Contraction of fibroblast-containing collagen gels: initial collagen concentration regulates the degree of contraction and cell survival

Enhancing the Repair of Substantial Volumetric Muscle Loss by Creating Different Levels of Blood Vessel Networks Using Pre-Vascularized Nerve Hydrogel Implants

Volumetric muscle loss (VML), a severe muscle tissue loss from trauma or surgery, results in scarring, limited regeneration, and significant fibrosis, leading to lasting reductions in muscle mass and function. A promising approach for VML recovery involves restoring vascular and neural networks at the injury site, a process not extensively studied yet. Collagen hydrogels have been investigated as scaffolds for blood vessel formation due to their biocompatibility, but reconstructing blood vessels and guiding innervation at the injury site is still difficult. In this study, collagen hydrogels with varied densities of vessel-forming cells are implanted subcutaneously in mice, generating pre-vascularized hydrogels with diverse vessel densities (0-145 numbers/mm2) within a week. These hydrogels, after being transplanted into muscle injury sites, are assessed for muscle repair capabilities. Results showed that hydrogels with high microvessel densities, filling the wound area, effectively reconnected with host vasculature and neural networks, promoting neovascularization and muscle integration, and addressing about 63% of the VML.

Engineering large and geometrically controlled vascularized nerve tissue in collagen hydrogels to restore large-sized volumetric muscle loss

Developing scalable vascularized and innervated tissue is a critical challenge for the successful clinical application of tissue-engineered constructs. Collagen hydrogels are extensively utilized in cell-mediated vascular network formation because of their naturally excellent biological properties. However, the substantial increase in hydrogel contraction induced by populated cells limits their long-term use. Previous studies attempted to mitigate this issue by concentrating collagen pre-polymer solutions or synthesizing covalently crosslinked collagen hydrogels. However, these methods only partially reduce hydrogel contraction while hindering blood vessel formation within the hydrogels. To address this challenge, we introduced additional support in the form of a supportive spacer to counteract the contraction forces of populated cells and prevent hydrogel contraction. This approach was found to promote cell spreading, resist hydrogel contraction, control hydrogel/tissue geometry, and even facilitate the engineering of functional blood vessels and host nerve growth in just one week. Subsequently, implanting these engineered tissues into muscle defect sites resulted in timely anastomosis with the host vasculature, leading to enhanced myogenesis, increased muscle innervation, and the restoration of injured muscle functionality. Overall, this innovative strategy expands the applicability of collagen hydrogels in fabricating large vascularized nerve tissue constructs for repairing volumetric muscle loss (∼63 %) and restoring muscle function.

A Novel Hydrogel-Based 3D In Vitro Tumor Panel of 30 PDX Models Incorporates Tumor, Stromal and Immune Cell Compartments of the TME for the Screening of Oncology and Immuno-Therapies

Human-relevant systems that mimic the 3D tumor microenvironment (TME), particularly the complex mechanisms of immuno-modulation in the tumor stroma, in a reproducible and scalable format are of high interest for the drug discovery industry. Here, we describe a novel 3D in vitro tumor panel comprising 30 distinct PDX models covering a range of histotypes and molecular subtypes and cocultured with fibroblasts and PBMCs in planar (flat) extracellular matrix hydrogels to reflect the three compartments of the TME-tumor, stroma, and immune cells. The panel was constructed in a 96-well plate format and assayed tumor size, tumor killing, and T-cell infiltration using high-content image analysis after 4 days of treatment. We screened the panel first against the chemotherapy drug Cisplatin to demonstrate feasibility and robustness, and subsequently assayed immuno-oncology agents Solitomab (CD3/EpCAM bispecific T-cell engager) and the immune checkpoint inhibitors (ICIs) Atezolizumab (anti-PDL1), Nivolumab (anti-PD1) and Ipilimumab (anti-CTLA4). Solitomab displayed a strong response across many PDX models in terms of tumor reduction and killing, allowing for its subsequent use as a positive control for ICIs. Interestingly, Atezolizumab and Nivolumab demonstrated a mild response compared to Ipilimumab in a subset of models from the panel. We later determined that PBMC spatial proximity in the assay setup was important for the PD1 inhibitor, hypothesizing that both duration and concentration of antigen exposure may be critical. The described 30-model panel represents a significant advancement toward screening in vitro models of the tumor microenvironment that include tumor, fibroblast, and immune cell populations in an extracellular matrix hydrogel, with robust and standardized high content image analysis in a planar hydrogel. The platform is aimed at rapidly screening various combinations and novel agents and forming a critical conduit to the clinic, thus accelerating drug discovery for the next generation of therapeutics.

Adipose-Derived Stem Cells in Reinforced Collagen Gel: A Comparison between Two Approaches to Differentiation towards Smooth Muscle Cells

Scaffolds made of degradable polymers, such as collagen, polyesters or polysaccharides, are promising matrices for fabrication of bioartificial vascular grafts or patches. In this study, collagen isolated from porcine skin was processed into a gel, reinforced with collagen particles and with incorporated adipose tissue-derived stem cells (ASCs). The cell-material constructs were then incubated in a DMEM medium with 2% of FS (DMEM_part), with added polyvinylalcohol nanofibers (PVA_part sample), and for ASCs differentiation towards smooth muscle cells (SMCs), the medium was supplemented either with human platelet lysate released from PVA nanofibers (PVA_PL_part) or with TGF-β1 + BMP-4 (TGF + BMP_part). The constructs were further endothelialised with human umbilical vein endothelial cells (ECs). The immunofluorescence staining of alpha-actin and calponin, and von Willebrand factor, was performed. The proteins involved in cell differentiation, the extracellular matrix (ECM) proteins, and ECM remodelling proteins were evaluated by mass spectrometry on day 12 of culture. Mechanical properties of the gels with ASCs were measured via an unconfined compression test on day 5. Gels evinced limited planar shrinkage, but it was higher in endothelialised TGF + BMP_part gel. Both PVA_PL_part samples and TGF + BMP_part samples supported ASC growth and differentiation towards SMCs, but only PVA_PL_part supported homogeneous endothelialisation. Young modulus of elasticity increased in all samples compared to day 0, and PVA_PL_part gel evinced a slightly higher ratio of elastic energy. The results suggest that PVA_PL_part collagen construct has the highest potential to remodel into a functional vascular wall.

Differential effects of acetylsalicylic acid and mitomycin C on cytokine-induced Tenon's capsule myofibroblast transdifferentiation and activity: Implications for glaucoma surgery

Inflammation-driven scarring is a major contributor to surgical failure after subconjunctival bleb forming glaucoma surgery. The current gold standard anti-scarring adjuvant mitomycin C (MMC) has variable effectiveness and is associated with significant risks. Acetylsalicylic acid (ASA), when delivered locally, repurposes the typically pro-inflammatory cyclooxygenase (COX-2) signaling for the resolution of inflammation and mitigating inflammation-mediated fibrosis. The aim of this study is to compare the effects of ASA and MMC in an in vitro model of subconjunctival scarring. Glaucoma patient-derived Tenon's capsule fibroblasts (HTCFs) were treated with TGFβ1 (2 ng/mL) plus or minus ASA (1600 μg/ml), or MMC (0.05, 0.1, 0.2 mg/mL). In vitro collagen contraction, MTT, LDH, immunofluorescence, and Western blot assays were performed. To elucidate the mechanistic effects of ASA in TGFβ1-induced HTCFs, liquid chromatography tandem mass spectrometry (LC-MS/MS) was used to identify and measure pro-inflammatory and pro-resolving lipid mediator secretion. ASA was at least as effective as MMC in reducing TGFβ1-induced HTCF-mediated collagen contraction, metabolic activity, and pro-fibrotic protein expression, with less cytotoxicity. Within cytokine-activated HTCFs, ASA significantly impaired secretion of pro-inflammatory lipid mediators prostaglandin E2 and 6-keto-prostaglandin F1α and significantly increased secretion of the pro-resolving mediators 5-hydroxyeicosatetraenoic acid (HETE), 15-HETE and 18-hydroxyeicosapentaenoic acid (HEPE). ASA reduces cytokine-induced myofibroblast transdifferentiation in HTCFs, being non-inferior to MMC in vitro. ASA's effects are associated with a unique lipid mediator expression profile, suggesting that the ASA-induced resolution of inflammation may be a promising strategy to mitigate inflammation-mediated scarring and could offer a novel alternative as a surgical adjuvant.

Antifibrosis Efficacy of Apo-9-Fucoxanthinone-Contained Sargassum horneri Ethanol Extract on Nasal Polyp: An In Vitro and Ex Vivo Organ Culture Assay

Sargassum horneri is a seaweed species with diverse bioactivities. However, its antifibrotic effects during nasal polyp (NP) formation are not clearly understood. Therefore, we investigated the inhibitory effect of S. horneri on fibrosis progression in NP-derived fibroblasts (NPDFs) and NP tissues ex vivo. NPDFs were stimulated with TGF-β1 in the presence or absence of S. horneri ethanol extract (SHE). The extracellular matrix (ECM) protein production levels, myofibroblast differentiation (α-smooth muscle actin, α-SMA), and phosphorylation of Smad 2/3 and -ERK in TGF-β1-stimulated NPDFs were investigated using western blotting. Further, the contractile activity of SHE was assessed by performing a collagen gel contraction assay. The expression levels of collagen-1, fibronectin, and α-SMA were investigated in NP organ cultures treated with SHE. TGF-β1 stimulated ECM protein expression, myofibroblast differentiation, and collagen contractile activity while these were attenuated by pretreatment with SHE. We also found antifibrotic effect of SHE on ex vivo NP tissues. The antifibrotic effects of SHE were modulated through the attenuation of Smad 2/3 and ERK signaling pathways in TGF-β1-stimulated NPDFs. In conclusion, SHE inhibited ECM protein accumulation and myofibroblast differentiation during NP remodeling. Thus, SHE may be helpful as a treatment for NP recurrence after endoscopic sinus surgery.

Advances in Scaffolds Used for Pulp-Dentine Complex Tissue Engineering - A Narrative Review

Pulp necrosis in immature teeth disrupts root development and predisposes roots to fracture as a consequence of their thin walls and open apices. Regenerative endodontics is a developing treatment modality whereby necrotic pulps are replaced with newly formed healthy tissue inside the root canal. Many clinical studies have demonstrated the potential of this strategy to stimulate root maturation and apical root-end closure. However, clinical outcomes are patient-dependent and unpredictable. The development of predictable clinical protocols is achieved through the interplay of the three classical elements of tissue engineering, namely, stem cells, signaling molecules, and scaffolds. Scaffolds provide structural support for cells to adhere and proliferate and also regulate cell differentiation and metabolism. Hence, designing and fabricating an appropriate scaffold is a crucial step in tissue engineering. In this review, four main classes of scaffolds used to engineer pulp-dentine complexes, including bioceramic-based scaffolds, synthetic polymer-based scaffolds, natural polymer-based scaffolds, and composite scaffolds, are covered. Additionally, recent advances in the design, fabrication, and application of such scaffolds are analysed along with their advantages and limitations. Finally, the importance of vascular network establishment in the success of pulp-dentine complex regeneration and strategies used to create scaffolds to address this challenge are discussed.

Melanoma spheroid-containing artificial dermis as an alternative approach to in vivo models

Melanoma spheroid-loaded 3D skin models allow for the study of crucial tumor characteristics and factors at a superior level because the neoplastic cells are integrated into essential human skin components, permitting tumor-skin model communication. Herein, we designed a melanoma-containing artificial dermis by inserting multicellular tumor spheroids from the metastatic phase of WM 1617 melanoma cells into an artificial dermis. We cultured multicellular melanoma spheroids by hanging drop method (250 cells per drop) with a size of 420 μm in diameter after incubation for 14 days. These spheroids were integrated into the dermal equivalents that had been previously preparedwith a type-I collagen matrix and healthy fibroblasts. The melanoma spheroid cells invaded and proliferated in the artificial dermis. Spheroids treated with a 1.0 μmol/L aluminum chloride phthalocyanine nanoemulsion in the absence of light showed high cell viability. In contrast, under irradiation with visible red light (660 nm) at 25 J/cm2, melanoma cells were killed and the healthy tissue was preserved, indicating that photodynamic therapy is effective in such a model. Therefore, the 3D skin melanoma model has potential to promote research in full-thickness skin model targeting optimized preclinical assays.

Mechanistic interrogation of mutation-independent disease modulators of RDEB identifies the small leucine-rich proteoglycan PRELP as a TGF-β antagonist and inhibitor of fibrosis

Recessive dystrophic epidermolysis bullosa (RDEB) is a genetic extracellular matrix disease caused by deficiency in type VII collagen (Col VII). The disease manifests with devastating mucocutaneous fragility leading to progressive fibrosis and metastatic squamous cell carcinomas. Although collagen VII abundance is considered the main predictor of symptom course, previous studies have revealed the existence of mutation-independent mechanisms that control disease progression. Here, to investigate and validate new molecular modifiers of wound healing and fibrosis in a natural human setting, and toward development of disease-modulating treatment of RDEB, we performed gene expression profiling of primary fibroblast from RDEB siblings with marked phenotypic variations, despite having equal COL7A1 genotype. Gene enrichment analysis suggested that severe RDEB was associated with enhanced response to TGF-β stimulus, oxidoreductase activity, and cell contraction. Consistently, we found an increased response to TGF-β, higher levels of basal and induced reactive oxygen species (ROS), and greater contractile ability in collagen lattices in RDEB fibroblasts (RDEBFs) from donors with severe RDEB vs mild RDEB. Treatment with antioxidants allowed a reduction of the pro-fibrotic and contractile phenotype. Importantly, our analyses revealed higher expression and deposition in skin of the relatively uncharacterized small leucine-rich extracellular proteoglycan PRELP/prolargin associated with milder RDEB manifestations. Mechanistic investigations showed that PRELP effectively attenuated fibroblasts' response to TGF-β1 stimulus and cell contractile capacity. Moreover, PRELP overexpression in RDEBFs enhanced RDEB keratinocyte attachment to fibroblast-derived extracellular matrix in the absence of Col VII. Our results highlight the clinical relevance of pro-oxidant status and hyper-responsiveness to TGF-β in RDEB severity and progression. Of note, our study also reveals PRELP as a novel and natural TGF-β antagonist with a likely dermo-epidermal pro-adhesive capacity.

A sonication-induced silk-collagen hydrogel for functional cartilage regeneration

Cartilage tissue has limited self-regeneration capacity and current treatment methods often result in fibrocartilage formation. Although collagen has shown the ability to induce chondrogenesis of mesenchymal stem cells (MSCs) and regenerate hyaline cartilage, the application of a pure collagen hydrogel is inherently limited by its fast degradation, poor mechanical properties and excessive cell-mediated shrinkage. To overcome this challenge, we developed a sonication-induced silk-collagen composite hydrogel (COL + SF(S)) and investigated its physicochemical and biological properties compared with a collagen hydrogel (COL) and a non-sonicated silk-collagen composite hydrogel (COL + SF(NS)). The results showed that the sonication treatment of silk fibroin induced antiparallel β-sheet formation and a stronger negative charge on the silk fibroin molecule, which resulted in improved mechanical properties of the COL + SF(S) hydrogel. The COL + SF(S) hydrogel exhibited superior stability during cell culture and promoted the gene expression of SOX9 at the early stage and sulfated glycosaminoglycan (sGAG) deposition without any exogenous growth factor. Moreover, the cartilage regeneration capacity of the COL + SF(S) group was evaluated in rabbit knee defects. The COL + SF(S) group exhibited well-integrated articular hyaline cartilage closely resembling native articular cartilage after 6 months. Overall, the COL + SF(S) hydrogel holds great potential as a scaffold material to regenerate functional hyaline cartilage.

Bioprinting and plastic compression of large pigmented and vascularized human dermo-epidermal skin substitutes by means of a new robotic platform

Extensive availability of engineered autologous dermo-epidermal skin substitutes (DESS) with functional and structural properties of normal human skin represents a goal for the treatment of large skin defects such as severe burns. Recently, a clinical phase I trial with this type of DESS was successfully completed, which included patients own keratinocytes and fibroblasts. Yet, two important features of natural skin were missing: pigmentation and vascularization. The first has important physiological and psychological implications for the patient, the second impacts survival and quality of the graft. Additionally, accurate reproduction of large amounts of patient’s skin in an automated way is essential for upscaling DESS production. Therefore, in the present study, we implemented a new robotic unit (called SkinFactory) for 3D bioprinting of pigmented and pre-vascularized DESS using normal human skin derived fibroblasts, blood- and lymphatic endothelial cells, keratinocytes, and melanocytes. We show the feasibility of our approach by demonstrating the viability of all the cells after printing in vitro, the integrity of the reconstituted capillary network in vivo after transplantation to immunodeficient rats and the anastomosis to the vascular plexus of the host. Our work has to be considered as a proof of concept in view of the implementation of an extended platform, which fully automatize the process of skin substitution: this would be a considerable improvement of the treatment of burn victims and patients with severe skin lesions based on patients own skin derived cells.

A mathematical model for cell-induced gel contraction incorporating osmotic effects

Biological tissues are composed of cells surrounded by the extracellular matrix (ECM). The ECM can be thought of as a fibrous polymer network, acting as a natural scaffolding to provide mechanical support to the cells. Reciprocal mechanical and chemical interactions between the cells and the ECM are crucial in regulating the development of tissues and maintaining their functionality. Hence, to maintain in vivo-like behaviour when cells are cultured in vitro, they are often seeded in a gel, which aims to mimic the ECM. In this paper, we present a mathematical model that incorporates cell-gel interactions together with osmotic pressure to study the mechanical behaviour of biological gels. In particular, we consider an experiment where cells are seeded within a gel, which gradually compacts due to forces exerted on it by the cells. Adopting a one-dimensional Cartesian geometry for simplicity, we use a combination of analytical techniques and numerical simulations to investigate how cell traction forces interact with osmotic effects (which can lead to either gel swelling or contraction depending on the gel’s composition). Our results show that a number of qualitatively different behaviours are possible, depending on the composition of the gel (i.e. its chemical potentials) and the strength of the cell traction forces. A novel prediction of our model is that there are cases where the gel oscillates between swelling and contraction; to our knowledge, this behaviour has not been reported in experiments. We also consider how physical parameters like drag and viscosity affect the manner in which the gel evolves.

Strategy for improving cell-mediated vascularized soft tissue formation in a hydrogen peroxide-triggered chemically-crosslinked hydrogel

The physically-crosslinked collagen hydrogels can provide suitable microenvironments for cell-based functional vascular network formation due to their biodegradability, biocompatibility, and good diffusion properties. However, encapsulation of cells into collagen hydrogels results in extensive contraction and rapid degradation of hydrogels, an effect known from their utilization as a pre-vascularized graft in vivo. Various types of chemically-crosslinked collagen-based hydrogels have been successfully synthesized to decrease volume contraction, retard the degradation rate, and increase mechanical tunability. However, these hydrogels failed to form vascularized tissues with uniformly distributed microvessels in vivo. Here, the enzymatically chemically-crosslinked collagen-Phenolic hydrogel was used as a model to determine and overcome the difficulties in engineering vascular networks. Results showed that a longer duration of inflammation and excessive levels of hydrogen peroxide limited the capability for blood vessel forming cells-mediated vasculature formation in vivo. Lowering the unreacted amount of crosslinkers reduced the densities of infiltrating host myeloid cells by half on days 2–4 after implantation, but blood vessels remained at low density and were mainly located on the edge of the implanted constructs. Co-implantation of a designed spacer with cell-laden hydrogel maintained the structural integrity of the hydrogel and increased the degree of hypoxia in embedded cells. These effects resulted in a two-fold increase in the density of perfused blood vessels in the hydrogel. Results agreed with computer-based simulations. Collectively, our findings suggest that simultaneous reduction of the crosslinker-induced host immune response and increase in hypoxia in hydrogen peroxide-triggered chemically-crosslinked hydrogels can effectively improve the formation of cell-mediated functional vascular networks.

The Role of the Dynamic Lung Extracellular Matrix Environment on Fibroblast Morphology and Inflammation

The extracellular matrix (ECM) supports lung tissue architecture and physiology by providing mechanical stability and elastic recoil. Over the last several decades, it has become increasingly clear that the stiffness of the ECM governs many cellular processes, including cell-phenotype and functions during development, healing, and disease. Of all the lung ECM proteins, collagen-I is the most abundant and provides tensile strength. In many fibrotic lung diseases, the expression of collagen is increased which affects the stiffness of the surrounding environment. The goal of this study was to assess the effect on fibroblast morphology, cell death, and inflammation when exposed to 2D and 3D low (0.4 mg/mL) versus high (2.0 mg/mL) collagen-I-matrix environments that model the mechanics of the breathing lung. This study demonstrates that human fetal lung fibroblasts (HFL1), grown in a 3D collagen type-I environment compared to a 2D one, do not form cells with a myofibroblast morphology, express less F-actin stress fibers, exhibit less cell death, and significantly produce less pro-inflammatory IL-6 and IL-8 cytokines. Exposure to mechanical strain to mimic breathing (0.2 Hz) led to the loss of HFL1 fibroblast dendritic extensions as well as F-actin stress fibers within the cell cytoskeleton, but did not influence cytokine production or cell death. This dynamic assay gives researchers the ability to consider the assessment of the mechanodynamic nature of the lung ECM environment in disease-relevant models and the potential of mechano-pharmacology to identify therapeutic targets for treatment.

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.

Application of Temporary, Cell-Containing Alginate Microcarriers to Facilitate the Fabrication of Spatially Defined Cell Pockets in 3D Collagen Hydrogels

Mimicking the complexity of natural tissue is a major challenge in the field of tissue engineering. Here, a facile 2-step fabrication method to prepare 3D constructs with distinct regions of high cell concentrations and without the need for elaborate equipment is proposed. The initial incorporation of cells in a sacrificial alginate matrix allows the addition of other, cell relevant biopolymers, such as, collagen to form a spatially confined, interpenetrating network at the microscale. A layered structure at the macroscale can be achieved by incorporating these cell-containing microspheres in thin collagen layers. Cells are locally released by de-gelling the alginate matrix and their attachment to the collagen hydrogel layers has been studied. The use of the murine pre-osteoblast cell line MC3T3-E1 as an example cell line shows that the cells behave differently in their cell migration pattern based on the initial composition of the alginate microspheres.

What do cells regulate in soft tissues on short time scales?

Cells within living soft biological tissues seem to promote the maintenance of a mechanical state within a defined range near a so-called set-point. This mechanobiological process is often referred to as mechanical homeostasis. During this process, cells interact with the fibers of the surrounding extracellular matrix (ECM). It remains poorly understood, however, what individual cells actually regulate during these interactions, and how these micromechanical regulations are translated to the tissue-level to lead to what we observe as biomaterial properties. Herein, we examine this question by a combination of experiments, theoretical analysis, and computational modeling. We demonstrate that on short time scales (hours) - during which deposition and degradation of ECM fibers can largely be neglected - cells appear to not regulate the stress / strain in the ECM or their own shape, but rather only the contractile forces that they exert on the surrounding ECM. STATEMENT OF SIGNIFICANCE: Cells in soft biological tissues sense and regulate the mechanical state of the extracellular matrix to ensure structural integrity and functionality. This so-called mechanical homeostasis plays an important role in the natural history of various diseases such as aneurysms in the cardiovascular system or cancer. Yet, it remains poorly understood to date which target quantity cells regulate on the mircroscale and how it translates to the macroscale. In this paper, we combine experiments, computer simulations, and theoretical analysis to compare different hypotheses about this target quantity. This allows us to identify a likely candidate for it at least on short time scales and in the simplified environment of tissue equivalents.

Soft Polymer-Based Technique for Cellular Force Sensing

Soft polymers have emerged as a vital type of material adopted in biomedical engineering to perform various biomechanical characterisations such as sensing cellular forces. Distinct advantages of these materials used in cellular force sensing include maintaining normal functions of cells, resembling in vivo mechanical characteristics, and adapting to the customised functionality demanded in individual applications. A wide range of techniques has been developed with various designs and fabrication processes for the desired soft polymeric structures, as well as measurement methodologies in sensing cellular forces. This review highlights the merits and demerits of these soft polymer-based techniques for measuring cellular contraction force with emphasis on their quantitativeness and cell-friendliness. Moreover, how the viscoelastic properties of soft polymers influence the force measurement is addressed. More importantly, the future trends and advancements of soft polymer-based techniques, such as new designs and fabrication processes for cellular force sensing, are also addressed in this review.

A computational framework for modeling cell-matrix interactions in soft biological tissues

Living soft tissues appear to promote the development and maintenance of a preferred mechanical state within a defined tolerance around a so-called set point. This phenomenon is often referred to as mechanical homeostasis. In contradiction to the prominent role of mechanical homeostasis in various (patho)physiological processes, its underlying micromechanical mechanisms acting on the level of individual cells and fibers remain poorly understood, especially how these mechanisms on the microscale lead to what we macroscopically call mechanical homeostasis. Here, we present a novel computational framework based on the finite element method that is constructed bottom up, that is, it models key mechanobiological mechanisms such as actin cytoskeleton contraction and molecular clutch behavior of individual cells interacting with a reconstructed three-dimensional extracellular fiber matrix. The framework reproduces many experimental observations regarding mechanical homeostasis on short time scales (hours), in which the deposition and degradation of extracellular matrix can largely be neglected. This model can serve as a systematic tool for future in silico studies of the origin of the numerous still unexplained experimental observations about mechanical homeostasis.

Mechanical homeostasis in tissue equivalents: a review

There is substantial evidence that growth and remodeling of load bearing soft biological tissues is to a large extent controlled by mechanical factors. Mechanical homeostasis, which describes the natural tendency of such tissues to establish, maintain, or restore a preferred mechanical state, is thought to be one mechanism by which such control is achieved across multiple scales. Yet, many questions remain regarding what promotes or prevents homeostasis. Tissue equivalents, such as collagen gels seeded with living cells, have become an important tool to address these open questions under well-defined, though limited, conditions. This article briefly reviews the current state of research in this area. It summarizes, categorizes, and compares experimental observations from the literature that focus on the development of tension in tissue equivalents. It focuses primarily on uniaxial and biaxial experimental studies, which are well-suited for quantifying interactions between mechanics and biology. The article concludes with a brief discussion of key questions for future research in this field.

Flexible Adipose-Vascular Tissue Assembly Using Combinational 3D Printing for Volume-Stable Soft Tissue Reconstruction

A new concept, assembling cell-laden tissue modules, is for the first time proposed for soft tissue engineering. Adipose-vascular tissue modules composed of a synthetic polymer-based substructure and customized bioinks using planar 3D cell printing are engineered. Such tissue modules are systematically assembled into a synthetic polymer-based module holder fabricated with rotational 3D printing, resulting in the development of a flexible and volumetric tissue assembly. Whereas most of the previous studies about the construction of adipose tissue are limited to hypoxia, poor vascularization, rapid resorption, and mismatch in mechanical properties, it is aimed to realize the construction of nonhypoxic, flexible, and volume-stable tissue assembly in this study. The significance of engineered tissue assembly is proven through various in vitro and in vivo evaluations. In particular, stable volume and remarkable neovascularization/adipogenesis are observed in the implanted assembly over four weeks. Interestingly, the size of newly formed lipid droplets and the remodeled morphology in the assembly are comparable to those in native adipose tissue. As far as it is known, this work is a first report suggesting a cell printing-based tissue assembly for functional reconstruction of soft tissue.

Dermal fibroblasts and triple-negative mammary epithelial cancer cells differentially stiffen their local matrix

The bulk measurement of extracellular matrix (ECM) stiffness is commonly used in mechanobiology. However, past studies by our group show that peri-cellular stiffness is quite heterogeneous and divergent from the bulk. We use optical tweezers active microrheology (AMR) to quantify how two phenotypically distinct migratory cell lines establish dissimilar patterns of peri-cellular stiffness. Dermal fibroblasts (DFs) and triple-negative human breast cancer cells MDA-MB-231 (MDAs) were embedded within type 1 collagen (T1C) hydrogels polymerized at two concentrations: 1.0 mg/ml and 1.5 mg/ml. We found DFs increase the local stiffness of 1.0 mg/ml T1C hydrogels but, surprisingly, do not alter the stiffness of 1.5 mg/ml T1C hydrogels. In contrast, MDAs predominantly do not stiffen T1C hydrogels as compared to cell-free controls. The results suggest that MDAs adapt to the bulk ECM stiffness, while DFs regulate local stiffness to levels they intrinsically prefer. In other experiments, cells were treated with transforming growth factor-β1 (TGF-β1), glucose, or ROCK inhibitor Y27632, which have known effects on DFs and MDAs related to migration, proliferation, and contractility. The results show that TGF-β1 alters stiffness anisotropy, while glucose increases stiffness magnitude around DFs but not MDAs and Y27632 treatment inhibits cell-mediated stiffening. Both cell lines exhibit an elongated morphology and local stiffness anisotropy, where the stiffer axis depends on the cell line, T1C concentration, and treatment. In summary, our findings demonstrate that AMR reveals otherwise masked mechanical properties such as spatial gradients and anisotropy, which are known to affect cell behavior at the macro-scale. The same properties manifest with similar magnitude around single cells.

Development of an Improved 3D in vitro Intestinal Model to Perform Permeability Studies of Paracellular Compounds

The small intestine is the primary site of drug absorption following oral administration, making paramount the proper monitoring of the absorption process. In vitro tools to predict intestinal absorption are particularly important in preclinical drug development since they are less laborious and cost-intensive and raise less ethical considerations compared to in vivo studies. The Caco-2 model is considered the gold standard of in vitro intestinal models regarding the prediction of absorption of orally delivered compounds. However, this model presents several drawbacks, such as the expression of tighter tight junctions, not being suitable to perform permeability of paracellular compounds. Besides, cells are representative of only one intestinal cell type, without considering the role of non-absorptive cells on the absorption pathway of drugs. In the present study, we developed a new three-dimensional (3D) intestinal model that aims to bridge the gap between in vitro tools and animal studies. Our 3D model comprises a collagen layer with human intestinal fibroblasts (HIFs) embedded, mimicking the intestinal lamina propria and providing 3D support for the epithelium, composed of Caco-2 cells and mucus-producing HT29-MTX cells, creating a model that can better resemble, both in terms of composition and regarding the outcomes of drug permeability when testing paracellular compounds, the human small intestine. The optimization of the collagen layer with HIFs was performed, testing different collagen concentrations and HIF seeding densities in order to avoid collagen contraction before day 14, maintaining HIF metabolically active inside the collagen disks during time in culture. HIF morphology and extracellular matrix (ECM) deposition were assessed, confirming that fibroblasts presented a normal and healthy elongated shape and secreted fibronectin and laminin, remodeling the collagen matrix. Regarding the epithelial layer, transepithelial electrical resistance (TEER) values decreased when cells were in the 3D configuration, comparing with the 2D analogs (Caco-2 and coculture of Caco-2+HT29-MTX models), becoming more similar with in vivo values. The permeability assay with fluorescein isothiocyanate (FITC)–Dextran 4 kDa showed that absorption in the 3D models is significantly higher than that in the 2D models, confirming the importance of using a more biorelevant model when testing the paracellular permeability of compounds.

Contraction of fibrin-derived matrices and its implications for in vitro human skin bioengineering

It is well-known that fibroblasts play a fundamental role in the contraction of collagen and fibrin hydrogels when used in the production of in vitro bilayered skin substitutes. However, little is known about the contribution of other factors, such as the hydrogel matrix itself, on this contraction. In this work, we studied the contraction of plasma-derived fibrin hydrogels at different temperatures (4, 23, and 37°C) in an isotonic buffer (phosphate-buffered saline). These types of hydrogels presented a contraction of approximately 30% during the first 24 hr, following a similar kinetics irrespectively of the temperature. This kinetics continued in a slowed down manner to reach a plateau value of 40% contraction after 10-15 days. Contraction of commercial fibrinogen hydrogels was studied under similar conditions and the kinetics was completed after 8 hr, reaching values between 20 and 70% depending on the temperature. We attribute these substantial differences to a modulatory effect on the contraction due to plasma proteins which are initially embedded in, and progressively released from, the plasma-based hydrogels. The elastic modulus of hydrogels measured at a constant frequency decreased with increasing temperature in 7-day gels. Rheological measurements showed the absence of a strain-hardening behavior in the plasma-derived fibrin hydrogels. Finally, plasma-derived fibrin hydrogels with and without human primary fibroblast and keratinocytes were prepared in transwell inserts and their height measured over time. Both cellular and acellular gels showed a height reduction of 30% during the first 24 hr likely due to the above-mentioned intrinsic fibrin matrix contraction.

Computer-Controlled Biaxial Bioreactor for Investigating Cell-Mediated Homeostasis in Tissue Equivalents

Soft biological tissues consist of cells and extracellular matrix (ECM), a network of diverse proteins, glycoproteins, and glycosaminoglycans that surround the cells. The cells actively sense the surrounding ECM and regulate its mechanical state. Cell-seeded collagen or fibrin gels, so-called tissue equivalents, are simple but powerful model systems to study this phenomenon. Nevertheless, few quantitative studies document the stresses that cells establish and maintain in such gels; moreover, most prior data were collected via uniaxial experiments whereas soft tissues are mainly subject to multiaxial loading in vivo. To begin to close this gap between existing experimental data and in vivo conditions, we describe here a computer-controlled bioreactor that enables accurate measurements of the evolution of mechanical tension and deformation of tissue equivalents under well-controlled biaxial loads. This device allows diverse studies, including how cells establish a homeostatic state of biaxial stress and if they maintain it in response to mechanical perturbations. It similarly allows, for example, studies of the impact of cell and matrix density, exogenous growth factors and cytokines, and different types of loading conditions (uniaxial, strip-biaxial, and biaxial) on these processes. As illustrative results, we show that NIH/3T3 fibroblasts establish a homeostatic mechanical state that depends on cell density and collagen concentration. Following perturbations from this homeostatic state, the cells were able to recover biaxial loading similar to homeostatic. Depending on the precise loads, however, they were not always able to fully maintain that state.

Organotypic breast tumor model elucidates dynamic remodeling of tumor microenvironment

Fibroblasts are a critical component of tumor microenvironments and associate with cancer cells physically and biochemically during different stages of the disease. Existing cell culture models to study interactions between fibroblasts and cancer cells lack native tumor architecture or scalability. We developed a scalable organotypic model by robotically encapsulating a triple negative breast cancer (TNBC) cell spheroid within a natural extracellular matrix containing dispersed fibroblasts. We utilized an established CXCL12 - CXCR4 chemokine-receptor signaling in breast tumors to validate our model. Using imaging techniques and molecular analyses, we demonstrated that CXCL12-secreting fibroblasts have elevated activity of RhoA/ROCK/myosin light chain-2 pathway and rapidly and significantly contract collagen matrices. Signaling between TNBC cells and CXCL12-producing fibroblasts promoted matrix invasion of cancer cells by activating oncogenic mitogen-activated protein kinase signaling, whereas normal fibroblasts significantly diminished TNBC cell invasiveness. We demonstrated that disrupting CXCL12 - CXCR4 signaling using a molecular inhibitor significantly inhibited invasiveness of cancer cells, suggesting blocking of tumor-stromal interactions as a therapeutic strategy especially for cancers such as TNBC that lack targeted therapies. Our organotypic tumor model mimics native solid tumors, enables modular addition of different stromal cells and extracellular matrix proteins, and allows high throughput compound screening against tumor-stromal interactions to identify novel therapeutics.

Thermally-controlled extrusion-based bioprinting of collagen

Thermally-crosslinked hydrogels in bioprinting have gained increasing attention due to their ability to undergo tunable crosslinking by modulating the temperature and time of crosslinking. In this paper, we present a new bioink composed of collagen type-I and Pluronic® F-127 hydrogels, which was bioprinted using a thermally-controlled bioprinting unit. Bioprintability and rheology of the composite bioink was studied in a thorough manner in order to determine the optimal bioprinting time and extrusion profile of the bioink for fabrication of three-dimensional (3D) constructs, respectively. It was observed that collagen fibers aligned themselves along the directions of the printed filaments after bioprinting based on the results on an anisotropy study. Furthermore, rat bone marrow-derived stem cells (rBMSCs) were bioprinted in order to determine the effect of thermally-controlled extrusion process. In vitro viability and proliferation study revealed that rBMSCs were able to maintain their viability after extrusion and attached to collagen fibers, spread and proliferated within the constructs up to seven days of culture.

3D human skin bioprinting: a view from the bio side

Based on the 3D printing technologies and the concepts developed in tissue engineering during the last decades, 3D bioprinting is emerging as the most innovative and promising technology for the generation of human tissues and organs. In the case of skin bioprinting, thanks to the research process carried out during the last years, interfollicular skin has been printed with a structural and functional quality that paves the way for clinical and industrial applications. This review analyzes the present achievements and the future improvements that this area must bring about if bioprinted skin is to become widely used. We have made an effort to integrate the technological and the biological/biomedical sides of the subject.

Modelling The Combined Effects Of Collagen and Cyclic Strain On Cellular Orientation In Collagenous Tissues

Adherent cells are generally able to reorient in response to cyclic strain. In three-dimensional tissues, however, extracellular collagen can affect this cellular response. In this study, a computational model able to predict the combined effects of mechanical stimuli and collagen on cellular (re)orientation was developed. In particular, a recently proposed computational model (which only accounts for mechanical stimuli) was extended by considering two hypotheses on how collagen influences cellular (re)orientation: collagen contributes to cell alignment by providing topographical cues (contact guidance); or collagen causes a spatial obstruction for cellular reorientation (steric hindrance). In addition, we developed an evolution law to predict cell-induced collagen realignment. The hypotheses were tested by simulating bi- or uniaxially constrained cell-populated collagen gels with different collagen densities, subjected to immediate or delayed uniaxial cyclic strain with varying strain amplitudes. The simulation outcomes are in agreement with previous experimental reports. Taken together, our computational approach is a promising tool to understand and predict the remodeling of collagenous tissues, such as native or tissue-engineered arteries and heart valves.

A retinoic acid-dependent stroma-leukemia crosstalk promotes chronic lymphocytic leukemia progression

In chronic lymphocytic leukemia (CLL), the non-hematopoietic stromal microenvironment plays a critical role in promoting tumor cell recruitment, activation, survival, and expansion. However, the nature of the stromal cells and molecular pathways involved remain largely unknown. Here, we demonstrate that leukemic B lymphocytes induce the activation of retinoid acid synthesis and signaling in the microenvironment. Inhibition of RA-signaling in stromal cells causes deregulation of genes associated with adhesion, tissue organization and chemokine secretion including the B-cell chemokine CXCL13. Notably, reducing retinoic acid precursors from the diet or inhibiting RA-signaling through retinoid-antagonist therapy prolong survival by preventing dissemination of leukemia cells into lymphoid tissues. Furthermore, mouse and human leukemia cells could be distinguished from normal B-cells by their increased expression of Rarγ2 and RXRα, respectively. These findings establish a role for retinoids in murine CLL pathogenesis, and provide new therapeutic strategies to target the microenvironment and to control disease progression.

The stromal microenvironment plays a key role in the expansion of chronic lymphocytic leukemia. Here, the authors use the Eµ-TCL1 mouse model to show that leukemic B-cells induce the activation of retinoic acid synthesis in stromal cells of the lymphoid microenvironment, and that impacting on retinoic acid signalling via diet or chemical inhibition prolonged survival by preventing leukemia dissemination and accumulation in lymphoid tissues.

Effects of Type I Collagen Concentration in Hydrogel on the Growth and Phenotypic Expression of Rat Chondrocytes

It is controversial whether type I collagen itself can maintain and improve chondrogenic phenotype of chondrocytes in a three-dimensional (3D) environment. In this study, we examined the effect of type I collagen concentration in hydrogel (0.5, 1, and 2 mg/ml) on the growth and phenotype expression of rat chondrocytes in vitro. All collagen hydrogels showed substantial contractions during culture, in a concentration-dependent manner, which was due to the cell proliferation. The cell viability was shown to be the highest in 2 mg/ml collagen gel. The mRNA expression of chondrogenic phenotypes, including SOX9, type II collagen, and aggrecan, was significantly up-regulated, particularly in 1 mg/ml collagen gel. Furthermore, the production of type II collagen and glycosaminoglycan (GAG) content was also enhanced. The results suggest that type I collagen hydrogel is not detrimental to, but may be useful for, the chondrocyte culture for cartilage tissue engineering.

Effect of Concentration of Collagen Gel on Functional Activity of Bone Marrow Mesenchymal Stromal Cells

Collagen I gels with protein concentrations of 1, 2, and 3.5 mg/ml were prepared and embedded in a porous polylactide scaffold to reduce their contraction. Concentration of the gel did not affect its degradation. Collagen gels promoted the formation of cell networks. The cells in the collagen gel with a concentration of 1 mg/ml embedded in polylactide scaffold had elongated spindle-like shape, in contrast to flattened cells in collagen gel of the same concentration not embedded in the scaffold. Stabilization of the collagen gel in the polylactide scaffold promoted active synthesis of laminin and fibronectin by cells as soon as on day 5 of culturing in comparison with that in free collagen substrate.

Three-Dimensional Assay for Studying Breast Cancer Cell Invasion

Cancer cell invasion is a complex process that naturally occurs in a three-dimensional (3-D) environment comprised of tumor cells and extracellular matrix components (ECM). Therefore, examining the invasive ability of breast cancer cells in a 3-D assay is imperative to discovering novel treatment strategies aimed at preventing cancer invasion and metastasis. Here, I describe a method to quantitatively measure the number of invaded cancer cells within a 3-D microenvironment and determine the effects of potential drugs on this cellular process.

A novel 3-dimensional approach for cardiac regeneration

Ischemic heart diseases, such as coronary artery disease and microvascular disease, are cardiovascular pathologies that cause reduced blood supply to the heart muscle. Acute and chronic ischemia cause cardiomyocytes to die, and these cells are not naturally replaced as part of the wound healing process in the heart. To promote neovascularization in the wound bed and in implanted engineered tissues, we have developed a collagen-alginate microspheres scaffold intended for local release of drugs and growth factors in order to recruit host endothelial cells to the area and provide them with geometrical cues to form new vessels. Optimization of alginate microspheres included modulation of nitrogen pressure, alginate and CaCl2 concentrations, nozzle size, and velocity of extrusion to achieve monodisperse populations of 100 μm diameter microspheres with protein release over 3 days. In vitro incorporation of fibroblasts in the bulk collagen demonstrated cellular compatibility with embedded alginate microspheres. An in vitro vessel formation assay, performed with human umbilical vein endothelial cells (HUVECs) immobilized in the collagen phase of the collagen-alginate microspheres scaffolds, showed that HUVECs formed networks following the 3-dimensional pattern of the microspheres even in the absence of growth factor. Implantation of acellular collagen-alginate microspheres scaffolds onto healthy rat hearts confirmed the invasion of host cells at one week. Together, these results suggest that the collagen-alginate microspheres scaffold is a viable, tunable therapeutic approach for directing neovascularization in engineered tissues and in the heart after ischemic events.

Evaluation of cell and matrix mechanics using fluorescence excitation spectroscopy: Feasibility study in collagen gels containing fibroblasts

BACKGROUND AND OBJECTIVE

Collagen gels containing cells are commonly used in tissue engineering, wound healing, and cancer research to investigate the interplay between cells and the extracellular matrix (ECM), as changes in the density and stiffness of the microenvironment are known to play a role in many diseases or pathological conditions. In these gels, the stiffness is regularly determined using destructive methods, such as indentation and tensile tests. Certain molecules native to cells and the ECM display fluorescence upon irradiation with ultraviolet light. The objective of the present study was to investigate the feasibility of using the endogenous, or innate, fluorescence of collagen gels containing fibroblasts as an optical marker to evaluate changes in the mechanical properties of the ECM.

MATERIALS AND METHODS

Human foreskin fibroblasts cells at concentrations of 50,000 and 100,000 cells/ml were cultured in three-dimensional gels of type I collagen for 16 days. Fibroblast cells remodeled the ECM, contracting and increasing the stiffness of the gel. During this remodeling process, changes in mechanical properties and fluorescence were measured with an indentation test and a spectrofluorometer, respectively. Force and displacement measurements from the indentation test were used to calculate the elastic modulus of the gel. Maps of fluorescence intensity, at excitation/emission of 240-520/290-530 nm, were used to identify the wavelengths of interest.

RESULTS

Fluorescence excitation/emission maps exhibited two distinct excitation/emission bands whose intensities increased as the fibroblasts remodeled and increased the stiffness of the ECM: The 290/340 nm band ascribed to tryptophan and the 330/390 nm band ascribed to cross-links of collagen (pepsin-digestible cross-links). A Spearman correlation analysis, between the elastic modulus of the gel containing fibroblasts and the fluorescence of cross-links of collagen, resulted in R = 0.95 (P < 0.05) and R = 0.77 (P = 0.12) for 50,000 and 100,000 cells/ml, respectively.

CONCLUSIONS

The endogenous fluorescence intensity ascribed to pepsin-digestible cross-links of collagen may serve as an optical marker to evaluate changes in the mechanical properties of the ECM; this is relevant to collagenous tissues for which pathological states are related to mechanical alterations, such as keratoconus in cornea and osteoarthritis in articular cartilage.

A novel collagen gel-based measurement technique for quantitation of cell contraction force

Cell contraction force plays an important role in wound healing, inflammation,angiogenesis and metastasis. This study describes a novel method to quantify single cell contraction force in vitro using human aortic adventitial fibroblasts embedded in a collagen gel. The technique is based on a depth sensing nano-indentation tester to measure the thickness and elasticity of collagen gels containing stimulated fibroblasts and a microscopy imaging system to estimate the gel area. In parallel, a simple theoretical model has been developed to calculate cell contraction force based on the measured parameters. Histamine (100 mM) was used to stimulate fibroblast contraction while the myosin light chain kinase inhibitor ML-7 (25 mM) was used to inhibit cell contraction. The collagen matrix used in the model provides a physiological environment for fibroblast contraction studies. Measurement of changes in collagen gel elasticity and thickness arising from histamine treatments provides a novel convenient technique to measure cell contraction force within a collagen matrix. This study demonstrates that histamine can elicit a significant increase in contraction force of fibroblasts embedded in collagen,while the Young's modulus of the gel decreases due to the gel degradation.

A Novel Collagen Dot Assay for Monitoring Cancer Cell Migration

Cell migration is a critical determinant of cancer invasion and metastasis. Drugs targeting cancer cell migration have been hindered due to the lack of effective assays for monitoring cancer cell migration. Here we describe a novel method to microscopically monitor cell migration in a quantitative fashion. This assay can be used to study genes involved in cancer cell migration, as well as screening anticancer drugs that target this cellular process.

Optimising contraction and alignment of cellular collagen hydrogels to achieve reliable and consistent engineered anisotropic tissue

Engineered anisotropic tissue constructs containing aligned cell and extracellular matrix structures are useful as in vitro models and for regenerative medicine. They are of particular interest for nervous system modelling and regeneration, where tracts of aligned neurons and glia are required. The self-alignment of cells and matrix due to tension within tethered collagen gels is a useful tool for generating anisotropic tissues, but requires an optimal balance between cell density, matrix concentration and time to be achieved for each specific cell type. The aim of this study was to develop an assay system based on contraction of free-floating cellular gels in 96-well plates that could be used to investigate cell–matrix interactions and to establish optimal parameters for subsequent self-alignment of cells in tethered gels. Using C6 glioma cells, the relationship between contraction and alignment was established, with 60–80% contraction in the 96-well plate assay corresponding to alignment throughout tethered gels made using the same parameters. The assay system was used to investigate the effect of C6 cell density, collagen concentration and time. It was also used to show that blocking α1 integrin reduced the contraction and self-alignment of these cells, whereas blocking α2 integrin had little effect. The approach was validated by using primary astrocytes in the assay system under culture conditions that modified their ability to contract collagen gels. This detailed investigation describes a robust assay for optimising cellular self-alignment and provides a useful reference framework for future development of self-aligned artificial tissue.

Tissue Transglutaminase, Not Lysyl Oxidase, Dominates Early Calcium-Dependent Remodeling of Fibroblast-Populated Collagen Lattices

Cell-populated collagen gels have provided significant insight into the cellular contractile mechanisms and cell-matrix interactions that are necessary for compacting and remodeling extant matrix. Nevertheless, little research has been devoted towards determining how cells entrench these deformations that contribute to establishing a preferred mechanical state. To this end, we examined the roles of two covalent matrix cross-linkers, i.e. tissue transglutaminase and lysyl oxidase, during global remodeling of the free-floating fibroblast-populated collagen lattice. Inhibition of tissue transglutaminase resulted in a reduced rate of compaction compared to controls during early remodeling (up to 2 days). In contrast, inhibition of lysyl oxidase did not alter the early compaction of these lattices, but it reduced the compaction after 2 days of culture. Acute inhibition of different contractile mechanisms suggested further that calcium-dependent contractility may have dominated during the initial remodeling of the collagen lattice before giving way to calcium-independent contractility at later times. In summary, these findings suggest that early remodeling of the free-floating collagen lattice is facilitated by calcium-dependent cell contraction while entrenchment is dominated by a tissue transglutaminase-mediated cross-linking of the extant matrix. As remodeling continues, however, lysyl oxidase increases its contribution, perhaps by consolidating de novo collagen fibrils into fibers to continue the remodeling while the cells transition to a more sustained, calcium-independent contractility. These results promise to influence future tissue engineering studies as well as computational simulations aimed at understanding matrix remodeling in complex in vivo situations.

Culturing fibroblasts in 3D human hair keratin hydrogels

Human hair keratins are readily available, easy to extract, and eco-friendly materials with natural bioactivities. Keratin-based materials have been studied for applications such as cell culture substrates, internal hemostats for liver injury, and conduits for peripheral nerve repair. However, there are limited reports of using keratin-based 3D scaffolds for cell culture in vitro. Here, we describe the development of a 3D hair keratin hydrogel, which allows for living cell encapsulation under near physiological conditions. The convenience of making the hydrogels from keratin solutions in a simple and controllable manner is demonstrated, giving rise to constructs with tunable physical properties. This keratin hydrogel is comparable to collagen hydrogels in supporting the viability and proliferation of L929 murine fibroblasts. Notably, the keratin hydrogels contract less significantly as compared to the collagen hydrogels, over a 16-day culture period. In addition, preliminary in vivo studies in immunocompetent animals show mild acute host tissue response. These results collectively demonstrate the potential of cell-loaded keratin hydrogels as 3D cell culture systems, which may be developed for clinically relevant applications.

Collagen matrix as a tool in studying fibroblastic cell behavior

Type I collagen is a fibrillar protein, a member of a large family of collagen proteins. It is present in most body tissues, usually in combination with other collagens and other components of extracellular matrix. Its synthesis is increased in various pathological situations, in healing wounds, in fibrotic tissues and in many tumors. After extraction from collagen-rich tissues it is widely used in studies of cell behavior, especially those of fibroblasts and myofibroblasts. Cells cultured in a classical way, on planar plastic dishes, lack the third dimension that is characteristic of body tissues. Collagen I forms gel at neutral pH and may become a basis of a 3D matrix that better mimics conditions in tissue than plastic dishes.

Chondrocytes behaviors within type I collagen microspheres and bulk hydrogels: an in vitro study

Cell niche, which is considered to be critical to the proliferation and differentiation of cells, is one of the most important aspects for the design and development of ideal scaffolds in tissue engineering.

Development of a high-throughput three-dimensional invasion assay for anti-cancer drug discovery

The lack of three-dimensional (3-D) high-throughput (HT) screening assays designed to identify anti-cancer invasion drugs is a major hurdle in reducing cancer-related mortality, with the key challenge being assay standardization. Presented is the development of a novel 3-D invasion assay with HT potential that involves surrounding cell-collagen spheres within collagen to create a 3-D environment through which cells can invade. Standardization was achieved by designing a tooled 96-well plate to create a precisely designated location for the cell-collagen spheres and by using dialdehyde dextran to inhibit collagen contraction, maintaining uniform size and shape. This permits automated readout for determination of the effect of inhibitory compounds on cancer cell invasion. Sensitivity was demonstrated by the ability to distinguish varying levels of invasiveness of cancer cell lines, and robustness was determined by calculating the Z-factor. A Z-factor of 0.65 was obtained by comparing the effects of DMSO and anti-β1-integrin antibody, an inhibitory reagent, on the invasion of Du145 cancer cells, suggesting this novel assay is suitable for large scale drug discovery. As proof of principle, the NCI Diversity Compound Library was screened against human invasive cancer cells. Nine compounds exhibiting high potency and low toxicity were identified, including DX-52-1, a compound previously reported to inhibit cell migration, a critical determinant of cancer invasion. The results indicate that this innovative HT platform is a simple, precise, and easy to replicate 3-D invasion assay for anti-cancer drug discovery.

Measurement of biomolecular diffusion in extracellular matrix condensed by fibroblasts using fluorescence correlation spectroscopy

The extracellular matrix (ECM) comprises the heterogeneous environment outside of cells in a biological system. The ECM is dynamically organized and regulated, and many biomolecules secreted from cells diffuse throughout the ECM, regulating a variety of cellular processes. Therefore, investigation of the diffusive behaviors of biomolecules in the extracellular environment is critical. In this study, we investigated the diffusion coefficients of biomolecules of various sizes, measuring from 1 to 10 nm in radius, by fluorescence correlation spectroscopy in contracted collagen gel caused by fibroblasts, a traditional culture model of dynamic rearrangement of collagen fibers. The diffusion coefficients of the biomolecules in control collagen gel without cells decreased slightly as compared to those in solution, while the diffusion coefficients of biomolecules in the contracted gel at the cell vicinity decreased dramatically. Additionally, the diffusion coefficients of biomolecules were inversely correlated with molecular radius. In collagen gels populated with fibroblasts, the diffusion coefficient at the cell vicinity clearly decreased in the first 24 h of culture. Furthermore, molecular diffusion was greatly restricted, with a central focus on the populated cells. By using the obtained diffusion coefficients of biomolecules, we calculated the collagen fiber condensation ratio by fibroblasts in the cell vicinity at 3 days of culture to represent a 52-fold concentration. Thus, biomolecular diffusion is restricted in the vicinity of the cells where collagen fibers are highly condensed.

Estradiol and tamoxifen enhance invasion of endometrial stromal cells in a three-dimensional coculture model of adenomyosis

OBJECTIVE

To examine the effect of estradiol alone or with progesterone and tamoxifen on the depth of invasion of endometrial stromal cells from women with and without adenomyosis in a three-dimensional (3D) coculture model that includes myocytes.

DESIGN

Case-controlled, blinded comparison.

SETTING

Medical school department.

PATIENT(S)

Premenopausal women with and without uterine adenomyosis.

INTERVENTION(S)

Human endometrial stromal and myometrial cells grown in a 3D coculture with crossover between cells from uteri with and without adenomyosis; cocultures treated with tamoxifen, estradiol alone, or estradiol with progesterone.

MAIN OUTCOME MEASURE(S)

Depth of stromal cell invasion into a collagen matrix.

RESULT(S)

The depth of invasion for adenomyotic stromal cells was statistically significantly higher than for the control stromal cells, whether grown on plain collagen, on collagen containing control or adenomyotic muscle cells. The addition of estradiol or tamoxifen, but not the estradiol and progesterone combination, increased the depth of invasion of both adenomyotic stromal cells and control stromal cells in all cell combinations. When grown on plain collagen, the depth of invasion for control stromal cells and adenomyotic stromal cells increased by 126% and 93% with the use of tamoxifen, and by 71% and 50%, with the use of estradiol.

CONCLUSION(S)

Both estradiol and tamoxifen enhance stromal cell invasion, but the greater depth of invasion of adenomyotic stromal cells and the enhancing effect of adenomyotic muscle were maintained under all experimental conditions, suggesting an inherent predisposition in affected women.