Gene transfer to epidermal stem cells: implications for tissue engineering

Bioengineered skin substitutes

Bioengineered skin has great potential for use in regenerative medicine for treatment of severe wounds such as burns or chronic ulcers. Genetically modified skin substitutes have also been used as cell-based devices or "live bioreactors" to deliver therapeutics locally or systemically. Finally, these tissue constructs are used as realistic models of human skin for toxicological testing, to speed drug development and replace traditional animal-based tests in a variety of industries. Here we describe a method of generating bioengineered skin based on a natural scaffold, namely, decellularized human dermis and epidermal stem cells.

Therapeutic potential of stem cells in skin repair and regeneration

Stem cells are defined by their capacity of self-renewal and multilineage differentiation, which make them uniquely situated to treat a broad spectrum of human diseases. Based on a series of remarkable studies in several fields of regenerative medicine, their application is not too far from the clinical practice. Full-thickness burns and severe traumas can injure skin and its appendages, which protect animals from water loss, temperature change, radiation, trauma and infection. In adults, the normal outcome of repair of massive full-thickness burns is fibrosis and scarring without any appendages, such as hair follicles, sweat and sebaceous glands. Perfect skin regeneration has been considered impossible due to the limited regenerative capacity of epidermal keratinocytes, which are generally thought to be the key source of the epidermis and skin appendages. Currently, researches on stem cells, such as epidermal stem cells, dermal stem cells, mesenchymal stem cells from bone marrow, and embryonic stem cells, bring promise to functional repair of skin after severe burn injury, namely, complete regeneration of skin and its appendages. In this study, we present an overview of the most recent advances in skin repair and regeneration by using stem cells.

Contractile smooth muscle cells derived from hair-follicle stem cells

AIMS

We hypothesized that hair-follicle stem cells can differentiate toward smooth contractile muscle cells, providing an autologous cell source for cardiovascular tissue regeneration.

METHODS AND RESULTS

Smooth muscle progenitor cells (SMPCs) were obtained from ovine hair follicles using a tissue-specific promoter and fluorescence-activated cell sorting. Hair-follicle smooth muscle progenitor cells (HF-SMPCs) expressed several markers of vascular smooth muscle including alpha-actin, calponin, myosin heavy chain (MHC), caldesmon, smoothelin, and SM22. HF-SMPCs were highly proliferative and showed high clonogenic potential without any signs of chromosomal abnormalities as evidenced by karyotype analysis. HF-SMPCs compacted fibrin hydrogels to a similar extent as vascular smooth muscle cells from ovine umbilical veins (V-SMCs), indicating the development of the force-generating machinery. In addition, cylindrical tissue equivalents prepared with HF-SMPCs displayed significant contractility in response to vasoactive agonists including KCl and the thromboxane A2 mimetic U46619, suggesting that these cells had developed receptor and non-receptor-mediated pathways of contractility. Finally, transforming growth factor-beta1 promoted differentiation of HF-SMPCs toward a mature SMC phenotype as suggested by increased expression of MHC and enhanced matrix compaction.

CONCLUSION

Our results suggest that hair follicles may be an easily accessible, autologous, and rich source of functional SMPC for cardiovascular tissue engineering and regenerative medicine.

Immune response to stem cells and strategies to induce tolerance

Although recent progress in cardiovascular tissue engineering has generated great expectations for the exploitation of stem cells to restore cardiac form and function, the prospects of a common mass-produced cell resource for clinically viable engineered tissues and organs remain problematic. The refinement of stem cell culture protocols to increase induction of the cardiomyocyte phenotype and the assembly of transplantable vascularized tissue are areas of intense current research, but the problem of immune rejection of heterologous cell type poses perhaps the most significant hurdle to overcome. This article focuses on the potential advantages and problems encountered with various stem cell sources for reconstruction of the damaged or failing myocardium or heart valves and also discusses the need for integrating advances in developmental and stem cell biology, immunology and tissue engineering to achieve the full potential of cardiac tissue engineering. The ultimate goal is to produce 'off-the-shelf' cells and tissues capable of inducing specific immune tolerance.

Directed three-dimensional growth of microvascular cells and isolated microvessel fragments

Tissue engineering has promise as a means for repairing diseased and damaged tissues. A significant challenge in tissue construction relates to the constraints placed on tissue geometries resulting from diffusion limitations. An ability to incorporate a premade vasculature would overcome these difficulties and promote construct viability once implanted. Most in vitro microvascular fabrication strategies rely on surface-associated cell growth, manipulated cell monolayers, or random arrangement of cells within matrix materials. In contrast, we successfully suspended microvascular cells and isolated microvessel fragments within collagen and then microfluidically drove the mixtures into microfabricated network topologies. Developing within the 3D collagen matrix, patterned cells progressed into cord-like morphologies. These geometries were directed by the surrounding elastomer mold. With similar techniques, suspended fragments formed endothelial sprouts. By avoiding the addition of exogenous growth factors, we allowed constituent cells and fragments to autonomously develop within the constructs, providing a more physiologically relevant system for in vitro microvascular development. In addition, we present the first examples of directed endothelial cell sprouting from parent microvessel fragments. We believe this system may serve as a foundation for future in vivo fabrication of microvascular networks for tissue engineering applications.

Gene transfer to ocular stem cells by early gestational intraamniotic injection of lentiviral vector

Ocular gene transfer has generally been approached by direct intraocular injection. In this study, we hypothesized that an opportunity exists during early gestation when specific ocular stem cell populations are accessible for gene transfer. These include the stem cell populations that maintain the cornea, lens, and retina throughout life. To test this hypothesis, we injected lentiviral vector encoding the green fluorescent protein (GFP) reporter gene into the murine amniotic space from the late head fold/early somite stage postcoital day 8 (E8) to E18 and performed sequential analysis of GFP expression in ocular tissues. Depending on the timing of vector exposure, significant GFP expression was observed in all ectoderm-derived tissues in the eye. With injection at early gestational time points, GFP expression persisted long term, with evidence of high efficiency stem cell transduction in the cornea, lens, and retina. The observed patterns and duration of gene expression confirm the accessibility of ocular stem cell populations for lentiviral vector-based gene transfer at specific developmental time points in early gestation. This model may be useful for the investigation of mechanisms of genetic and/or developmental ocular disease and for the development of prenatal gene therapy for specific ocular disorders.

Microengineered surface topography facilitates cell grafting from a prototype hydrogel wound dressing with antibacterial capability

Skin wounds derive therapeutic benefit from redeployment of dermal tissues, whether as split-thickness allo- and autografts or as biological dressings comprising cultured cells. However, the clinical outcome is strongly influenced by the techniques used for cell/tissue grafting and also the microbiological status of the wound. Here we report that microtopography incorporated into the surface of a novel polymeric material, derivatized with fibronectin to promote attachment and encourage motility, improved the efficiency of cell transfer onto de-epithelialized human skin ex vivo. The microtopography had two functions, first as a conduit for migrating cells to cross between the vehicle and recipient surface and second to shield adherent cells from destruction by mechanical shearing during handling and application. Quantitative analysis showed that topographic projections (columns) rather than recesses (pits) in the hydrogel surface achieved the highest efficiency of cell transfer. In order to address the crucial relevance of microbiological contamination to the success of wound grafting, the effect of iodine on several common bacterial pathogens was examined using an XTT+C(Q10) kinetic cell viability assay. Increasing concentrations of iodine initially stressed and after 0.5% v/v were subsequently bacteriocidal for Gram-negative Pseudomonas aeruginosa and Escherichia coli and Gram-positive Bacillus subtillis and Staphylococcus aureus. Slightly higher doses of iodine (approx 1-1.5% v/v) were required to kill HaCaT cells outright, but for both pro- and eukaryotes the major determinant of cytotoxicity was absolute dose rather than duration of exposure. Iodine delivered by the hydrogel at low concentration was bacteriostatic but not apparently cytotoxic to epithelial cells as measured by MTT end-point cell viability assay. Zone of inhibition studies confirmed that bacteriocidal quantities of neomycin, phenol red, and silver could also be delivered using the same hydrogel. This research suggests that grafting cell-based biological dressings to wounds using a topographically modified hydrogel dressing capable of simultaneous reducing the microbiological threat to a successful outcome may be a realistic clinical proposition.

Experimental models and high-throughput diagnostics for tissue regeneration

During wound healing, cells recreate functional structures to regenerate the injured tissue. Understanding the healing process is essential for the development of new concepts and the design of novel biomimetic approaches for delivery of cells, genes and growth factors to accelerate tissue regeneration. To this end, realistic experimental models and high-throughput diagnostics are necessary to understand the molecular mechanisms of healing and reveal the genetic networks that determine tissue repair versus regeneration. Following a brief overview of the biology of wound healing, this review covers the in vitro and in vivo models that are employed at present to study the healing process. Discussion then covers the application of high-throughput genomic and proteomic technologies in epithelial development, living skin substitutes and wound healing. Finally, this review provides a perspective on novel technologies that should be developed to facilitate the understanding of wound healing complications and the design of therapeutics that target the underlying deficiencies.

Biomimetic approaches to protein and gene delivery for tissue regeneration

Novel therapeutic strategies that promote wound healing seek to mimic the response of the body to wounding, to regenerate rather than repair injured tissues. Many synthetic or natural biomaterials have been developed for this purpose and are used to deliver wound therapeutics in a controlled manner that prevents unwanted and potentially harmful side-effects. Here, we review the natural and synthetic biomaterials that have been developed for protein and gene delivery to enhance tissue regeneration. Particular emphasis is placed on novel biomimetic materials that respond to environmental stimuli or release their cargo according to cellular demand. Engineering biomaterials to release therapeutic agents in response to physiologic signals mimics the natural healing process and can promote faster tissue regeneration and reduce scarring in severe acute or chronic wounds.

Epidermale Stammzellen. Epidermal stem cells

Current understanding of the biology of epidermal stem cells opens a totally new perspective in the function of the epidermis and adjacent epithelial structures. A number of pathogenetic as well as clinical-therapeutic approaches against a variety of dermatoses may become possible with knowledge about keratinocyte proliferation, differentiation and regeneration. The reservoir of epidermal stem cells is located in the interfollicular epidermis, the hair follicle area and the germinal hair follicle matrix. Endogenous stem cell clones exist here, giving rise to transient amplifying cells and postmitotic cells. The stem cell clones are organized in clusters and display high expression of adhesion proteins, which guarantee their stability in a specific environment consisting of different cell types and extracellular substrates in the stratum basale. Differentiation is determined by a specific cascade of chemical signals from the stem cell environment and from the genetic program of the cell. The clinical relevance of stem cells lies primarily in their therapeutic potential with reconstruction of epithelia by reimplantation of autologous stem cells or gene therapeutic applications such as targeted transfection. However, the benefit-to-risk ratio cannot yet be accurately estimated.

Tissue engineering of cultured skin substitutes

Skin replacement has been a challenging task for surgeons ever since the introduction of skin grafts by Reverdin in 1871. Recently, skin grafting has evolved from the initial autograft and allograft preparations to biosynthetic and tissue-engineered living skin replacements. This has been fostered by the dramatically improved survival rates of major burns where the availability of autologous normal skin for grafting has become one of the limiting factors. The ideal properties of a temporary and a permanent skin substitute have been well defined. Tissue-engineered skin replacements: cultured autologous keratinocyte grafts, cultured allogeneic keratinocyte grafts, autologous/allogeneic composites, acellular biological matrices, and cellular matrices including such biological substances as fibrin sealant and various types of collagen, hyaluronic acid etc. have opened new horizons to deal with such massive skin loss. In extensive burns it has been shown that skin substitution with cultured grafts can be a life-saving measure where few alternatives exist. Future research will aim to create skin substitutes with cultured epidermis that under appropriate circumstances may provide a wound cover that could be just as durable and esthetically acceptable as conventional split-thickness skin grafts. Genetic manipulation may in addition enhance the performance of such cultured skin substitutes. If cell science, molecular biology, genetic engineering, material science and clinical expertise join their efforts to develop optimized cell culture techniques and synthetic or biological matrices then further technical advances might well lead to the production of almost skin like new tissue-engineered human skin products resembling natural human skin.

Microbubble-enhanced ultrasound for gene transfer into living skin equivalents

BACKGROUND

Gene transfer to skin is an attractive therapeutic approach because of the accessibility of the skin and the high rate of cure for many cutaneous diseases. However, safety concerns over viral vectors and the low efficiency of most non-viral gene transfer techniques have encumbered their clinical application for gene transfer. By contrast, efficient gene transfers into various cell types using microbubble-enhanced ultrasound has been reported.

OBJECTIVES

The purpose of this study was to investigate whether ultrasound with microbubble enhancement allowed effective transfer of foreign genes into living skin equivalents (LSEs).

METHODS

Microbubbles and plasmid DNA encoding green fluorescent protein (GFP) were added to the dermal-epidermal junctions of LSEs, which were then exposed to ultrasound. The LSEs were harvested at different time points to investigate transgene expression using confocal laser microscopy. Transfected LSEs were also transplanted onto nude mice, and the in vivo transgene expression was observed.

RESULTS

From days 2 to 7 after transfection, most GFP-positive cells continued to migrate upward from the basal layer, while other GFP-positive cells lagged behind or remained in the basal layer on days 5 and 7. Transfection resulted in 20-30% GFP-positive cells. Multiple transfections further increased the percentage of transfected cells and resulted in multi-layer transgene expression. Grafts from the transfected LSEs survived on nude mice and continued to express GFP up to 2 weeks post-transplantation.

CONCLUSION

Gene transfer into LSE using ultrasound with microbubble enhancement is an effective alternative to viral and non-viral methods.