cell fate

Direct reprogramming which changes the fate of matured cell is a very useful technique with a great interest recently. This approach can eliminate the drawbacks of direct usage of stem cells and allow the patient specific treatment in regenerative medicine. Overexpression of diverse factors such as general reprogramming factors or lineage specific transcription factors can change the fate of already differentiated cells. On the other hand, biomaterials can provide physical and topographical cues or biochemical cues on cells, which can dictate or significantly affect the differentiation of stem cells. The role of biomaterials on direct reprogramming has not been elucidated much, but will be potentially significant to improve the efficiency or specificity of direct reprogramming. In this review, the strategies for general direct reprogramming and biomaterials-guided stem cell differentiation are summarized with the addition of the up-to-date progress on biomaterials for direct reprogramming.