A new in vitro wound model based on the co-culture of human dermal microvascular endothelial cells and human dermal fibroblasts

Comparison of three in vitro keratinocytes-fibroblasts wound healing models commonly used in pharmaceutical research

OBJECTIVES

Several common wound healing models have been used to evaluate wound healing agents and formulations, namely: conditioned media (CM), transwell co-cultures (TWCC) and co-cultures (CC) in a monolayer. However, no study has been conducted to compare the relevance of these models in the keratinocytes and fibroblasts interaction physiologically. Therefore, this study aimed to compare these models based on cell migration and proliferation, and matrix metalloproteinase (MMP) expression.

METHODS

Cell migration was analysed by scratch assay and MMP-7, while cell proliferation was analysed by (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) tetrazolium reduction assay.

KEY FINDINGS

Increased cell migration was observed in CM and TWCC models, while varied results were obtained in CC. Cell migration was increased due to upregulation of MMP-7 in CM and TWCC models, while it was downregulated in CC, which might have hindered migration of both cells in monolayers.

CONCLUSIONS

CM and TWCC are more suitable than CC for wound healing research and for evaluating wound healing agents or formulations, as they can better simulate the layered tissue constructs and paracrine interactions in the physiological environment.

Collagen gel contraction assays: From modelling wound healing to quantifying cellular interactions with three-dimensional extracellular matrices

Cells respond to and actively remodel the extracellular matrix (ECM). The dynamic and bidirectional interaction between cells and ECM, especially their mechanical interactions, has been found to play an essential role in triggering a series of complex biochemical and biomechanical signal pathways and in regulating cellular functions and behaviours. The collagen gel contraction assay (CGCA) is a widely used method to investigate cell-ECM interactions in 3D environments and provides a mechanically associated readout reflecting 3D cellular contractility. In this review, we summarize various versions of CGCA, with an emphasis on recent high-throughput and low-consumption CGCA techniques. More importantly, we focus on the technique of force monitoring during the contraction of collagen gel, which provides a quantitative characterization of the overall forces generated by all the resident cells in the collagen hydrogel. Accordingly, we present recent biological applications of the CGCA, which have expanded from the initial wound healing model to other studies concerning cell-ECM interactions, including fibrosis, cancer, tissue repair and the preparation of biomimetic microtissues.

Dose-dependent dominance: How cell densities design stromal cell functions during soft tissue healing

Regular soft tissue healing relies on the well-organized interaction of different stromal cell types with endothelial cells. However, spatiotemporal conditions might provoke high densities of one special stromal cell type, potentially leading to impaired healing. Detailed knowledge of the functions of rivaling stromal cell types aiming for tissue contraction and stabilization as well as vascular support is mandatory. By the application of an in vitro approach comprising the evaluation of cell proliferation, cell morphology, myofibroblastoid differentiation, and cytokine release, we verified a density-dependent modulation of these functions among juvenile and adult fibroblasts, pericytes, and adipose-derived stem cells during their interaction with microvascular endothelial cells in cocultures. Results indicate that juvenile fibroblasts rather support angiogenesis via paracrine regulation at the early stage of healing, a role potentially compromised in adult fibroblasts. In contrast, pericytes showed a more versatile character aiming at angiogenesis, vessel stabilization, and tissue contraction. Such a universal character was even more pronounced among adipose-derived stem cells. The explicit knowledge of the characteristic functions of stromal cell types is a prerequisite for the development of new analytical and therapeutic approaches for impaired soft tissue healing. The present study delivers new considerations concerning the roles of rivaling stromal cell types within a granulation tissue, pointing to extraordinary properties of pericytes and adipose-derived stem cells.

Bioengineered Wound Healing Skin Models: The Role of Immune Response and Endogenous ECM to Fully Replicate the Dynamic of Scar Tissue Formation In Vitro

The healing of deep skin wounds is a complex phenomenon evolving according with a fine spatiotemporal regulation of different biological events (hemostasis, inflammation, proliferation, remodeling). Due to the spontaneous evolution of damaged human dermis toward a fibrotic scar, the treatment of deep wounds still represents a clinical concern. Bioengineered full-thickness skin models may play a crucial role in this direction by providing a deep understanding of the process that leads to the formation of fibrotic scars. This will allow (i) to identify new drugs and targets/biomarkers, (ii) to test new therapeutic approaches, and (iii) to develop more accurate in silico models, with the final aim to guide the closure process toward a scar-free closure and, in a more general sense, (iv) to understand the mechanisms involved in the intrinsic and extrinsic aging of the skin. In this work, the complex dynamic of events underlaying the closure of deep skin wound is presented and the engineered models that aim at replicating such complex phenomenon are reviewed. Despite the complexity of the cellular and extracellular events occurring during the skin wound healing the gold standard assay used to replicate such a process is still represented by planar in vitro models that have been largely used to identify the key factors regulating the involved cellular processes. However, the lack of the main constituents of the extracellular matrix (ECM) makes these over-simplistic 2D models unable to predict the complexity of the closure process. Three-dimensional bioengineered models, which aim at recreating the closure dynamics of the human dermis by using exogenous biomaterials, have been developed to fill such a gap. Although interesting mechanistic effects have been figured out, the effect of the inflammatory response on the ECM remodelling is not replicated yet. We discuss how more faithful wound healing models can be obtained by creating immunocompetent 3D dermis models featuring an endogenous ECM.

Strategies to Induce Blood Vessel Ingrowth into Skin Grafts and Tissue-Engineered Substitutes

Skin is a multilayer organ consisting of several tissues and appendages residing in a complex niche. Adequate and physiologically regulated vascularization is an absolute requirement for skin homeostasis, regeneration, and wound healing. The lack of vascular networks and ischemia results in delayed wound closure. In addition, vascularization is critical for the prolonged function and survival of skin grafts and tissue-engineered skin substitutes. This study highlights the clinical challenges associated with the limited vascularization in the cutaneous wounds. Then, we highlight the novel approaches for the development of vascular networks in the skin autografts, allografts, and artificial substitutes. Also, the future directions to overcome the existing vascularization complications in skin grafting and synthetic skin substitutes are presented. Statement of Significance Delayed closure of large dermal wounds, such as burn injuries, results from the lack of vascular networks and ischemia. The amount of blood supply in the skin graft is the primary factor determining the quality of the transplanted grafts. The current skin grafts and their fabrication methods lack the appropriate features that contribute to the vascularization and integration of the wound bed and graft and adherence to the skin layers. Therefore, the new generation of skin grafts should consider advanced technologies to induce vascularization and overcome current challenges.

Apoptotic bodies extracted from adipose mesenchymal stem cells carry microRNA-21-5p to induce M2 polarization of macrophages and augment skin wound healing by targeting KLF6

BACKGROUND

Adipose-derived mesenchymal stem cells (adMSCs) are suggested as potential tools for the treatment of regenerative diseases, including tissue repair. This study aimed to explore the function of adMSC-derived apoptotic bodies in skin wound healing and the molecules of action.

METHODS

The acquired adMSCs and their-derived apoptotic bodies were identified. A murine model of full-thickness skin wounds was treated with apoptotic bodies. The wound healing process of mice and the pathological changes in wound tissues were examined. Ana-1 macrophages were treated with lipopolysaccharide (LPS) and apoptotic bodies for in vitro experiments. Polarization of macrophages was examined by immunofluorescence staining of the specific biomarkers and ELISA kits. Dermal microvascular endothelial cells (DMECs) or dermal fibroblasts (DFs) were co-cultured with apoptotic bodies or the LPS- and apoptotic bodies-treated Ana-1 cells. Downstream molecules mediated by apoptotic bodies were screened by microarray and bioinformatic analyses.

RESULTS

Apoptotic bodies treatment accelerated skin wound healing in mice and promoted formation of granulation tissues and blood vessels in wound tissues. Apoptotic bodies treatment induced M2 polarization of macrophages. The angiogenesis ability of DMECs, and the viability and migration of DFs were increased when co-cultured with the apoptotic bodies-treated Ana-1 cells. MicroRNA (miR)-21-5p was abundantly expressed in ABs, and kruppel like factor 6 (KLF6) mRNA was confirmed as a target of miR-21-5p. Overexpression of KLF6 reduced M2 polarization of macrophages and blocked the promoting effect of apoptotic bodies on wound healing in vitro and in vivo.

CONCLUSION

miR-21-5p carried by adMSC-derived apoptotic bodies targets KLF6 to induce M2 polarization of macrophages and augment skin wound healing.

Peptide nanogels as a scaffold for fabricating dermal grafts and 3D vascularized skin models

Millions of people worldwide suffer from skin injuries, which create significant problems in their lives and are costly to cure. Tissue engineering is a promising approach that aims to fabricate functional organs using biocompatible scaffolds. We designed ultrashort tetrameric peptides with promising properties required for skin tissue engineering. Our work aimed to test the efficacy of these scaffolds for the fabrication of dermal grafts and 3D vascularized skin tissue models. We found that the direct contact of keratinocytes and fibroblasts enhanced the proliferation of the keratinocytes. Moreover, the expression levels of TGF-β1, b-FGF, IL-6, and IL-1α is correlated with the growth of the fibroblasts and keratinocytes in the co-culture. Furthermore, we successfully produced a 3D vascularized skin co-culture model using these peptide scaffolds. We believe that the described results represent an advancement in the fabrication of skin tissue equivalent, thereby providing the opportunity to rebuild missing, failing, or damaged parts.

Peptide-modified substrate enhances cell migration and migrasome formation

Extracellular vesicles (EVs) are cell-to-cell communication tools. Migrasomes are recently discovered microscale EVs formed at the rear ends of migrating cells, and thus are suggested to be involved in communicating with neighboring cells. In cell culture, peptide scaffolds on substrates have been used to demonstrate cellular function for regenerative medicine. In this study, we evaluated peptide scaffolds, including cell penetrating, virus fusion, and integrin-binding peptides, for their potential to enable the formation of migrasome-like vesicles. Through structural and functional analyses, we confirmed that the EVs formed on these peptide-modified substrates were migrasomes. We further noted that the peptide interface comprising cell-penetrating peptides (pVEC and R9) and virus fusion peptide (SIV) have superior properties for enabling cell migration and migrasome formation than fibronectin protein, integrin-binding peptide (RGD), or bare substrate. This is the first report of migrasome formation on peptide-modified substrates. Additionally, the combination of 95% RGD and 5% pVEC peptides provided a functional interface for effective migrasome formation and desorption of cells from the substrate via a simple ethylenediaminetetraacetic acid treatment. These results provide a functional substrate for the enhancement of migrasome formation and functional analysis.

Co-culture Model for Cutaneous Wound Healing to Assess a Porous Fiber-Based Drug Delivery System

In vitro tissue-engineered cell culture models are an essential instrument to investigate physiological and pathophysiological wound healing mechanisms and to evaluate new beneficial wound dressing materials and therapeutics to identify possible drug targets and to improve regeneration processes in nonhealing and chronic wounds. In this study, the authors established an in vitro model for cutaneous wound healing, based on primary human dermal microvascular endothelial cells (HDMEC) and primary human dermal fibroblasts (HDF) to study wound healing-associated processes. Co-cultivation of HDMEC and HDF results in the formation of microvessel-like structures in long-term co-cultures. The proposed in vitro co-culture model can be easily modified by adding macrophages to simulate the process of inflammation, thus allowing in vitro investigation of pathophysiological wound healing processes present in nonhealing wounds. Furthermore, the beneficial in vitro wound healing model was used to evaluate a porous fiber-based drug delivery dressing material consisting of melt-spun porous fibers that were filled with a hydrogel carrier (gellan gum) containing vascular endothelial growth factor (VEGF). Angiogenic capability was chosen as functional parameter for improved wound healing, and release of deposited VEGF from the dressing material was evaluated up to 7 days of cultivation. The experiments demonstrated that the porous fiber-based drug delivery dressing material for dermal wound healing with incorporated VEGF strongly enhances the process of angiogenesis in the in vitro co-culture model through a release of VEGF over 7 days of cultivation. In conclusion, tissue-engineered human skin equivalents could contribute significantly to the understanding and improvement of drug releasing dressing materials in the context of treating chronic wounds.

Interacting adipose-derived stem cells and microvascular endothelial cells provide a beneficial milieu for soft tissue healing

There is growing evidence suggesting that healing of chronic soft tissue wounds profits from the presence of adipose-derived stem cells (ADSC). Among the large spectrum of mechanisms by which ADSC might act, especially the interaction with the microvascular endothelial cell, a main player during angiogenesis, is of special interest. In the present 2D model on the basis of endothelial cell ADSC co-cultures, we focused on the identification of characteristics of both cell types in response to a typical condition in acute and chronic wounds: hypoxia. Parameters like proliferation capacity, migration, myofibroblastoid differentiation of ADSC and the quantification of important paracrine factors related to angiogenesis and inflammation were used to correlate our experimental model with the in vivo situation of soft tissue healing. ADSC were not negatively affected by hypoxia in terms of proliferation, referring to their excellent hypoxia tolerance. Myofibroblastoid differentiation among ADSC was enhanced by hypoxia in mono- but not in co-culture. Furthermore, co-cultures were able to migrate under hypoxia. These effects might be caused to some extent by the distinct milieu created by interacting ADSC and endothelial cells, which was characterized by modulated levels of interleukin-6, interleukin-8, monocyte chemoattractant protein-1 and vascular endothelial growth factor. The identification of these cell characteristics in the present 2D in vitro model provide new insights into the process of human soft tissue healing, and underpin a beneficial role of ADSC by regulating inflammation and angiogenesis.

Observation of co-culturing cells on porous silk fibroin films

Porous silk fibroin films (PSFFs) are widely used in skin regeneration. Prevascularization of PSFFs is a successful strategy for enhancing the survival of engineered tissues in vivo. The adhesion and migration of human umbilical vein endothelial cells (HUVECs) and fibroblasts on PSFFs were observed by scanning electron microscopy and confocal laser scanning microscopy after vital staining of the cells. PSFFs could attract a large number of HUVECs and fibroblasts to pores in an interesting alignment and support them to spread well from the outside of pores to the center of PSFFs to form cell layers. PSFFs showed minor structural changes due to less degradation for 12 d culture, and cell layers overlapped from pores to the center in PSFFs. The total DNA assay indicated excellent cell proliferation on PSFFs on days 1 and 6 and no difference between co-culturing HUVECs and mono-HUVECs and monofibroblasts. PSFFs could guide cell migration and arrangement to form pore-centered and vessel-like structures without additional coating of proteins. The authors’ aim was to study the potential prevascularization of porous silk fibroin scaffolds by co-culturing HUVECs and fibroblasts. This model has potential application for angiogenesis in dermis regeneration in the future and provides a suitable microenvironment for the development of capillary-like structures.

Endothelial calpain systems orchestrate myofibroblast differentiation during wound healing

The transformation of fibroblasts to myofibroblasts plays a major role in fibrogenic responses during dermal wound healing. We show a contribution of calpain systems (intracellular regulatory protease systems) in vascular endothelial cells (ECs) to myofibroblast differentiation in wound sites. Dermal wound healing experiments in mice found that calpastatin (an endogenous inhibitor of calpains) is enriched in preexisting vessels but not in newly formed capillaries. Transgenic overexpression of calpastatin in ECs delayed wound healing in mice as well as reducing the keratinocyte layer, extracellular matrix deposition, and myofibroblast accumulation in wound sites. EC and leukocyte markers, however, remain unchanged. Calpastatin overexpression reduced the expression of genes encoding platelet-derived growth factor-B and PDGF receptor-β (PDGFR-β). Topical application of platelet-derived growth factor-BB-containing ointment to wounds accelerated healing in control mice, but calpastatin overexpression prevented this acceleration. In cultured human dermal fibroblasts, α-smooth muscle actin and PDGFR-β were up-regulated by coculturing with ECs, but this action was inhibited by suppression of EC calpain activity. EC-driven transformation of mouse dermal fibroblasts was also suppressed by calpastatin overexpression in ECs. These results suggest that endothelial calpain systems influence PDGFR-β signaling in fibroblasts, EC-driven myofibroblast differentiation, and subsequent fibrogenic responses in wounds.-Miyazaki, T., Haraguchi, S., Kim-Kaneyama, J.-R., Miyazaki, A. Endothelial calpain systems orchestrate myofibroblast differentiation during wound healing.

Development of a novel in situ gelling skin dressing: Delivering high levels of dissolved oxygen at pH 5.5

BACKGROUND AND AIMS

Wound healing requires appropriate oxygen and pH levels. Oxygen therapy and pH-modulating treatments have shown positive effects on wound healing. Thus, a dressing, which combines high levels of dissolved oxygen (DO) with the pH of intact skin, may improve wound healing. Our aims were to (1) formulate an in situ gelling dressing with high levels of DO and with the pH level of intact skin, (2) evaluate physical and chemical properties of the dressing, and (3) elucidate basic effects of elevated levels of DO on human skin cells in vitro.

METHODS

A dressing was formulated with 15 to 16 wt% poloxamer 407, acetate buffer, and oxygenated water. Stability of pH and DO, rheology, and shelf life were analysed. Furthermore, in vitro studies of the effect of increased levels of DO were performed.

RESULTS

An in situ gelling wound dressing, with a DO concentration ranging between 25 and 35 mg/L and a pH of 5.5, was formulated. The DO concentration was stable above 22 mg/L for at least 30 hours when applied on a surface at 35°C and covered for directed diffusion into the intended wound area. At storage, the dressing had stable pH for 3 months and stable DO concentration over 30 mg/L for 7 weeks. Increasing DO significantly enhanced intracellular ATP in human skin cells, without changing reactive oxygen species production, proliferation rate, or viability.

CONCLUSION

The developed dressing may facilitate wound healing by delivering controlled and stable oxygen levels, providing adjustable pH for optimized healing, and increasing intracellular ATP availability.

Studies on wound healing potential of polyherbal formulation using in vitro and in vivo assays

Background

The use of herbal plant extracts in wound healing is known through decades, but it is necessary to provide scientific data through reverse pharmacology.

Objective

The aim of the present study is to find the mechanism behind the healing of wounds using in vitro and in vivo assays.

Material and methods

The study was designed to determine proliferation and mobilization of fibroblast and keratinocytes at the site of injury, angiogenesis at the site of healing and reduction in oxidative stress while healing. In our earlier studies it was observed that herbal extract of Vitex negundo L. (VN), Emblica officinalis Gaertn (EO), and Tridax procumbens L. (TP) showed rapid regeneration of skin, wound contraction and collagen synthesis at the site of injury in excision wound model. In the present study the cell mobilization was monitored in the scratch assay on L929 fibroblastic cell line and HaCaT keratinocytes cell line under the influence of aqueous plant extracts and its formulation. This formulation was also assessed for its angiogenic potential using CAM assay. Study was carried out to probe synergistic effect of polyherbal formulation using excision model in rat.

Results

The formulation was found to contain high amount of flavonoids, tannins and phenols which facilitate wound healing. At 20 μg/ml concentration of formulation, significant increase in tertiary and quaternary vessels were observed due to angiogenic potential of formulation. Formulation at the concentration of 3 μg/ml and 5 μg/ml showed significant mobilization of keratinocytes and fibroblasts respectively at the site of injury. Polyherbal formulation showed rapid regeneration of skin and wound contraction. Biochemical parameters like hydroxyproline, hexosamine and collagen turnover was increased in test drug treated animals as compared to untreated, whereas antioxidants such as catalase and GSH were increased significantly and decreased amount of tissue MDA was observed.

Conclusion

Polyherbal formulation prepared from the plant extracts accelerates wound healing process by proliferation and mobilization of fibroblast and keratinocytes, and angiogenesis at the site of injury. It also shows fast contraction of wound with its beneficial improvement in tissue biochemical and antioxidant parameters.

On-chip assay of the effect of topographical microenvironment on cell growth and cell-cell interactions during wound healing

Wound healing is an essential physiological process for tissue homeostasis, involving multiple types of cells, extracellular matrices, and growth factor/chemokine interactions. Many in vitro studies have investigated the interactions between cues mentioned above; however, most of them only focused on a single factor. In the present study, we design a wound healing device to recapitulate in vivo complex microenvironments and heterogeneous cell situations to investigate how three types of physiologically related cells interact with their microenvironments around and with each other during a wound healing process. Briefly, a microfluidic device with a micropillar substrate, where diameter and interspacing can be tuned to mimic the topographical features of the 3D extracellular matrix, was designed to perform positional cell loading on the micropillar substrate, co-culture of three types of physiologically related cells, keratinocytes, dermal fibroblasts, and human umbilical vein endothelial cells, as well as an investigation of their interactions during wound healing. The result showed that cell attachment, morphology, cytoskeleton distribution, and nucleus shape were strongly affected by the micropillars, and these cells showed collaborative response to heal the wound. Taken together, these findings highlight the dynamic relationship between cells and their microenvironments. Also, this reproducible device may facilitate the in vitro investigation of numerous physiological and pathological processes such as cancer metastasis, angiogenesis, and tissue engineering.

In vitro analysis of the effects on wound healing of high- and low-molecular weight chains of hyaluronan and their hybrid H-HA/L-HA complexes

BACKGROUND

Recent studies have reported the roles of Hyaluronic acid (HA) chains of diverse length in wound repair, especially considering the simultaneous occurrence in vivo of both high- (H-HA) and low-molecular weight (L-HA) hyaluronan at an injury site. It has been shown that HA fragments (5 ≤ MW ≤ 20 kDa) usually trigger an inflammatory response that, on one hand, is the first signal in the activation of a repair mechanism but on the other, when it's overexpressed, it may promote unwanted side effects. The present experimental research has aimed to investigate H-HA, L-HA and of a newly developed complex of the two (H-HA/L-HA) for stability (e.g. hyaluronidases digestion), for their ability to promote wound healing of human keratinocytes in vitro and for their effect on cellular biomarker expression trends.

RESULTS

Time-lapse video microscopy studies proved that the diverse HA was capable of restoring the monolayer integrity of HaCat. The H-HA/L-HA complex (0.1 and 1%w/v) proved faster in regeneration also in co-culture scratch test where wound closure was achieved in half the time of H-HA stimulated cells and 2.5-fold faster than the control. Gene expression was evaluated for transformation growth factor beta 1 (TGF-β1) proving that L-HA alone increased its expression at 4 h followed by restoration of similar trends for all the stimuli. Depending on the diverse stimulation (H-HA, L-HA or the complex), metalloproteinases (MMP-2, -9, -13) were also modulated differently. Furthermore, type I collagen expression and production were evaluated. Compared to the others, persistence of a significant higher expression level at 24 h for the H-HA/L-HA complex was found.

CONCLUSIONS

The outcomes of this research showed that, both at high and low concentrations, hybrid complexes proved to perform better than HA alone thus suggesting their potential as medical devices in aesthetic and regenerative medicine.

Response of endothelial cells and pericytes to hypoxia and erythropoietin in a co-culture assay dedicated to soft tissue repair

The increasing mean life expectancy of the citizens of the western world countries leads to an increase of the age-related diseases, among them soft tissue defects exhibiting inadequate healing. In order to develop new therapeutic strategies to support disturbed soft tissue repair, there is a strong need of sophisticated in vitro assays. A new assay combining scratch wounding with co-cultures of primary human microvascular endothelial cells (HDMEC) and pericytes (HPC) focuses on basic characteristics of cell interaction against the background of soft tissue repair. The cell parameters proliferation, migration and differentiation, and the release of monocyte chemoattractant protein-1 (MCP-1) were analysed in response to hypoxia (pO2 < 5 mmHg) and to erythropoietin (EPO; 50 IU/ml), a glycoprotein hormone having shown promising effects in soft tissue repair. As basic characteristics of the assay, direct cell contact in co-culture led to a weakened proliferation of both cell types, an increase of the percentage of myofibroblast-like pericytes and to a higher release of MCP-1. Hypoxia caused a proliferation decrease of HPC in co-culture, which was slightly attenuated by EPO. Hypoxia also reduced the MCP-1 release of co-cultured cells, when EPO had been added. In addition, EPO had a rather positive effect on HPC migration under hypoxia. These in vitro results allow new insights into the interaction of pericytes with endothelial cells in the context of soft tissue repair.

Generation of a three-dimensional full thickness skin equivalent and automated wounding

In vitro models are a cost effective and ethical alternative to study cutaneous wound healing processes. Moreover, by using human cells, these models reflect the human wound situation better than animal models. Although two-dimensional models are widely used to investigate processes such as cellular migration and proliferation, models that are more complex are required to gain a deeper knowledge about wound healing. Besides a suitable model system, the generation of precise and reproducible wounds is crucial to ensure comparable results between different test runs. In this study, the generation of a three-dimensional full thickness skin equivalent to study wound healing is shown. The dermal part of the models is comprised of human dermal fibroblast embedded in a rat-tail collagen type I hydrogel. Following the inoculation with human epidermal keratinocytes and consequent culture at the air-liquid interface, a multilayered epidermis is formed on top of the models. To study the wound healing process, we additionally developed an automated wounding device, which generates standardized wounds in a sterile atmosphere.

Mechanisms of tubulogenesis and endothelial phenotype expression by MSCs

Stem cell-based therapies are a promising new avenue for treating ischemic disease and chronic wounds. Mesenchymal stem cells (MSCs) have a proven ability to augment the neovascularization processes necessary for wound healing and are widely popular as an autologous source of progenitor cells. Our lab has previously reported on PEGylated fibrin as a unique hydrogel that promotes spontaneous tubulogenesis of encapsulated MSCs without exogenous factors. However, the mechanisms underlying this process have remained unknown. To better understand the therapeutic value of PEGylated fibrin delivery of MSCs, we sought to clarify the relationship between biomaterial properties and cell behavior. Here we find that fibrin PEGylation does not dramatically alter the macroscopic mechanical properties of the fibrin-based matrix (less than 10% difference). It does, however, dramatically reduce the rate of diffusion through the gel matrix. PEGylated fibrin enhances the tubulogenic growth of encapsulated MSCs demonstrating fluid-filled lumens by interconnected MSCs. Image analysis gave a value of 4320 ± 1770 μm total network length versus 618 ± 443 μm for unmodified fibrin. PEGylation promotes the endothelial phenotype of encapsulated MSCs--compared to unmodified fibrin--as evidenced by higher levels of endothelial markers (von Willebrand factor, 2.2-fold; vascular endothelial cadherin, 1.8-fold) and vascular endothelial growth factor (VEGF, up to 1.8-fold). Prospective analysis of underlying molecular pathways demonstrated that this endothelial-like MSC behavior is sensitively modulated by hypoxic stress, but not VEGF supplementation as evidenced by a significant increase in VEGF and MMP-2 secretion per cell under hypoxia. Further gain-of-function studies under hypoxic stress demonstrated that hypoxia culture of MSCs in unmodified fibrin could increase both vWF and VE-cadherin levels to values that were not significantly different than cells cultured in PEGylated fibrin. This result corroborated our hypothesis that the diffusion-limited environment of PEGylated fibrin is augmenting endothelial differentiation cues provided by unmodified fibrin. However, MSC networks lack platelet endothelial cell adhesion molecule-1 (PECAM-1) expression, which indicates incomplete differentiation towards an endothelial cell type. Collectively, the data here supports a revised understanding of MSC-derived neovascularization that contextualizes their behavior and utility as a hybrid endothelial-stromal cell type, with mixed characteristics of both populations.

Simultaneous isolation of enriched myoblasts and fibroblasts for migration analysis within a novel co-culture assay

Skeletal muscle injury elicits the activation of satellite cells and their migration to the wound area for subsequent terminal differentiation and tissue integration. However, interstitial fibroblasts recruited to the site of injury promote deposition of fibrotic tissue, which hampers myoblast-mediated muscle regeneration. Currently, analysis of myoblast migration in vitro can be accomplished using chemotactic, cell-exclusion, or wound healing assays. Yet, to investigate cell motility following skeletal muscle damage more accurately, migration assays need to better simulate the repair process. Here we present a protocol for the simultaneous isolation of myoblasts and fibroblasts from the same muscle tissue, ensuring the consistent generation of enriched, purified, and matched cell populations at a low passage number. We then describe a wound assay that uses a novel approach to the co-culture of myoblasts and fibroblasts to mimic the injured environment more closely than other established methods. Using this assay, we demonstrate that fibroblasts are able to increase myoblast migration significantly, validating our new in vitro method. As the observed effect on migration is most likely mediated by secreted factors, our assay could easily be extended to include antibody-based protein analysis of secreted factors in animal or human systems.

Cell sheet technology-driven re-epithelialization and neovascularization of skin wounds

Skin regeneration remains a challenge, requiring a well-orchestrated interplay of cell-cell and cell-matrix signalling. Cell sheet (CS) engineering, which has the major advantage of allowing the retrieval of the intact cell layers along with their naturally organized extracellular matrix (ECM), has been poorly explored for the purpose of creating skin substitutes and skin regeneration. This work proposes the use of CS technology to engineer cellular constructs based on human keratinocytes (hKC), key players in wound re-epithelialization, dermal fibroblasts (hDFb), responsible for ECM remodelling, and dermal microvascular endothelial cells (hDMEC), part of the dermal vascular network and modulators of angiogenesis. Homotypic and heterotypic three-dimensional (3-D) CS-based constructs were developed simultaneously to target wound re-vascularization and re-epithelialization. After implantation of the constructs in murine full-thickness wounds, human cells were engrafted into the host wound bed and were present in the neotissue formed up to 14 days post-implantation. Different outcomes were obtained by varying the composition and organization of the 3-D constructs. Both hKC and hDMEC significantly contributed to re-epithelialization by promoting rapid wound closure and early epithelial coverage. Moreover, a significant increase in the density of vessels at day 7 and the incorporation of hDMEC in the neoformed vasculature confirmed its role over neotissue vacularization. As a whole, the obtained results confirmed that the proposed 3-D CS-based constructs provided the necessary cell machinery, when in a specific microenvironment, guiding both re-vascularization and re-epithelialization. Although dependent on the nature of the constructs, the results obtained sustain the hypothesis that different CS-based constructs lead to improved skin healing.

Comparison of interleukin 10 homologs on dermal wound healing using a novel human skin ex vivo organ culture model

BACKGROUND

Anti-inflammatory cytokine interleukin (IL)-10 has been shown to induce regenerative healing in postnatal wounds. A viral homolog of IL-10 produced by human cytomegalovirus (CMV IL-10) similarly generates potent immunoregulatory effects, but its effects on wound healing have not been investigated. Currently, there are limited cost-effective methods of screening vulnerary therapeutics. Taken together, we aim to develop and validate a novel human ex vivo dermal wound model and hypothesize that CMV IL-10 will enhance dermal wound healing.

METHODS

Full-thickness circular (6-mm) explants were taken from surgical skin samples and 3-mm full-thickness wounds were created. Explants were embedded in collagen I matrix and maintained in specially formulated media with the epidermis at air-liquid interface, and treated with human IL-10 or CMV IL-10 (200 ng/mL). The viability of cultured explants was validated by histology and lactate dehydrogenase (LDH) activity. Epithelial gap, epithelial height, basal keratinocyte migration, vascular endothelial growth factor levels, and neovascularization were measured at days 3 and 7 to determine IL-10 effects on wound healing.

RESULTS

Culture explants at day 7 appeared similar to fresh skin in morphology, cell, and vessel density. By day 14, the epidermis separated from the dermis and the cell density diminished. Day 7 wounds appeared viable with advancing epithelial and basal keratinocyte migration with no evidence of necrosis. Cytotoxicity analysis via the quantification of LDH revealed no differences between controls and treated groups. There was a slight increase in the quantity of LDH in media at day 3; however, this decreased at day 5 and continued to decline up to day 21. CMV IL-10 treatment resulted in a significant decrease in the epithelial gap and an increase in epithelial height. There were no differences in the rates of basal keratinocyte migration at day 7 between treated and control groups. Interestingly, human IL-10 increased vascular endothelial growth factor expression and neovascularization compared with controls.

CONCLUSIONS

The human ex vivo wound model provides a simple and viable design to study dermal wound healing. Both IL-10 homologs demonstrate vulnerary effects. The viral homolog demonstrates enhanced effects on wound closure compared with human IL-10. These data represent a novel tool that can be used to screen therapeutics, such as CMV IL-10, before preclinical studies.

Toxicity of functional nano-micro zinc oxide tetrapods: impact of cell culture conditions, cellular age and material properties

With increasing production and applications of nanostructured zinc oxide, e.g., for biomedical and consumer products, the question of safety is getting more and more important. Different morphologies of zinc oxide structures have been synthesized and accordingly investigated. In this study, we have particularly focused on nano-micro ZnO tetrapods (ZnO-T), because their large scale fabrication has been made possible by a newly introduced flame transport synthesis approach which will probably lead to several new applications. Moreover, ZnO-T provide a completely different morphology then classical spherical ZnO nanoparticles. To get a better understanding of parameters that affect the interactions between ZnO-T and mammalian cells, and thus their biocompatibility, we have examined the impact of cell culture conditions as well as of material properties on cytotoxicity. Our results demonstrate that the cell density of fibroblasts in culture along with their age, i.e., the number of preceding cell divisions, strongly affect the cytotoxic potency of ZnO-T. Concerning the material properties, the toxic potency of ZnO-T is found to be significantly lower than that of spherical ZnO nanoparticles. Furthermore, the morphology of the ZnO-T influenced cellular toxicity in contrast to surface charges modified by UV illumination or O₂ treatment and to the material age. Finally, we have observed that direct contact between tetrapods and cells increases their toxicity compared to transwell culture models which allow only an indirect effect via released zinc ions. The results reveal several parameters that can be of importance for the assessment of ZnO-T toxicity in cell cultures and for particle development.

Bromelain down-regulates myofibroblast differentiation in an in vitro wound healing assay

Bromelain, a pineapple-derived enzyme mixture, is a widely used drug to improve tissue regeneration. Clinical and experimental data indicate a better outcome of soft tissue healing under the influence of bromelain. Proteolytic, anti-bacterial, anti-inflammatory, and anti-oedematogenic effects account for this improvement on the systemic level. It remains unknown, whether involved tissue cells are directly influenced by bromelain. In order to gain more insight into those mechanisms by which bromelain modulates tissue regeneration at the cellular level, we applied a well-established in vitro wound healing assay. Two main players of soft tissue healing--fibroblasts and microvascular endothelial cells--were used as mono- and co-cultures. Cell migration, proliferation, apoptosis, and the differentiation of fibroblasts to myofibroblasts as well as interleukin-6 were quantified in response to bromelain (36 × 10(-3) IU/ml) under normoxia and hypoxia. Bromelain attenuated endothelial cell and fibroblast proliferation in a moderate way. This proliferation decrease was not caused by apoptosis, rather, by driving cells into the resting state G0 of the cell cycle. Endothelial cell migration was not influenced by bromelain, whereas fibroblast migration was clearly slowed down, especially under hypoxia. Bromelain led to a significant decrease of myofibroblasts under both normoxic (from 19 to 12 %) and hypoxic conditions (from 22 to 15 %), coincident with higher levels of interleukin-6. Myofibroblast differentiation, a clear sign of fibrotic development, can be attenuated by the application of bromelain in vitro. Usage of bromelain as a therapeutic drug for chronic human wounds thus remains a very promising concept for the future.

Expansion and differentiation of human primary osteoblasts in two- and three-dimensional culture

Despite the regenerative capability of bone, treatment of large defects often requires bone grafts. The challenge for bone grafting is to establish rapid and sufficient vascularization. Three-dimensional (3D) multicellular spheroids consisting of the relevant cell types can be used as "mini tissues" to study the complexity of angiogenesis. We investigated two-dimensional (2D) expansion, differentiation and characterization of primary osteoblasts as steps toward the establishment of 3D multicellular spheroids. Supplementation of cell culture medium with vitamin D(3) induces the osteocalcin expression of osteoblasts. An increased osteocalcin concentration of 10.8 ± 0.58 ng/ml could be measured after 19 days in supplemented medium. Vitamin D(3) has no influence on the expression of alkaline phosphatase or the deposition of calcium. Expression of these additional osteogenic markers requires addition of a cocktail of osteogenic factors that, conversely, have no influence on the expression of osteocalcin. Supplementation of the cell culture medium with both vitamin D(3) and a cocktail of osteogenic factors is recommended to produce an osteoblast phenotype that secretes osteocalcin, expresses alkaline phosphatase and deposits calcium. In such a supplemented medium, a mean osteocalcin concentration of 11.63 ± 4.85 ng/ml was secreted by the osteoblasts. Distinguishing osteoblasts and fibroblasts remains a challenge. Neither differentiated nor undifferentiated osteoblasts can be distinguished from fibroblasts by the expression of CD90, ED-A-fibronectin or α-smooth muscle actin; however, these cell types exhibit clear differences in their growth characteristics. Osteoblasts can be arranged as 3D spheroids by coating the bottom of the cell culture device with agarose. The cellular composition of 3D multicellular spheroids can be evaluated quantitatively using vital fluorescence labeling techniques. Spheroids are a promising tool for studying angiogenic and osteogenic phenomena in vivo and in vitro.

Modelling the interaction of keratinocytes and fibroblasts during normal and abnormal wound healing processes

The crosstalk between fibroblasts and keratinocytes is a vital component of the wound healing process, and involves the activity of a number of growth factors and cytokines. In this work, we develop a mathematical model of this crosstalk in order to elucidate the effects of these interactions on the regeneration of collagen in a wound that heals by second intention. We consider the role of four components that strongly affect this process: transforming growth factor-β, platelet-derived growth factor, interleukin-1 and keratinocyte growth factor. The impact of this network of interactions on the degradation of an initial fibrin clot, as well as its subsequent replacement by a matrix that is mainly composed of collagen, is described through an eight-component system of nonlinear partial differential equations. Numerical results, obtained in a two-dimensional domain, highlight key aspects of this multifarious process, such as re-epithelialization. The model is shown to reproduce many of the important features of normal wound healing. In addition, we use the model to simulate the treatment of two pathological cases: chronic hypoxia, which can lead to chronic wounds; and prolonged inflammation, which has been shown to lead to hypertrophic scarring. We find that our model predictions are qualitatively in agreement with previously reported observations and provide an alternative pathway for gaining insight into this complex biological process.

Application of a partial-thickness human ex vivo skin culture model in cutaneous wound healing study

A number of in vivo and ex vivo skin models have been applied to human wound healing studies. A reliable skin model, which recapitulates the features of human wound repair, is essential for the clinical and mechanical investigation of human cutaneous wound healing. Full-skin ex vivo culture systems have been used in wound healing studies. However, important structures of the skin, such as the differentiation of keratinocytes and epidermis-dermis junction, are poorly characterized in this model. This study aims to develop an optimized partial-thickness human ex vivo skin culture (HESC) model to maintain human skin characteristics in vitro. During our culture, the basal layer, suprabasal layer, and stratum granulosum layer of epidermis were preserved until day 8. Analyses of hemidesmosome proteins, bullous pemphigoid antigen 1 (BP180) and 2 (BP230), showed that the integrity of the basement membrane of the epidermis was well preserved in the HESC model. In contrast, an organotypic culture with human keratinocytes and fibroblasts failed to show an integrated basement membrane. Maintenance of skin structure by histological analysis and proliferation of epidermal keratinocytes by Ki67 staining were observed in our model for 12 days. Complete re-epithelialization of the wounding area was observed at day 6 post wounding when a superficial incisional wound was created. The expression of Ki-67 and keratin 6, indicators of activated keratinocytes in epidermis, was significantly upregulated and new collagen synthesis was found in the dermis during the wound healing process. As control, we also used organotypic culture in studying the differentiation of the keratinocyte layers and incisional wound repair. It turned out that our model has advantage in these study fields. The results suggest that our HESC model retains important elements of in vivo skin and has significant advantages for the wound healing studies in vitro.

Stimulation of skin and wound fibroblast migration by mesenchymal stem cells derived from normal donors and chronic wound patients

Chronic wounds continue to be a major cause of morbidity for patients and an economic burden on the health care system. Novel therapeutic approaches to improved wound healing will need, however, to address cellular changes induced by a number of systemic comorbidities seen in chronic wound patients, such as diabetes, chronic renal failure, and arterial or venous insufficiency. These effects likely include impaired inflammatory cell migration, reduced growth factor production, and poor tissue remodeling. The multifunctional properties of bone marrow-derived mesenchymal stem cells (MSCs), including their ability to differentiate into various cell types and capacity to secrete factors important in accelerating healing of cutaneous wounds, have made MSCs a promising agent for tissue repair and regeneration. In this study we have used an in vitro scratch assay procedure incorporating labeled MSCs and fibroblasts derived from normal donors and chronic wound patients in order to characterize the induction of mobilization when these cells are mixed. A modified Boyden chamber assay was also used to examine the effect of soluble factors on fibroblast migration. These studies suggest that MSCs play a role in skin wound closure by affecting dermal fibroblast migration in a dose-dependent manner. Deficiencies were noted, however, in chronic wound patient fibroblasts and MSCs as compared with those derived from normal donors. These findings provide a foundation to develop therapies targeted specifically to the use of bone marrow-derived MSCs in wound healing and may provide insight into why some wounds do not heal.

Development of a three-dimensional human skin equivalent wound model for investigating novel wound healing therapies

Numerous difficulties are associated with the conduct of preclinical studies related to skin and wound repair. Use of small animal models such as rodents is not optimal because of their physiological differences to human skin and mode of wound healing. Although pigs have previously been used because of their human-like mode of healing, the expense and logistics related to their use also renders them suboptimal. In view of this, alternatives are urgently required to advance the field. The experiments reported herein were aimed at developing and validating a simple, reproducible, three-dimensional ex vivo de-epidermised dermis human skin equivalent wound model for the preclinical evaluation of novel wound therapies. Having established that the human skin equivalent wound model does in fact &#x201C;heal," we tested the effect of two novel wound healing therapies. We also examined the utility of the model for studies exploring the mechanisms underpinning these therapies. Taken together the data demonstrate that these new models will have wide-spread application for the generation of fundamental new information on wound healing processes and also hold potential in facilitating preclinical optimization of dosage, duration of therapies, and treatment strategies prior to clinical trials.

Stromal cells differentially regulate neutrophil and lymphocyte recruitment through the endothelium

Stromal fibroblasts modify the initial recruitment of leucocytes by endothelial cells (EC), but their effects on subsequent transendothelial migration remain unclear. Here, EC and dermal or synovial fibroblasts were cultured on opposite surfaces of 3-μm pore filters and incorporated in static or flow-based migration assays. Fibroblasts had little effect on tumour necrosis factor-α-induced transendothelial migration of neutrophils, but tended to increase the efficiency of migration away from the endothelium. Surprisingly, similar close contact between EC and fibroblasts strongly reduced lymphocyte migration in static assays, and nearly abolished stable lymphocyte adhesion from flow. Fibroblasts did not alter endothelial surface expression of adhesion molecules or messenger RNA for chemokines. Inhibition of attachment did not occur when EC-fibroblast contact was restricted by using 0.4-μm pore filters, but under these conditions pre-treatment with heparinase partially inhibited adhesion. In the 3-μm pore co-cultures, inhibition of metalloproteinase activity partially recovered lymphocyte adhesion, but addition of CXCL12 (SDF-1α) to the endothelial surface did not. Hence, the ability of EC to present activating chemokines for lymphocytes may have been enzymatically inhibited by direct contact with fibroblasts. To avoid contact, we cultured EC and fibroblasts on separate 3-μm pore filters one above the other. Here, fibroblasts promoted the transendothelial migration of lymphocytes. Fibroblasts generate CXCL12, but blockade of CXCL12 receptor had no effect on lymphocyte migration. While stromal cells can provide signal(s) promoting leucocyte migration away from the sub-endothelial space, direct cell contact (which might occur in damaged tissue) may cause disruption of chemokine signalling, specifically inhibiting lymphocyte rather than neutrophil recruitment.

Mathematical model of hyperbaric oxygen therapy applied to chronic diabetic wounds

The failure of certain wounds to heal (including diabetic foot ulcers) is a significant socioeconomic issue for countries worldwide. There is much debate about the best way to treat these wounds and one approach that is shrouded with controversy is hyperbaric oxygen therapy (HBOT), a technique that can reduce the risk of amputation in diabetic patients.In this paper, we develop a six species mathematical model of wound healing angiogenesis and use it to investigate the effectiveness of HBOT, compare the response to different HBOT protocols and study the effect of HBOT on the healing of diabetic wounds that fail to heal for a variety of reasons. We vary the pressure level (1 atm-3 atm), percentage of oxygen inspired by the patient (21%-100%), session duration (0-180 minutes) and frequency (twice per day-once per week) and compare the simulated wound areas associated with different protocols after three weeks of treatment.We consider a variety of etiologies of wound chronicity and show that HBOT is only effective in treating certain causes of chronic wounds. For a wound that fails to heal due to excessive, oxygen-consuming bacteria, we show that intermittent HBOT can accelerate the healing of a chronic wound but that sessions should be continued until complete healing is observed. Importantly, we also demonstrate that normobaric oxygen is not a replacement for HBOT and supernormal healing is not an expected outcome. Our simulations illustrate that HBOT has little benefit for treating normal wounds, and that exposing a patient to fewer, longer sessions of oxygen is not an appropriate treatment option.

In vitro assays for endothelial cell functions related to angiogenesis: proliferation, motility, tubular differentiation, and proteolysis

This chapter covers the breakdown of the process of angiogenesis into simple assays to measure discrete endothelial cell functions. The techniques described are suitable for studying stimulators or inhibitors of angiogenesis and determining which aspect of the process is modulated. The procedures outlined are robust and straightforward but cannot cover the complexity of the angiogenic process as a whole, incorporating as it does myriad positive and negative signals, three-dimensional interactions with host tissues and many accessory cells, including fibroblasts, macrophages, pericytes, and platelets. The extent to which in vitro assays predict responses in vivo (e.g., wound healing, tumor angiogenesis, or surrogate techniques such as Matrigel plugs, sponge implants, corneal assays, etc.) remains to be determined.

In vitro wounding: effects of hypoxia and transforming growth factor β1 on proliferation, migration and myofibroblastic differentiation in an endothelial cell-fibroblast co-culture model

The adequate reconstitution of human soft tissue wounds requires the coordinated interaction of endothelial cells and fibroblasts during the proliferation phase of healing. Endothelial cells assure neoangiogenesis, fibroblasts fill the defect and provide extracellular matrix proteins, and myofibroblasts are believed to support the reconstitution of microvessels. In the present study, we combined in vitro-wound size measurement and multicolour immunocytochemical staining of co-cultured human dermal microvascular endothelial cells and normal human dermal fibroblasts, recently introduced as co-culture scratch-wound migration assay. Applying antibodies for alpha-smooth-muscle actin, von Willebrand factor, extra domain A fibronectin and endothelin-1, we were able to monitor proliferation, migration and the differentiation process from fibroblasts to myofibroblasts as a response to hypoxia. Furthermore, we verified, whether transforming growth factor beta1 (TGFbeta1) and endothelin-1 are able to mediate this response. We show, that proliferation and migration of endothelial cells and fibroblasts decreased under hypoxia. The additional administration of TGFbeta1 did not significantly attenuate this decrease. Solely the myofibroblast population in co-culture adapted well to hypoxia, when cultures were supplemented with TGFbeta1. Considerating the data concerning TGFbeta1 and endothelin-1, we propose a model explaining the cellular interaction during early and late proliferation phase of human wound healing.

Time-dependent responses of wounded human skin fibroblasts following phototherapy

BACKGROUND AND OBJECTIVE

The penetration and distribution of laser light in target tissue is dependent on the wavelength of the light. One problem with most of the published data on laser irradiation is that most studies do not record the duration between the exposure and the evaluation. This study aimed to establish if the dose, wavelength or duration of effect (1h or 24h) influences the biological responses of irradiated fibroblasts.

MATERIALS AND METHODS

The study established cellular responses of normal and wounded human skin fibroblasts to helium-neon (632.8 nm), diode (830 nm) and Nd:YAG (1064 nm) laser irradiation using one exposure of 5 J/cm(2) or 16 J/cm(2) on day 1 and again on day 4. Cellular responses to laser irradiation were evaluated by measuring changes in cell viability (ATP viability and caspase 3/7 activity) and cell proliferation (ALP enzyme activity and bFGF expression), 1h and 24h post irradiation.

RESULTS

Wounded cells exposed to 5 J/cm(2) using 632.8 nm showed an increase in ATP viability after 1h, a decrease in caspase 3/7 activity after 24h and an increase in cell proliferation after 24h. The results suggest that changes in parameters such as ATP viability should be observed directly after laser irradiation (1h) whereas other parameters such as caspase 3/7 activity, bFGF expression and ALP enzyme activity should be measured at least 24h after the final exposure.

CONCLUSION

This study confirms that the duration of effect should be included as one of the main laser parameters when reporting on the effects of laser irradiation. It is important to establish time-dependent responses as the results may provide an understanding of the cellular responses following laser irradiation.