Overexpression of vascular endothelial growth factor accelerates early vascularization and improves healing of genetically modified cultured skin substitutes

Fibroblast growth factor-impregnated collagen-gelatin sponge improves keratinocyte sheet survival

Commercially available cultured epithelial keratinocyte sheets (KSs) have played an essential role in wound healing over the last four decades. Despite the initial uptake by the dermal elements, the survival rate of KS on the dermis-like tissue generated by conventional artificial dermis (AD) is low, making this method unsuitable for standard treatments. Therefore, an innovative AD such as collagen/gelatin sponge (CGS) that maintains the release of human recombinant basic fibroblast growth factor (bFGF) may promote wound healing. In this study, we examined whether combination therapy with KSs and CGS with bFGF (bFGF-CGS) could enhance KS survival by heterologous grafting by transplantation of human-derived KSs in an athymic nude rat wound model of staged skin reconstruction. The CGSs were implanted into skin defect wounds on athymic nude rats, which were then divided into two experimental groups: the bFGF group (CGSs containing bFGF, n = 8) and control group (CGSs with saline, n = 8). Two weeks after implantation, human epithelial cell-derived KSs were grafted onto the dermis-like tissue, followed by assessment of the survival and morphology at one week later using digital imaging, histology (hematoxylin and eosin and Masson's trichrome staining), immunohistology (von Willebrand factor), immunohistochemistry (cytokeratin 1-5-6, Ki-67), and immunofluorescence (collagen IV, pan-cytokeratins) analyses. The bFGF group showed a significantly higher KS survival area (86 ± 58 vs. 32 ± 22 mm2; p < 0.05) and increased epidermal thickness (158 ± 66 vs. 86 ± 40 µm; p < 0.05) compared with the control group, along with higher dermis-like tissue regeneration, neovascularization, epidermal maturation, and basement membrane development. These results indicate that the survival rate of KSs in the dermis-like tissue formed by bFGF-CGS was significantly increased. Therefore, combination treatment of bFGF-CGS and KSs shows potential for full-thickness skin defect reconstruction in clinical situations.

Vascularization in skin wound healing: where do we stand and where do we go?

Cutaneous healing is a highly complex process that, if altered due to, for example, impaired vascularization, results in chronic wounds or repaired neotissue of poor quality. Significant progress has been achieved in promoting neotissue vascularization during tissue repair/regeneration. In this review, we discuss the strategies that have been explored and how each one of them contributes to regulate vascularization in the context of cutaneous wound healing from two different perspectives - biomaterial-based and a cell-based approaches. Finally, we discuss the implications of these findings on the development of the 'next generation' approaches to target vascularization in wound healing highlighting the importance of going beyond its contribution to regulate vascularization and take into consideration the temporal features of the healing process and of different types of wounds.

Bioactive antibacterial bilayer PCL/gelatin nanofibrous scaffold promotes full-thickness wound healing

Treatment of diabetic, chronic, and full-thickness wounds is a challenge as these injuries usually lead to infections that cause delayed and inappropriate healing. Therefore, fabrication of skin scaffolds with prolonged antibacterial properties are of great interest. Due to this demand, bilayered nanofibrous scaffolds were fabricated based on polycaprolactone and gelatin. The top layer of these scaffolds contained amoxicillin as a model drug and the bottom layer was loaded with zinc oxide nanoparticles to accelerate wound healing. Several characterization techniques including FTIR, SEM, swelling, tensile test, in vitro degradation, drug release, antibacterial activity, and MTT assay were used to assess physical, mechanical, and biological properties of produced nanofibers. SEM results demonstrated that bilayered scaffolds have smooth bead-free microstructures while in vitro release test showed that samples have a sustained release for amoxicillin up to 144 h (tested time). Disk diffusion assessment confirmed the potency of scaffolds for hindering bacterial growth while results of cytotoxicity evaluation revealed that scaffolds could effectively accelerate cell proliferation. Finally, in vivo tests on full-thickness rat models revealed that fabricated nanofibers accelerate wound contraction, increase collagen deposition and angiogenesis, and prevent scar formation. Altogether, results showed that fabricated scaffolds are promising candidates for treatment of full-thickness wounds.

Auto Micro Atomization Delivery of Human Epidermal Organoids Improves Therapeutic Effects for Skin Wound Healing

Severe skin wounds are often associated with large areas of damaged tissue, resulting in substantial loss of fluids containing electrolytes and proteins. The net result is a vulnerability clinically to skin infections. Therapies aiming to close these large openings are effective in reducing the complications of severe skin wounds. Recently, cell transplantation therapy showed the potential for rapid re-epithelialization of severe skin wounds. Here, we show the improved effects of cell transplantation therapy using a robust protocol of efficient expansion and delivery of epidermal cells for treatment of severe skin wounds. Human skin tissues were used to generate human epidermal organoids maintained under newly established culture conditions. The human epidermal organoids showed an improved capacity of passaging for at least 10 rounds, enabling organoids to expand to cell numbers required for clinical applications. A newly designed auto micro-atomization device (AMAD) was developed for delivery of human epidermal organoids onto the sites of severe skin wounds enhancing uniform and concentrated delivery of organoids, facilitating their engraftment and differentiation for skin reconstitution. With the optimal design and using pneumatic AMAD, both survival and functions of organoids were effectively protected during the spraying process. Cells in the sprayed human epidermal organoids participated in the regeneration of the epidermis at wound sites in a mouse model and accelerated wound healing significantly. The novel AMAD and out new protocol with enhanced effects with respect to both organoid expansion and efficient transplantation will be used for clincal treatments of complex, uneven, or large-area severe skin wounds.

Vascularization strategies for skin tissue engineering

Lack of proper vascularization after skin trauma causes delayed wound healing. This has sparked the development of various tissue engineering strategies to improve vascularization.

Experimental models for cutaneous hypertrophic scar research

Human skin wound repair may result in various outcomes with most of them leading to scar formation. Commonly seen in many cutaneous wound healing cases, hypertrophic scars are considered as phenotypes of abnormal wound repair. To prevent the formation of hypertrophic scars, efforts have been made to understand the mechanism of scarring following wound closure. Numerous in vivo and in vitro models have been created to facilitate investigations into cutaneous scarring and the development of antiscarring treatments. To select the best model for a specific study, background knowledge of the current models of hypertrophic scars is necessary. In this review, we describe in vivo and in vitro models for studying hypertrophic scars, as well as the distinct characteristics of these models. The choice of models for a specific study should be based on the characteristics of the model and the goal of the study. In general, in vivo animal models are often used in phenotypical scar formation analysis, development of antiscarring treatment, and functional analyses of individual genes. In contrast, in vitro models are chosen to pathway identification during scar formation as well as in high-throughput analysis in drug development. Besides helping investigators choose the best scarring model for their research, the goal of this review is to provide knowledge for improving the existing models and development of new models. These will contribute to the progress of scarring studies.

Animal Models for Studies of Keloid Scarring

Significance: Keloid scarring is a disfiguring fibroproliferative disorder that can significantly impair the quality of life in affected individuals. The mechanisms that initiate keloid scarring are incompletely understood, and keloids remain one of the most challenging skin conditions to treat. Keloids are unique to humans; thus, the lack of adequate animal models has hindered research efforts aimed at prevention and effective therapeutic intervention. Recent Advances: In the absence of a suitable animal model, keloid researchers often rely on studying excised keloid scar tissue and keloid-derived cultured cells. Recently, in vivo models have been described that involve transplantation to mice of reconstructed skin containing keloid-derived fibroblasts and/or keratinocytes. These mouse-human hybrid animal models display some similarities with keloids and may enable investigation of novel therapies, although no model yet recapitulates all the features of human keloid scarring. Critical Issues: Differences in skin physiology and modes of healing contribute to challenges in modeling keloids in laboratory animals. Furthermore, recent studies suggest that cells of the immune system contribute to keloid pathology. The need to use immunodeficient hosts for transplanted human keloid cells in recently described animal models precludes studying the role of the immune system in keloid scarring. Future Directions: Future animal models may take advantage of humanized mice with immune systems reconstituted using human immune cells. Such models, when combined with grafted tissues prepared using keloid-derived cells, might enable investigation of complex interactions between systemic and local factors that combine to promote keloid scar formation and may aid in the development of novel therapies.

Soluble Epoxide Hydrolase Inhibition and Epoxyeicosatrienoic Acid Treatment Improve Vascularization of Engineered Skin Substitutes

BACKGROUND

Autologous engineered skin substitutes comprised of keratinocytes, fibroblasts, and biopolymers can serve as an adjunctive treatment for excised burns. However, engineered skin lacks a vascular plexus at the time of grafting, leading to slower vascularization and reduced rates of engraftment compared with autograft. Hypothetically, vascularization of engineered skin grafts can be improved by treatment with proangiogenic agents at the time of grafting. Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are inactivated by soluble epoxide hydrolase (sEH). EETs have multiple biological activities and have been shown to promote angiogenesis. Inhibitors of sEH (sEHIs) represent attractive therapeutic agents because they increase endogenous EET levels. We investigated sEHI administration, alone or combined with EET treatment, for improved vascularization of engineered skin after grafting to mice.

METHODS

Engineered skin substitutes, prepared using primary human fibroblasts and keratinocytes, were grafted to full-thickness surgical wounds in immunodeficient mice. Mice were treated with the sEHI 1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl) urea (TPPU), which was administered in drinking water throughout the study period, with or without topical EET treatment, and were compared with vehicle-treated controls. Vascularization was quantified by image analysis of CD31-positive areas in tissue sections.

RESULTS

At 2 weeks after grafting, significantly increased vascularization was observed in the TPPU and TPPU + EET groups compared with controls, with no evidence of toxicity.

CONCLUSIONS

The results suggest that sEH inhibition can increase vascularization of engineered skin grafts after transplantation, which may contribute to enhanced engraftment and improved treatment of full-thickness wounds.

Gellan gum-hyaluronic acid spongy-like hydrogels and cells from adipose tissue synergize promoting neoskin vascularization

Currently available substitutes for skin wound healing often result in the formation of nonfunctional neotissue. Thus, urgent care is still needed to promote an effective and complete regeneration. To meet this need, we proposed the assembling of a construct that takes advantage of cell-adhesive gellan gum-hyaluronic acid (GG-HA) spongy-like hydrogels and a powerful cell-machinery obtained from adipose tissue, human adipose stem cells (hASCs), and microvascular endothelial cells (hAMECs). In addition to a cell-adhesive character, GG-HA spongy-like hydrogels overpass limitations of traditional hydrogels, such as reduced physical stability and limited manipulation, due to improved microstructural arrangement characterized by pore wall thickening and increased mean pore size. The proposed constructs combining cellular mediators of the healing process within the spongy-like hydrogels that intend to recapitulate skin matrix aim to promote neoskin vascularization. Stable and off-the-shelf dried GG-HA polymeric networks, rapidly rehydrated at the time of cell seeding then depicting features of both sponges and hydrogels, enabled the natural cell entrapment/encapsulation and attachment supported by cell-polymer interactions. Upon transplantation into mice full-thickness excisional wounds, GG-HA spongy-like hydrogels absorbed the early inflammatory cell infiltrate and led to the formation of a dense granulation tissue. Consequently, spongy-like hydrogel degradation was observed, and progressive wound closure, re-epithelialization, and matrix remodelling was improved in relation to the control condition. More importantly, GG-HA spongy-like hydrogels promoted a superior neovascularization, which was enhanced in the presence of human hAMECs, also found in the formed neovessels. These observations highlight the successful integration of a valuable matrix and prevascularization cues to target angiogenesis/neovascularization in skin full-thickness excisional wounds.

Wound coverage technologies in burn care: novel techniques

Improvements in burn wound care have vastly decreased morbidity and mortality in severely burned patients. Development of new therapeutic approaches to increase wound repair has the potential to reduce infection, graft rejection, and hypertrophic scarring. The incorporation of tissue-engineering techniques, along with the use of exogenous proteins, genes, or stem cells to enhance wound healing, heralds new treatment regimens based on the modification of already existing biological activity. Refinements to surgical techniques have enabled the creation of protocols for full facial transplantation. With new technologies and advances such as these, care of the severely burned will undergo massive changes over the next decade. This review centers on new developments that have recently shown great promise in the investigational arena.

Models of abnormal scarring

Keloids and hypertrophic scars are thick, raised dermal scars, caused by derailing of the normal scarring process. Extensive research on such abnormal scarring has been done; however, these being refractory disorders specific to humans, it has been difficult to establish a universal animal model. A wide variety of animal models have been used. These include the athymic mouse, rats, rabbits, and pigs. Although these models have provided valuable insight into abnormal scarring, there is currently still no ideal model. This paper reviews the models that have been developed.

Characterization of hair follicle development in engineered skin substitutes

Generation of skin appendages in engineered skin substitutes has been limited by lack of trichogenic potency in cultured postnatal cells. To investigate the feasibility and the limitation of hair regeneration, engineered skin substitutes were prepared with chimeric populations of cultured human keratinocytes from neonatal foreskins and cultured murine dermal papilla cells from adult GFP transgenic mice and grafted orthotopically to full-thickness wounds on athymic mice. Non-cultured dissociated neonatal murine-only skin cells, or cultured human-only skin keratinocytes and fibroblasts without dermal papilla cells served as positive and negative controls respectively. In this study, neonatal murine-only skin substitutes formed external hairs and sebaceous glands, chimeric skin substitutes formed pigmented hairs without sebaceous glands, and human-only skin substitutes formed no follicles or glands. Although chimeric hair cannot erupt readily, removal of upper skin layer exposed keratinized hair shafts at the skin surface. Development of incomplete pilosebaceous units in chimeric hair corresponded with upregulation of hair-related genes, LEF1 and WNT10B, and downregulation of a marker of sebaceous glands, Steroyl-CoA desaturase. Transepidermal water loss was normal in all conditions. This study demonstrated that while sebaceous glands may be involved in hair eruption, they are not required for hair development in engineered skin substitutes.

VEGF(165) expressing bone marrow mesenchymal stem cells differentiate into hepatocytes under HGF and EGF induction in vitro

A short half-life and low levels of growth factors in an injured microenvironment necessitates the sustainable delivery of growth factors and stem cells to augment the regeneration of injured tissues. Our aim was to investigate the ability of VEGF(165) expressing bone marrow mesenchymal stem cells (BMMSCs) to differentiate into hepatocytes when cultured with hepatocyte growth factor (HGF) and epidermal growth factor (EGF) in vitro. We isolated, cultured and identified rabbit BMMSCs, then electroporated the BMMSCs with VEGF(165)-pCMV6-AC-GFP plasmid. G418 was used to select transfected cells and the efficiency was up to 70%. The groups were then divided as follows: Group A was electroporated with pCMV6-AC-GFP plasmid + HGF + EGF and Group B was electroporated with VEGF(165)-pCMV6-AC-GFP plasmid +HGF + EGF. After 14 days, BMMSCs were induced into short spindle and polygonal cells. Alpha-fetoprotein (AFP) was positive and albumin (ALB) was negative in Group A, while both AFP and ALB were positive in group B on day 10. AFP and ALB in both groups were positive on day 20, but the quantity of AFP in group B decreased with prolonged time and was about 43.5% less than group A. The quantity of the ALB gene was increased with prolonged time in both groups. However, there was no significant difference between group A and B on day 10 and 20. Our results demonstrated that VEGF(165)-pCMV6-AC-GFP plasmid modified BMMSCs still had the ability to differentiate into hepatocytes. The VEGF(165) gene promoted BMMSCs to differentiate into hepatocyte-like cells under the induction of HGF and EGF, and reduced the differentiation time. These results have implications for cell therapies.

Hypoxia preconditioning of tissue-engineered mucosa enhances its angiogenic capacity in vitro

Improving vascularization of tissue-engineered oral mucosa (TEM) is a major challenge in the field of plastic surgery. Hypoxia is a stimulator of angiogenesis through a number of mechanisms. Therefore, hypoxia is a critical parameter that can be controlled in an effort to improve angiogenesis. In the present study we studied the secretion of a number of angiogenic factors during hypoxia exposure and evaluated the effect of TEM conditioned medium on endothelial cells. TEM was constructed by seeding human oral mucosa keratinocytes and fibroblasts on acellular human donor skin. TEM was exposed to hypoxia during 6, 12, and 24 h. Cellular hypoxia was assessed by immunolocalization of the hypoxia-inducible factor-1α. Secretion of vascular endothelial growth factor, placental growth factor (PlGF), tissue inhibitors of matrix metalloproteinases-1 and -2, and the activity of matrix metalloproteinase-9 significantly increased during hypoxia exposure. Moreover, conditioned medium from hypoxic TEM strongly enhanced endothelial cell proliferation and migration. In vitro exposure of TEM to hypoxia improves its capacity to support endothelial cell proliferation and migration, which suggests that hypoxia preconditioning of TEM potentially improves angiogenic responses for in vivo implantation.

Formation of human capillaries in vitro: the engineering of prevascularized matrices

Initial take, development, and function of transplanted engineered tissue substitutes are crucially dependent on rapid and adequate blood perfusion. Therefore, the development of rapidly and efficiently vascularized tissue grafts is vital for tissue engineering and regenerative medicine. Here we report on the construction of a network of highly organotypic capillaries in engineered tissue substitutes. We employed a three-dimensional culture system consisting of human microvascular endothelial cells. These were reproducibly expanded at high purity and subsequently seeded into biodegradable, fibrin-based hydrogels. The process of capillary formation in vitro followed the principles of both angiogenesis and postnatal vasculogenesis and a distinct sequence of other developmental steps that closely resemble embryonic neovascularization. Capillary lumen formation in vitro was initiated by the deposition of a basement membrane and intensive pinocytosis, followed by the generation of intracellular vacuoles, successive fusion of these vacuoles, and finally the formation of a long, continuous lumen. After transplantation the vascular structures were stabilized by mural cells of the recipient animal. Our findings suggest that the in vitro engineering of prevascularized matrices is within reach.

The effectiveness of basic fibroblast growth factor in fibrin-based cultured skin substitute in vivo

Cultured skin substitute (CSS), comprised keratinocytes and fibroblasts in a biopolymer matrix, is useful for adjunctive burn therapy. However, the vascularization of CSS is much slower than split-thickness autografts, because it lacks a vascular plexus. This study evaluated the influence of basic fibroblast growth factor (bFGF) on fibrin-based CSS grafting in vivo. Fibrin-based CSS treated with 0, 0.26, 1.3, 6.5, 13, or 130 microg/cm bFGF was transplanted into athymic mice, and macroscopic and histologic examinations of the graft were performed on day 21 posttransplantation. Engrafted CSS of the 0.26 to 6.5 microg/cm bFGF treatment groups were similar to the untreated control. However, the engrafted area was significantly suppressed in the 13 microg/cm bFGF treatment group, and the 130 microg/cm bFGF treatment group was not engrafted. Neovascularization of CSS was significantly increased in the 1.3 microg/cm bFGF treatment group compared with the control (P < .05). The number of human fibroblastic cells in CSS that were positive for vimentin increased significantly in the 0.26 and 1.3 microg/cm bFGF treatment groups (P < .01). CSS treated with 0.26 to 6.5 microg/cm bFGF showed normal epidermis with keratinizing stratified squamous epithelium, whereas the thickness of the epidermis and proliferation of keratinocytes in the basal layer was decreased. These results demonstrated that bFGF treatment (1.3 microg/cm) in fibrin-based CSS may enhance angiogenesis and fibroblast proliferation after transplantation.

Primary Study on Transplantation of Endothelialized Dermal Equivalents Into Normal Rats

This study was designed to determine the ability of human umbilical vein endothelial cells (HUVEC) in dermal equivalent (DE) to form microvessel-like tubes after transplantation into normal rats. A mixture of rat fibroblasts and HUVEC was inosculated into collagen-chitosan sponges to prepare endothelialized dermal equivalents (EDE). After culture in vitro for 24 hours, inosculated cells dispersed throughout the sponges and the equivalents were transplanted subcutaneously into the back of normal Lewis rats. Anti-human specific CD31 antibody was used for immunohistochemical localization of human endothelial cells in sections of EDE excised from rats after grafting. HUVEC in EDE organized into microvessel-like tubes at the end of the first week after transplantation, which still persisted after two weeks. The host microvessels began to pervade both DE and EDE during the second week after transplantation. These results demonstrated that HUVEC in EDE was able to persist and form microvessel-like tubes after transplantation into normal rats, and this is the first time to transplant DE containing HUVEC into normal rats.

Phase I/II clinical evaluation of StrataGraft: a consistent, pathogen-free human skin substitute

BACKGROUND

Large wounds often require temporary allograft placement to optimize the wound bed and prevent infection until permanent closure is feasible. We developed and clinically tested a second-generation living human skin substitute (StrataGraft). StrataGraft provides both a dermis and a fully-stratified, biologically-functional epidermis generated from a pathogen-free, long-lived human keratinocyte progenitor cell line, Neonatal Immortalized KeratinocyteS (NIKS).

METHODS

Histology, electron microscopy, quantitative polymerase chain reaction, and bacterial growth in vitro were used to analyze human skin substitutes generated from primary human keratinocytes or NIKS cells. A phase I/II, National Institute of Health-funded, randomized, safety, and dose escalation trial was performed to assess autograft take in 15 patients 2 weeks after coverage with StrataGraft skin substitute or cryopreserved cadaver allograft.

RESULTS

StrataGraft skin substitute exhibited a fully stratified epidermis with multilamellar lipid sheets and barrier function as well as robust human beta defensin-3 mRNA levels. Analysis of the primary endpoint in the clinical study revealed no differences in autograft take between wound sites pretreated with StrataGraft skin substitute or cadaver allograft. No StrataGraft-related adverse events or serious adverse events were observed.

CONCLUSIONS

The major finding of this phase I/II clinical study is that performance of StrataGraft skin substitute was comparable to cadaver allograft for the temporary management of complex skin defects. StrataGraft skin substitute may also eliminate the risk for disease transmission associated with allograft tissue and offer additional protection to the wound bed through inherent antimicrobial properties. StrataGraft is a pathogen-free human skin substitute that is ideal for the management of severe skin wounds before autografting.

Engineering tissue from human embryonic stem cells

Recent advances in human embryonic stem cell (hESC) biology now offer an alternative cell source for tissue engineers, as these cells are capable of proliferating indefinitely and differentiating to many clinically relevant cell types. Novel culture methods capable of exerting spatial and temporal control over the stem cell microenvironment allow for more efficient expansion of hESCs, and significant advances have been made toward improving our understanding of the biophysical and biochemical cues that direct stem cell fate choices. Effective production of lineage specific progenitors or terminally differentiated cells enables researchers to incorporate hESC derivatives into engineered tissue constructs. Here, we describe current efforts using hESCs as a cell source for tissue engineering applications, highlighting potential advantages of hESCs over current practices as well as challenges which must be overcome.

Efficient retroviral gene transfer to epidermal stem cells

Skin is an attractive target for gene modification to treat skin diseases, wound healing, or even systemic disorders. Although retroviral transduction results in permanent genetic modification, differentiation and eventually loss of the transduced cells from the epidermis and in temporary transgene expression. Therefore, it is important to develop methods that promote gene transfer to epidermal stem cells, which self-renew and regenerate the epidermis for extended periods of time. Here we describe an efficient protocol that results in high levels of retroviral gene transfer to human epidermal stem cells by immobilizing retrovirus on a recombinant fibronectin (rFN) fragment. In contrast to the traditional method, transduction on rFN promotes gene transfer to epidermal stem cells and prevents loss of clonogenic potential due to exposure of cells to retroviral supernatant. Notably, transduction on rFN does not require addition of toxic polycations such as polybrene. Overall this method provides a simple, fast, and efficient means to modify human epidermal stem cells for cutaneous gene therapy and for biological studies that require stable genetic modification.

Cultured epithelial autograft (CEA) in burn treatment: three decades later

Methods for handling burn wounds have changed in recent decades and increasingly aggressive surgical approach with early tangential excision and wound closure is being applied. Split-thickness skin (STSG) autografts are the "gold standard" for burn wound closure and remain the mainstay of treatment to provide permanent wound coverage and achieve healing. In some massively burned patients, however, the burns are so extensive that donor site availability is limited. Fortunately, considerable progress has been made in the culture of human keratinocytes and it is now possible to obtain large amounts of cultured epithelium from a small skin biopsy within 3-4 weeks. Questions related to optimal cell type for culture, culture techniques, transplantation of confluent sheets or non-confluent cells, immediate and late final take, carrier and transfer modality, as well as final outcome, ability to generate an epithelium after transplantation, and scar quality are still not fully answered. Progress accomplished since Reinwald and Green first described their keratinocyte culture technique is reviewed.

Gene-modified tissue-engineered skin: the next generation of skin substitutes

Tissue engineering combines the principles of cell biology, engineering and materials science to develop three-dimensional tissues to replace or restore tissue function. Tissue engineered skin is one of most advanced tissue constructs, yet it lacks several important functions including those provided by hair follicles, sebaceous glands, sweat glands and dendritic cells. Although the complexity of skin may be difficult to recapitulate entirely, new or improved functions can be provided by genetic modification of the cells that make up the tissues. Gene therapy can also be used in wound healing to promote tissue regeneration or prevent healing abnormalities such as formation of scars and keloids. Finally, gene-enhanced skin substitutes have great potential as cell-based devices to deliver therapeutics locally or systemically. Although significant progress has been made in the development of gene transfer technologies, several challenges have to be met before clinical application of genetically modified skin tissue. Engineering challenges include methods for improved efficiency and targeted gene delivery; efficient gene transfer to the stem cells that constantly regenerate the dynamic epidermal tissue; and development of novel biomaterials for controlled gene delivery. In addition, advances in regulatable vectors to achieve spatially and temporally controlled gene expression by physiological or exogenous signals may facilitate pharmacological administration of therapeutics through genetically engineered skin. Gene modified skin substitutes are also employed as biological models to understand tissue development or disease progression in a realistic three-dimensional context. In summary, gene therapy has the potential to generate the next generation of skin substitutes with enhanced capacity for treatment of burns, chronic wounds and even systemic diseases.

Exogenous pro-angiogenic stimuli cannot prevent physiologic vessel regression

BACKGROUND

In healing wounds, rising levels of vascular endothelial growth factor (VEGF) induce a period of robust angiogenesis. The levels of pro-angiogenic factors in the wound begin to decline just before a period of vascular regression, suggesting that these mediators are necessary to sustain vessel density. The purpose of this study was to determine if the maintenance of pro-angiogenic stimuli in the wound would prevent physiological vessel regression.

MATERIALS AND METHODS

A standard subcutaneous sponge wound model was modified by the addition of a mini-osmotic pump, allowing manipulation of the wound milieu by the addition of exogenous growth factors. After initial characterization of this model, exogenous VEGF (10 microg/mL), FGF (10 microg/mL), PDGF (10 microg/mL), or VEGF (10 microg/mL) plus FGF (10 microg/mL) were delivered to wounds and blood vessel density analyzed by immunohistochemistry.

RESULTS

VEGF administration resulted in a transient increase in wound vessel density (P < 0.05). None of the pro-angiogenic growth factors (VEGF, FGF, PDGF, VEGF/FGF) were able to prevent vascular regression (P = NS).

CONCLUSIONS

These findings suggest that the anti-angiogenic signals that mediate physiological vascular regression in wounds are strongly dominant over pro-angiogenic stimuli during the later phases of wound healing. Clinical manipulation of anti-angiogenic signals in addition to the currently used pro-angiogenic targets may be needed to achieve therapeutic modulation of blood vessel density.

Local expression of indoleamine 2,3-dioxygenase protects engraftment of xenogeneic skin substitute

The expression of indoleamine 2,3-dioxygenase (IDO), which metabolizes tryptophan, an essential amino acid, into kynurenine, has been identified as having a key role in the prevention of the immune rejection of the semi-allogeneic fetus during pregnancy. We have previously demonstrated that IDO expressed in fibroblasts causes bystander CD4(+) T cell damage as well as THP-1 cell damage by apoptosis. As T cells are primarily responsible for graft rejection, here, we asked the question of whether engraftment of IDO-expressing xenogeneic fibroblasts populated in a collagen matrix can be immuno-protected in an animal model. The results show a significant reduction in the number of infiltrated CD3(+) T lymphocytes on days 14 and 28 post-transplantation in the wounds receiving IDO-expressing fibroblasts relative to controls. IDO-expressing human fibroblasts embedded in bovine collagen on wounds in a rat model accelerates wound healing by promoting neovascularization during the early stages and providing protection of the xenograft fibroblasts. Using a co-culture system, we further confirm that IDO can induce angiogenesis through the depletion of tryptophan. These findings suggest that IDO may have an application in promoting the engraftment of skin substitutes and other transplanted organs.

Engineered skin substitutes: practices and potentials

Wound healing can be problematic in several clinical settings because of massive tissue injury (burns), wound healing deficiencies (chronic wounds), or congenital conditions and diseases. Engineered skin substitutes have been developed to address the medical need for wound coverage and tissue repair. Currently, no engineered skin substitute can replace all of the functions of intact human skin. A variety of biologic dressings and skin substitutes have however contributed to improved outcomes for patients suffering from acute and chronic wounds. These include acellular biomaterials and composite cultured skin analogs containing allogeneic or autologous cultured skin cells.

[Regenerative medicine and plastic surgery]

Regenerative medicine recently evolved as a new medical field that includes tissue engineering, cell/system biology, nanotechnology, pharmacology, stem-cell biology, and bioengineering. Regenerative medicine targets new forms of therapy to promote and support the intrinsic, autologous, regenerative potential of human biological systems. All fields of surgery have profited from these developments, and spectacular experimental results and clinical benefits have been obtained. Plastic surgery has shown interest in regenerative medicine due to its focus on reconstructive surgery. Early on, several interdisciplinary experimental working groups were founded including plastic surgery. This overview takes a closer look at common experimental and clinical results of regenerative medicine and plastic surgery.

In vivo gene delivery of Ad-VEGF121 to full-thickness wounds in aged pigs results in high levels of VEGF expression but not in accelerated healing

We have previously reported that endogenous vascular endothelial growth factor (VEGF) concentration in older pig wounds peaked later and at one-fourth the level of young pigs. These data suggested that VEGF might play a major role in the healing of full-thickness wounds in the aged pig. By in vivo gene transfer using the microseeding technique, we treated full-thickness wounds with different doses of VEGF-expressing adenoviral vector (Ad-VEGF) varying from 1 x 10(7) to 2.7 x 10(11) particles per wound (ppw). We found that the VEGF expression in wound fluid followed a dose-response pattern. However, when wounds were microseeded with the highest concentration of Ad-VEGF (2.7 x 10(11) ppw), diminished healing rates were found. We then determined the minimal functional concentrations of Ad-VEGF. We used five aged Yucatan minipigs, all retired breeders, to analyze the role of over-expression of 1 x 10(8) and 1 x 10(9) ppw of Ad-VEGF (n= 78) in terms of healing of full-thickness wounds, all 2.5 x 2.5 x 1 cm in size (n= 158). The Ad-VEGF solutions were delivered to the wound floor and borders by in vivo microseeding. Control wounds (n= 80) were microseeded with Ad-Lac-Z (n= 25), treated with saline (n= 49) or treated dry (n= 6). All wounds except for the dry-treated ones were covered with a wound chamber and a wet environment was created by injecting 2.5 ml saline into the chamber. Peak VEGF expression (2300-4000 pg/ml) was detected on days 2 or 3 post gene delivery. This level of VEGF expression was not seen in the saline (n= 49) or Ad-null (n= 25) control groups. The VEGF expression in wounds treated with 1 x 10(8) and 3 x 10(8) ppw (n= 39) exhibited a slower onset with a peak concentration of 400-920 pg/ml on days 5-7. Although high levels of VEGF expression were achieved in the local wound environment, we could not show a significant increase in neovascularization as compared to saline-treated wounds. No significant differences were observed in the rate of reepithelialization and wound contraction among groups of full-thickness wounds treated with Ad-VEGF, Ad-null mutant, or saline in the aged "wet wound healing" pig model. These results indicate that increased levels of VEGF in wounds produced by in vivo gene transfer have little effect on the healing of full-thickness wounds in the aged pig model. Moreover, significantly higher levels of VEGF expression by Ad-VEGF could lead to impaired wound healing.

The effects of systemic hypoxia on colon anastomotic healing: an animal model

PURPOSE

Acute postoperative systemic hypoxia occurs frequently in the clinical setting following intestinal resection, as a result of complications such as pneumonia, pulmonary edema, or the acute respiratory distress syndrome. Although it is well established that oxygen is essential for metabolism in general and intestinal anastomotic healing, the mechanisms by which systemic hypoxia affect this process are not clear. The purpose of this study was to establish an animal model to simulate acute systemic hypoxia and to examine the effects on anastomotic healing. We investigated the hypothesis that systemic hypoxia impairs anastomotic healing in the colon by disrupting revascularization via changes in the expression of two putative angiogenic factors: inducible nitric oxide synthase and vascular endothelial growth factor.

METHODS

Phase I: Juvenile male Sprague-Dawley rats underwent carotid artery cannulation. In a controlled environment the FiO2 was incrementally decreased from 21 to 9 percent and the resultant PaO2 measured. Phase II: Animals underwent colonic transection with immediate reanastomosis and were placed in either a normoxic (FiO2 21 percent) or hypoxic (FiO2 11 percent) environment for seven days. Perianastomotic in vivo tissue oxygen saturation was measured before segmental colon resection in each of the animals and at seven days before measurement of anastomotic bursting pressure. Perianastomotic tissue samples were assessed by Western blot assay for the expression of vascular endothelial growth factor and inducible nitric oxide synthase protein. Sections from each tissue sample were taken and evaluated by a pathologist blinded to treatment group for determination of anastomotic healing score.

RESULTS

Phase I: Incrementally decreasing the FiO2 resulted in a progressive decrease in PaO2 (r2 = 0.77). Phase II: Animals maintained in a hypoxic environment had a significant decrease in tissue oxygen saturation (73 +/- 9 percent vs. 94 +/- 3 percent; P < 0.0001) and anastomotic bursting pressure (118 +/- 18 mmHg vs. 207 +/- 30 mmHg; P < 0.0001) compared with normoxic controls. Systemic hypoxia induced a significant increase, when compared with normoxic controls, in vascular endothelial growth factor (247.1 +/- 9.5 vs. 142.2 +/- 10.6; P < 0.0001) and inducible nitric oxide synthase (259.6 +/- 21.1 vs. 120.2 +/- 10.9; P < 0.0001) protein expression and led to a significant decrease in the overall wound-healing score.

CONCLUSION

This study validates a new animal model to study the effects of acute systemic hypoxia on colonic anastomotic healing. In this model, systemic hypoxia directly translated into local tissue hypoxia, and anastomotic healing was impaired. Contrary to our original hypothesis, hypoxia led to a significant increase in vascular endothelial growth factor and inducible nitric oxide synthase protein expression at the colonic anastomotic site. Impairment in anastomotic integrity despite upregulation of these angiogenic factors could be a result of the inability of wounded tissue to respond to vascular endothelial growth factor and inducible nitric oxide synthase or alternatively, hypoxia may adversely affect collagen synthesis and deposition directly.

Efficient Gene Transfer to Human Epidermal Keratinocytes on Fibronectin: In Vitro Evidence for Transduction of Epidermal Stem Cells

The epidermis is an attractive target for gene therapy because it is easily accessible and has great potential as an ectopic site for protein delivery in vivo. Genetically modified keratinocytes can be expanded in culture and used to generate three-dimensional skin equivalents, which can deliver therapeutic proteins either locally or systemically for the treatment of wounds or systemic diseases. Here we present an optimum protocol that yields consistently high retroviral gene transfer on a substrate of recombinant fibronectin (rFN). Gene transfer on rFN depends strongly on virus concentration and the density of target cells. Interestingly, the kinetics of gene transfer varies depending upon the origin--mouse or human--of virus-producer cells. Most notably, long-term growth and clonogenic assays show that transduction on rFN promotes gene transfer to epidermal stem cells and prevents loss of clonogenic potential due to exposure of cells to retroviral supernatant. In contrast, the traditional protocol transduces mostly differentiated keratinocytes. We also show that skin equivalents prepared from genetically modified keratinocytes display high levels of transgene expression, mainly in the suprabasal layers. Our results are important for cutaneous gene therapy and for biological studies that require efficient and permanent genetic modification.

Skin grafts and skin flaps in the horse

Although most equine wounds can be easily treated and heal without consequence, there are many that present special challenges. Skin grafts area valuable part of the veterinarian's armamentarium for treatment of complicated wounds, particularly limb wounds. Attention to preparation of the recipient site and proper aftercare are critical to successful grafting. With better understanding of equine wound and graft physiology and the promise foretold by advances in human skin grafting, the outcome of treatments of difficult equine wounds should continue to improve in the future.

Gene transfer to epidermal stem cells: implications for tissue engineering

The skin is an attractive target for gene therapy because it is easily accessible and shows great potential as an ectopic site for protein delivery in vivo. Genetically modified epidermal cells can be used to engineer three-dimensional skin substitutes, which when transplanted can act as in vivo 'bioreactors' for delivery of therapeutic proteins locally or systemically. Although some gene transfer technologies have the potential to afford permanent genetic modification, differentiation and eventual loss of genetically modified cells from the epidermis results in temporary transgene expression. Therefore, to achieve stable long-term gene expression, it is critical to deliver genes to epidermal stem cells, which possess unlimited growth potential and self-renewal capacity. This review discusses the recent advances in epidermal stem cell isolation, gene transfer and engineering of skin substitutes. Recent efforts that employ gene therapy and tissue engineering for the treatment of genetic diseases, chronic wounds and systemic disorders, such as leptin deficiency or diabetes, are reviewed. Finally, the use of gene-modified tissue-engineered skin as a biological model for understanding tissue development, wound healing and epithelial carcinogenesis is also discussed.

An In Vivo Model of Wound Healing in Genetically Modified Skin-Humanized Mice

Cutaneous wound-healing disorders are a major health problem that requires the development of innovative treatments. Whithin this context, the search for reliable human wound-healing models that allow us to address both mechanistic and therapeutic matters is warranted. In this study, we have developed a novel invivo wound-healing model in a genetically modified human context. Our model is based on the regeneration of human skin on the back of nude mice by transplantation of a cultured bioengineered skin equivalent previously designed in our laboratory. In this setting, human keratinocytes in the epidermal compartment were genetically modified with a retroviral vector encoding the enhanced green fluorescent protein (EGFP). After stable engraftment of the EGFP skin was achieved (9-12 wk after grafting), a small circular full thickness wound was performed on this mature human skin. A wide variety of parameters involved in wound healing were monitored, including tissue architecture, cell proliferation, epidermal differentiation, dermal remodelling, and basement membrane regeneration. Wounded gene-targeted skin-humanized mice re-capitulated native skin wound-healing features. In addition, when keratinocyte growth factor (KGF), a growth factor that has been shown to improve wound healing, was added to wounds during 3 d, the re-epithelialization was significantly accelerated. The present wound-healing model system provides a suitable in vivo tool to test gene transfer strategies for human skin repair. It also serves as a complementary platform for studies in genetically modified mice and as a model to evaluate pharmaceutical therapeutic approaches for impaired wound healing.

Current approaches and perspectives in human keratinocyte-based gene therapies

Inherited and acquired disorders are liable to treatment with somatic gene therapy. The skin, and in particular epidermal cells, are particularly suited to genetic manipulation and follow-up of therapeutic effects. Cutaneous gene therapy may be effective for skin defects and systemic abnormalities. The robust basic and preclinical data available today would support the application of keratinocyte-based gene therapy to patients.

A review of keratinocyte delivery to the wound bed

Over the last 20 years, confluent sheets of cultured epithelial autograft have been used for patients with major burns. Problems with the lack of "take" and long-term durability, as well as the time delay to produce such grafts, have led to the development of delivery systems to transfer keratinocytes to the wound bed. This review article describes the problems of using cultured epithelial autograft and the advantages of using preconfluent keratinocytes. Despite the numerous delivery systems that have been reported, most studies are limited to animal wound bed models. There are a few small clinical studies that have demonstrated enhanced healing using mainly subjective methods. There is a need for controlled, randomized clinical trials to prove the efficacy of keratinocyte delivery systems. Proposals for the use of this technology are made.

Vascular endothelial growth factor overexpression increases vascularization by murine but not human endothelial cells in cultured skin substitutes grafted to athymic mice

Cultured skin substitutes (CSS) consisting of fibroblasts, keratinocytes, and biopolymers are an adjunctive treatment for large burns. Because CSS lack a vascular plexus, they vascularize more slowly than split-thickness autografts. Previously, CSS were prepared with dermal microvascular endothelial cells (ECs), which formed vascular analogs at a low frequency but did not contribute to increased vascularization after grafting. The present study addressed whether keratinocytes genetically modified to overexpress vascular endothelial growth factor (VEGF), an endothelial cell mitogen, could improve the persistence and organization of ECs in CSS. CSS were prepared with control or VEGF-modified keratinocytes, with (CSS + ECs) or without added ECs, and were grafted to full-thickness wounds in athymic mice. Elevated VEGF expression was detected in VEGF-modified CSS and CSS + ECs compared with controls, but no significant difference in EC density in vitro was observed. After grafting, VEGF-modified CSS and CSS + ECs showed enhanced vascularization, and organization of human ECs into multicellular structures in CSS + ECs was observed. However, VEGF overexpression did not significantly enhance the proliferation of human ECs, suggesting that other factors may be required. Improved persistence and organization of human ECs in vitro will likely be required for their participation in vascularization of CSS + ECs after grafting.

Engraftment of a vascularized human skin equivalent

Clinical performance of currently available human skin equivalents is limited by failure to develop perfusion. To address this problem we have developed a method of endothelial cell transplantation that promotes vascularization of human skin equivalents in vivo. Enhancement of vascularization by Bcl-2 overexpression was demonstrated by seeding human acellular dermis grafts with human umbilical vein endothelial cells (HUVEC) transduced with the survival gene Bcl-2 or an EGFP control transgene, and subcutaneous implantation in immunodeficient mice (n=18). After 1 month the grafts with Bcl-2-transduced cells contained a significantly greater density of perfused HUVEC-lined microvessels (55.0/mm3) than controls (25.4/mm3,P=0.026). Vascularized skin equivalents were then constructed by sequentially seeding the apical and basal surfaces of acellular dermis with cultured human keratinocytes and Bcl-2-transduced HUVEC, respectively. Two weeks after orthotopic implantation onto mice, 75% of grafts (n=16) displayed both a differentiated human epidermis and perfusion through HUVEC-lined microvessels. These vessels, which showed evidence of progressive maturation, accelerated the rate of graft vascularization. Successful transplantation of such vascularized human skin equivalents should enhance clinical utility, especially in recipients with impaired angiogenesis.

Improved free vascular graft survival in an irradiated surgical site following topical application of rVEGF

PURPOSE

Wound healing disorders following surgery in preirradiated tissue are clinically well known and may even become more crucial with the increasing use of neoadjuvant chemoradiation protocols. Both the expression of vascular endothelial growth factor (VEGF) and endoglin (CD105) play a key role in neovascularization and wound healing after soft tissue grafts in irradiated and nonirradiated tissue. Modulation of neovascularization through the application of recombinant VEGF (rVEGF) may be a therapeutic option to reduce wound healing disorders in irradiated tissue. An experimental in vivo model was used to study the possible role of rVEGF for reduction of wound healing disorders and the promotion of neovascularization.

METHODS AND MATERIALS

A free myocutaneous gracilis flap was transplanted from the groin into the neck region of Wistar rats (weight 300-500 g) with and without previous irradiation of the neck region with 40 Gy: Group 1 (n = 7) radiotherapy alone; Group 2 (n = 14) flap transplantation alone and rVEGF; Group 3 (n = 14) radiotherapy, transplantation, and rVEGF. Time interval between irradiation and grafting was 10 +/- 1 day. 1.0 micro g rVEGF/500 microL phosphate-buffered saline was applied s.c. intraoperatively and on Days 1 through 7. Neovascularization (CD105) and endogenous VEGF expression were analyzed by means of immunohistochemistry on Days 3, 5, 7, 14, and 28 postoperatively and quantified as labeling indices (LI).

RESULTS

After irradiation there was a continuous significant reduction of the cytoplasmic VEGF expression (MEAN LI: 0.018 +/- 0.048) compared with the nonirradiated control (mean LI: 0.042 +/- 0.006) (p < 0.001). VEGF expression after flap transplantation without irradiation after VEGF application was at a constantly higher level from Day 3 (mean LI: 0.044 +/- 0.01) to Day 28 postoperatively compared with the control group (Day 3, mean LI: 0.028 +/- 0.006) (p < 0.001). As an indication of increased neovascularization after the local application of rVEGF, a significantly increased expression of CD105 was found in the transition area and graft bed from Day 7 on (p < 0.001). After irradiation and grafting there was a significant overall increase in the VEGF- and CD105-expression throughout Day 28 after rVEGF in the transition area (p < 0.001).

CONCLUSION

Whereas irradiation alone led to a downregulation of the endogenous VEGF expression, rVEGF application resulted in an increased expression and in a CD105 associated neovascularization after soft tissue grafting in irradiated tissues. Application of rVEGF may enable modulation of wound healing by influencing neovascularization. This could indicate a possible clinical approach for reducing fibrosis and chronic wound healing disorders in irradiated tissues.

Cytokines and growth factors in keratinocytes and sweat glands in chronic venous leg ulcers. An immunohistochemical study

The role of growth factors and cytokines in the impaired healing of chronic leg ulcers remains uncertain. The aim of this study was to determine whether changes in the amount and location of cytokines and growth factors may be associated with impaired healing in chronic leg ulcers. Biopsies from leg ulcers of 21 patients and from normal skin of nine healthy volunteers were examined immunohistochemically for selected growth factors and cytokines. Greater staining intensity was found in keratinocytes at the edges of ulcers compared to normal skin, or skin adjacent to the ulcers. Staining at the ulcer edge was more intense in nonhealing ulcers for only vascular endothelial growth factor and platelet-derived growth factor, whereas staining in the adjacent skin was more intense for all factors in the nonhealing phase. For all factors staining was cytoplasmic, suggesting production in these areas. This study shows up-regulation of the production of cytokines and growth factors in keratinocytes of chronic leg ulcers that is greater when the ulcers are nonhealing.

Long-term remodeling of a bilayered living human skin equivalent (Apligraf) grafted onto nude mice: immunolocalization of human cells and characterization of extracellular matrix

Type I collagen is a clinically approved biomaterial largely used in tissue engineering. It acts as a regenerative template in which the implanted collagen is progressively degraded and replaced by new cell-synthesized tissue. Apligraf, a bioengineered living skin, is composed of a bovine collagen lattice containing living human fibroblasts overlaid with a fully differentiated epithelium made of human keratinocytes. To investigate its progressive remodeling, athymic mice were grafted and the cellular and the extracellular matrix components were studied from 0 to 365 days after grafting. Biopsies were analyzed using immunohistochemistry with species-specific antibodies and electron microscopy techniques. We observed that this bioengineered tissue provided living and bioactive cells to the wound site up to 1 year after grafting. The graft was rapidly incorporated within the host tissue and the bovine collagen present in the graft was progressively replaced by human and mouse collagens. A normal healing process was observed, i.e., type III collagen appeared transiently with type I collagen, the major collagen isoform present at later stages. New molecules, such as elastin, were produced by the living human cells contained within the graft. This animal model combined with species-specific immunohistochemistry tools is thus very useful for studying long-term tissue remodeling of bioengineered living tissues.