Particle-mediated gene transfer of PDGF isoforms promotes wound repair

Skin permeation, biodistribution, and expression of topically applied plasmid DNA

BACKGROUND

Topical application is emerging as a new route of gene delivery. However, the extent of skin permeation and the in vivo fate of topically applied plasmid DNA are not fully understood.

METHODS

In vitro permeation of plasmid DNA across human skin and keratinocyte layers was tested using Franz diffusion cells. In vivo absorption and biodistribution of topically applied plasmid in mice were determined using quantitative polymerase chain reaction (PCR). The expression levels of plasmid DNA in various tissues were measured by semiquantitative reverse transcription PCR.

RESULTS

In vitro, topically applied DNA was capable of penetrating human skin and keratinocyte layers. Following topical application of plasmid DNA onto murine skin, the levels of plasmid DNA in the serum peaked at 4 hr. At 24 hr post-dose, topically applied DNA existed at higher levels than intravenously administered DNA in almost all tissues, and induced 11.4- and 22-fold higher mRNA expression in muscle and skin, respectively. Moreover, the topical route showed sustained expression of plasmid DNA in the regional lymph nodes over 5 days, whereas the intravenous route did not.

CONCLUSIONS

Taken together, our results show that topically applied plasmid DNA is capable of permeating the skin and being expressed for prolonged periods in various tissues including lymph nodes. This suggests that skin may provide an appealing, noninvasive route of delivery for DNA vaccines and other therapeutic genes.

Gene therapy applications for spine fusion

OBJECTIVE

This article reviews the potential utilization of various growth factors to enhance spinal fusion and outlines the principles of gene therapy and its application to spinal fusion surgery.

SUMMARY OF BACKGROUND DATA

Gene therapy offers an exciting new way to potentially deliver growth factors locally in a targeted fashion with physiologic doses. In its current definition, gene therapy is defined as the use of nucleic acid transfer, either RNA or DNA, to treat or prevent a disease. The scope of gene therapy has expanded beyond its initial application as a method of replacing genetic defects, and its potential to facilitate spinal fusions is currently being evaluated.

CONCLUSIONS

Gene therapy strategies for spine fusion are appealing because the setting is uniquely suited for genetic manipulation. The intervention is locally applied. Only a short duration of transgene response by the cells is necessary to establish a spine fusion, and a variety of osteoinductive growth factors have been identified and are available for use. Attempts at spine fusion using gene therapy in the lower animals have been successful using both in vivo and ex vivo approaches. Before human clinical trials can be established, further testing is required in more challenging animal models of bone induction such as nonhuman primates. Should a successful clinical program of gene therapy for spine fusion be established, the use of autograft and its associated morbidities could be eliminated. In fact, gene therapy offers the potential for minimally invasive applications that could bypass the need for an open procedure altogether. It is likely that gene therapy will be a powerful therapeutic tool for the spine surgeon in the new millennium.

Growth factors in wound healing

Enhancement of wound healing was limited to good surgical technique, maintenance of a clean wound with appropriate dressings, and debridement. The ability to heal wounds has been advanced through the recognition that healing in a moist environment is improved over that of a desiccated wound. Pharmacologic approaches to wound healing did not exist until the last few decades, when it was recognized that growth factors are normally present in the wound environment and that in animal models and a few clinical studies, the addition of growth factors could enhance healing. In 1998, platelet-derived growth factor was approved for clinical use. This approach is still the subject of intense investigation and clinical trials. This article analyzes current knowledge on growth factors as therapeutic agents and speculates on their future potential, with an analysis of successes and failures to date.

Gene therapy in wound healing

Gene therapy is a new and emerging technology that has been catalyzed by the progress of the Human Genome Project. It employs the process of manipulating genes to achieve a clinically beneficial alteration in gene product. Wound healing lends itself to the application of gene therapy by virtue of the vast array of proteins involved in its complex cascade. This article provides an overview of the background to gene therapy and describes current techniques in use as applied to wound healing. The authors show the potential role that many candidate genes may offer in the future for optimizing wound healing through gene therapy.

SPARC-null mice display abnormalities in the dermis characterized by decreased collagen fibril diameter and reduced tensile strength

Although collagen and elastic fibers are among the major structural constituents responsible for the mechanical properties of skin, proteins that associate with these components are also important for directing formation and maintaining the stability of these fibers. We present evidence that SPARC (secreted protein acidic and rich in cysteine) contributes to collagen fibril formation in the dermis. The skin of SPARC-null adult mice had approximately half the tensile strength as that of wild-type skin. Moreover, the collagen content of SPARC-null skin, as measured by hydroxyproline analysis, was substantially reduced in adult mice. At 2 weeks of age, no differences in collagen content were observed; within 2 months, however, the dermis of SPARC-null mice displayed a reduced collagen content that persisted through adulthood until approximately 20 months, when collagen levels of SPARC-null skin approximated those of wild-type controls. The collagen fibrils present in SPARC-null skin were smaller and more uniform in diameter, in comparison with those of wild-type skin. At 5 months of age, the average fibril diameter in SPARC-null versus wild-type skin was 60.2 nm versus 87.9 nm, respectively. Extraction of soluble dermal collagen revealed a relative increase in collagen VI, accompanied by a decrease in collagen I, in SPARC-null mice. A reduction in the relative amounts of higher-molecular weight collagen complexes was also observed in extracts of dermis from SPARC-null animals. Thus the absence of SPARC compromises the mechanical properties of the dermis, an effect that we attribute, at least in part, to the changes in the structure and composition of its collagenous extracellular matrix.

Cellular, biochemical, and clinical aspects of wound healing

BACKGROUND

The response to tissue injury requires the symphonious interaction of immune cells, keratinocytes, fibroblasts, and endothelial cells, which unite to regenerate the damaged epithelium. Recent insights have elucidated the cellular and molecular mechanisms required for wound healing and have raised the prospect of novel therapeutic targets.

METHODS

Review of the pertinent literature.

RESULTS

The initial inflammatory response leads to the influx of macrophages and neutrophils, which release cytokines, growth factors, and nitric oxide, and induce nearby keratinocytes to migrate across the wounded epithelium. This process, known as re-epithelialization, requires integrin-mediated activation of Rho-GTPases. The subsequent influx of fibroblasts and endothelial cells results in the production of tissue stroma and formation of new blood vessels, which lead to the generation of functional tissue. Importantly, disease states associated with impaired or excessive wound healing can be attributed to defects in these responses, providing a rationale for the use of evidence-based biological therapies.

CONCLUSION

The elucidation of the cellular and biochemical response to wound healing is essential for an understanding to the treatment of clinical conditions during which impaired healing is encountered.

Intradermal injection of transforming growth factor-beta1 gene enhances wound healing in genetically diabetic mice

PURPOSE

To evaluate the biologic effect of direct cutaneous TGF-beta1 gene delivery on impaired wound healing models using genetically diabetic mice.

METHODS

Diabetic mice (C57BKS.Cg-m +/+ Leprdb female mice) with 1 cm x 1 cm excisional wounds were intradermally injected with 60 microg of plasmid DNA encoding TGF-beta1 gene. The wound closure was measured up to 14 days postwounding. At days 7 and 14 postwounding, sections of skin were taken for hematoxylin and eosin and Masson's trichome staining to examine the morphology and collagen deposition. The cell proliferation and TGF-beta1 gene expression were studied using immunohistochemical stainings for 5-bromo-2-deoxyuridine and for TGF-beta1.

RESULTS

A higher cell proliferation rate and a denser and more organized new extracellular matrix were observed in the treated wound site. Complete wound closure was detected as early as 7 days for TGF-beta1-treated group in comparison with 11-14 days for the untreated, control plasmid DNA- and PBS-treated groups.

CONCLUSION

A single intradermal injection of TGF-beta1 plasmid DNA was sufficient to enhance wound healing. This approach represents a new strategy that may be applied to the treatment of excisional wounds in human diabetic patients.

Growth factors in the treatment of diabetic foot ulcers

BACKGROUND

Chronic foot ulceration is a major source of morbidity in diabetic patients. Despite traditional comprehensive wound management, including vascular reconstruction, there remains a cohort of patients with non-responding wounds, often resulting in amputation. These wounds may benefit from molecular manipulation of growth factors to enhance the microcirculation.

METHODS

A review of the current literature was performed using Pubmed, with secondary references obtained from key articles.

RESULTS AND CONCLUSION

There has been a generally disappointing clinical outcome from growth factor trials, although topical platelet-derived growth factor has shown significant benefit and should be considered in non-healing, well perfused ulcers after failure of conventional wound care. The modulatory role of the extracellular matrix in the cellular response to growth factors and data from regenerative-type fetal wound healing are further areas of interest. The chemical induction of microvessel formation may become a future therapeutic option.

Gene transfer of human hepatocyte growth factor into rat skin wounds mediated by liposomes coated with the sendai virus (hemagglutinating virus of Japan)

Hepatocyte growth factor (HGF) regulates cell growth, cell motility, and morphogenesis in various types of cells, including epithelial and endothelial cells, indicating that it probably promotes epithelial repair and neovascularization during wound healing. To better understand the effects of HGF on wound healing, we performed human HGF-gene transfer into skin wounds in rats. The rat HGF mRNA levels, and human and rat HGF protein concentrations in the wounds in HGF gene-transfer rats were significantly elevated at 3 days, 3 to 14 days, and 3 and 14 days after gene transfer, respectively. An expression of human HGF mRNA and protein was revealed in squamous cells in the epidermis, in endothelial cells and smooth muscle cells in blood vessels, and in fibroblasts in granulation tissues at 3, 7, and 14 days after gene transfer in HGF gene-transfer rats. The wound lesion area in HGF gene-transfer rats was significantly less than that in control rats from 3 to 7 days after gene transfer. The re-epithelialization rate, microvessel counts in granulation tissues, proliferating cell nuclear antigen index of fibroblasts in granulation tissues, and the proliferating cell nuclear antigen index in the epidermis of HGF gene-transfer rats were significantly increased at 3 and 7 days after gene transfer. Semiquantitative reverse transcriptase-polymerase chain reaction revealed that the expression levels of transforming growth factor-beta1 and Colalpha2(I) mRNAs in the wounds of HGF gene-transfer rats were significantly decreased at 7 and 14 days, respectively. The hydroxyproline concentration in the wound was significantly less in HGF gene-transfer rats than in control rats at 3 days after gene transfer. These results suggest that HGF gene transfer into a skin wound may aid re-epithelialization and neovascularization in the early phase of wound healing, and that HGF may play a role in modulating cutaneous wound healing.

Augmentation of wound healing with translation initiation factor eIF4E mRNA

BACKGROUND

Initiation of translation is the rate-limiting step in protein synthesis; eIF4E increases translational efficiency by facilitating ribosome scanning. eIF4E is present in cells in rate-limiting amounts; chronic overexpression of eIF4E causes cell transformation by upregulating growth-related proteins. Biolistic delivery of epidermal growth factor (EGF) increases wound healing; transiently increasing wound eIF4E levels with biolistic mRNA transmission may further augment wound healing without oncogenesis.

PATIENTS AND METHODS

Midline fascial wounds were created in rats and biolistically treated with gold particles carrying mRNA encoding for hEGF with or without eIF4E prior to suture closure; control animals received blank bullets. The animals were sacrificed at 7 or 14 days for determination of peak wound bursting strength on a tensiometer. Results are expressed as means +/- standard deviation; statistics were via analysis of variance.

RESULTS

[Table: see text].

CONCLUSIONS

Simultaneous biolistic delivery of EGF mRNA with eIF4E mRNA significantly increases wound breaking strength compared to that in control animals or treatment with EGF mRNA alone without risk of cellular transformation. Further studies of translational activation to augment wound healing are warranted.

Adenoviral gene transfer of PDGF downregulates gas gene product PDGFalphaR and prolongs ERK and Akt/PKB activation

The delivery of platelet-derived growth factor (PDGF) for tissue engineering of skin and periodontal wounds has become an active area of interest. However, little is known regarding the extended effects of PDGF on cell signaling via gene therapy and how such an approach facilitates the exiting of cells from growth arrest and entry to competence required for cell cycling. We show in vitro expression and secretion of PDGF-AA by recombinant adenovirus encoding the PDGF-A gene (Ad-PDGF-A). The bioactive PDGF-AA protein released induces sustained downregulation of PDGFalphaR that is encoded by a growth arrest-specific (gas) gene. Ad-PDGF-A induces sustained phosphorylation of PDGFalphaR as well as prolonged phosphorylation of downstream extracellular signal-regulated kinase 1/2 and Akt signaling pathways. Furthermore, the phosphorylation of PDGFalphaR is abolished by cotransducing cells with adenovirus encoding a dominant negative mutant of the PDGF-A gene that disrupts PDGF bioactivity. These findings demonstrate the prolonged effects of adenoviral delivery of PDGF and aid in the better understanding of sustained PDGF signaling.

Adenovirus vector-mediated transfer of the vascular endothelial growth factor cDNA to healing abdominal fascia enhances vascularity and bursting strength in mice with normal and impaired wound healing

BACKGROUND

We hypothesized that adenovirus-mediated transfer of the vascular endothelial growth factor (VEGF121) complementary DNA (cDNA) to murine laparotomy fascial wounds would enhance vascularity and bursting strength.

METHODS

Microfibrillar collagen sponges saturated with adenovirus (Ad) vectors encoding for the human VEGF121 cDNA (Ad(CU)VEGF121.1), a control marker gene (Ad beta gal, AdLuc) or no transgene (AdNull) were sutured to fascial edges during laparotomy closure in normal mice and mice treated with dexamethasone. Endpoints addressed included transgene expression in the fascia and surrounding tissue, the number of blood vessels in the healing wound determined using immunostaining, and wound bursting strength using a calibrated tensinometer.

RESULTS

Transgene expression was detected readily in the fascial edges, but only marginally detectable in neighboring tissues. In normal mice and mice treated with dexamethasone, no differences were observed at 7 days. Strikingly, however, 21 days after wound closure/therapy, significantly more blood vessels were present in the wounds that received the VEGF121 vector compared with controls (normal: AdNull: 4.2 +/- 1.8; Ad(CU)VEGF121.1: 11.2 +/- 1.2; P <.05; dexamethasone: AdNull: 1.4 +/- 0.8; Ad(CU)VEGF121.1: 5.4 +/- 1.2; P <.05), and bursting strength was significantly higher in VEGF121-treated wounds (normal: AdNull: 665 +/- 68 mN/mm; Ad(CU)VEGF121.1: 956 +/- 82 mN/mm; P <.0005; dexamethasone: AdNull: 234 +/- 111 mN/mm; Ad(CU)VEGF121.1: 592 +/- 121 mN/mm; P <.03).

CONCLUSIONS

Adenovirus-mediated gene transfer to healing fascial wounds is achieved readily using a microfibrillar collagen sponge, with transfer of the human VEGF121 cDNA significantly enhancing wound vascularity and bursting strength in normal mice, as well as in mice treated with dexamethasone.

Tissue-engineered skin substitutes

The last two years have seen new tissue-engineered skin substitutes come onto the market and begin to resolve the various roles to which each is best suited. It is becoming evident that some of the very expensive cell-based products have cost-benefit advantage despite their high price and are valuable within the restricted applications for which they are intended. The use of skin substitutes for testing purposes has extended from epidermal keratinocytes to other integumentary epithelia and into preparations containing multiple cell types in which reactions resulting from paracrine interactions can be examined. Challenges remain in the application of gene therapy techniques to skin substitutes, both the control of transgene expression and in the selection of suitable genes to transfect. A coming challenge is the production of tissue-engineered products without the use of animal products other than human cells. A challenge that may be diminishing is the importance of acute rejection of allogeneic tissue-engineered skin substitutes.

Success and limitations of a naked plasmid transfection protocol for keratinocyte growth factor-1 to enhance cutaneous wound healing

Our group and others have previously reported enhancement of cutaneous wound healing following the transfection of tissue with plasmid vectors expressing the DNA for growth factors. In these experiments, growth factor treated animals were usually compared to animals treated with control plasmid vector. To achieve consistent transfection, high DNA plasmid load and repeated penetrations of the wound by needle or gene gun were required. In the current experiments, we assessed the effect of the plasmid load and repeated tissue penetrations on wound healing of excisional wounds in diabetic C57 mice. Animals received 5 mm excisional wounds, and were assigned to the following groups, no treatment, phosphate buffered saline solution injections, and plasmid vector injection with and without the keratinocyte growth factor-1 gene. Intradermal injections of 100 microg plasmid were given adjacent to the wounds at days 1-5, 7 and 11. At day 9, wound closure was more advanced in keratinocyte growth factor-1 treated animals compared to those treated with control plasmid. But a detrimental effect of the DNA plasmid injection was evident from a comparison of the DNA control group versus the non-injected group. Therefore, the challenge for developing an effective system for the enhancement of wound healing lies in improving transfection efficiency.

Platelet-derived growth factor (PDGF) gene delivery for application in periodontal tissue engineering

BACKGROUND

A challenge in the reconstruction of periodontal structures is the targeted delivery of growth-promoting molecules to the tooth root surface. Polypeptide growth factors such as platelet-derived growth factor (PDGF) stimulate both cementogenesis and osteogenesis. Recent advances in gene therapy offer the advantage of delivering recombinant proteins to tissues for extended periods of time in vivo.

METHODS

Recombinant adenoviral vectors encoding for the PDGF-A gene were constructed to allow delivery of PDGF transgenes to cells. The recombinant adenoviruses were assembled using the viral backbone of Ad2/CMV/EGFP and replacing GFP (reporter gene encoding green fluorescent protein driven by the cytomegalovirus promoter [CMV] within adenovirus type 2) with the PDGF-A gene. Root lining cells (cloned cementoblasts) were transduced with Ad2/PDGF-A and evaluated for gene expression, DNA synthesis, and cell proliferation. PDGF-inducible genes, c-myc and osteopontin, were also evaluated following gene delivery of Ad2/PDGF-A.

RESULTS

The results revealed high level transduction of cementoblasts by gene transfer for 7 days as evidenced by flow cytometry and Northern blotting. Cementoblast DNA synthesis and subsequent proliferation were stimulated by Ad2/PDGF-A at levels equal to or greater than continuous rhPDGF-AA application. Strong message for the PDGF-A gene and protein as evidenced by Northern blotting and immunocytochemistry was noted. Furthermore, the potent induction of c-myc and osteopontin mRNA was found after PDGF gene delivery to cementoblasts.

CONCLUSIONS

These findings demonstrate that gene delivery of platelet-derived growth factor stimulates cementoblast activity that is sustained above that of rhPDGF-AA application. The use of gene therapy as a mode of growth factor delivery offers a novel approach to periodontal tissue engineering.

The transcriptional corepressor NAB2 blocks Egr-1-mediated growth factor activation and angiogenesis

Effective tissue repair results from a rapid, temporally orchestrated series of events. At the site of local tissue injury, the production of many growth factors and cytokines is, in part, stimulated by the early growth response transcription factors such as Egr-1. Egr-1 protein binds to a family of corepressor proteins called NAB which function to block or limit Egr-1 trans-activation of cognate target genes. NAB2 blocks Egr-1 activation of the tissue factor (TF) promoter, Egr-1 stimulated production of PDGF-AB, HGF, TGFbeta(1), and VEGF and the endogenous expression of PDGF-AB and TGFbeta(1). Expression of a wild-type NAB2 but not a dominant negative NAB2 mutant abrogates Egr-1 driven TF promoter activity and tubule formation in an in vitro model of angiogenesis. These findings may have importance in any tissue that is subject to scarring after acute or chronic injury.

Matrix immobilization enhances the tissue repair activity of growth factor gene therapy vectors

Although growth factor proteins display potent tissue repair activities, difficulty in sustaining localized therapeutic concentrations limits their therapeutic activity. We reasoned that enhanced histogenesis might be achieved by combining growth factor genes with biocompatible matrices capable of immobilizing vectors at delivery sites. When delivered to subcutaneously implanted sponges, a platelet-derived growth factor B-encoding adenovirus (AdPDGF-B) formulated in a collagen matrix enhanced granulation tissue deposition 3- to 4-fold (p < or = 0.0002), whereas vectors encoding fibroblast growth factor 2 or vascular endothelial growth factor promoted primarily angiogenic responses. By day 8 posttreatment of ischemic excisional wounds, collagen-formulated AdPDGF-B enhanced granulation tissue and epithelial areas up to 13- and 6-fold (p < 0.009), respectively, and wound closure up to 2-fold (p < 0.05). At longer times, complete healing without excessive scar formation was achieved. Collagen matrices were shown to retain both vector and transgene products within delivery sites, enabling the transduction and stimulation of infiltrating repair cells. Quantitative PCR and RT-PCR demonstrated both vector DNA and transgene mRNA within wound beds as late as 28 days posttreatment. By contrast, aqueous formulations allowed vector seepage from application sites, leading to PDGF-induced hyperplasia in surrounding tissues but not wound beds. Finally, repeated applications of PDGF-BB protein were required for neotissue induction approaching equivalence to a single application of collagen-immobilized AdPDGF-B, confirming the utility of this gene transfer approach. Overall, these studies demonstrate that immobilizing matrices enable the controlled delivery and activity of tissue promoting genes for the effective regeneration of injured tissues.

Involvement of platelet-derived growth factor in disease: development of specific antagonists

Platelet-derived growth factor (PDGF) is a family of dimeric isoforms that stimulates, e.g., growth, chemotaxis and cell shape changes of various connective tissue cell types and certain other cells. The cellular effects of PDGF isoforms are exerted through binding to two structurally related tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation. This enables a number of SH2 domain containing signal transduction molecules to bind to the receptors, thereby initiating various signaling pathways. PDGF isoforms have important roles during the embryonic development, particularly in the formation of connective tissue in various organs. In the adult, PDGF stimulates wound healing. Overactivity of PDGF has been implicated in certain disorders, including fibrotic conditions, atherosclerosis, and malignancies. Different kinds of PDGF antagonists are currently being developed and evaluated in different animal disease models, as well as in clinical trials.

Gene transfer and expression of platelet-derived growth factors modulate periodontal cellular activity

Platelet-derived growth factor (PDGF) is a potent stimulator of wound healing. PDGF gene therapy may promote greater periodontal regeneration than local protein application, due to sustained growth factor delivery to the target tissue. This investigation tested the ability of recombinant adenoviruses (rAds) encoding PDGF-A or PDGF-1308 (a PDGF-A dominant-negative mutant that disrupts endogenous PDGF bioactivity) to affect cells derived from the periodontium. Osteoblasts, periodontal ligament fibroblasts, and gingival fibroblasts were transduced with rAds, and gene expression, DNA synthesis, and cell proliferation were evaluated. The results revealed strong message for the PDGF-A gene for 7 days following gene delivery. Ad2/PDGF-A enhanced the mitogenic and proliferative response in all cell types, while Ad2/PDGF-1308 potently inhibited mitogenesis and proliferation. In conclusion, Ad2/PDGF can effectively transduce cells derived from the periodontium and promote biological activity equivalent to PDGF-AA. These studies support the potential use of gene therapy for sustained PDGF release in periodontal tissues.

Particle-mediated gene therapy of wounds

Gene therapy is becoming a reality, and it is a particularly attractive approach for wound healing, because the wound site is often exposed, the treatment and condition should be transient, and gene products such as growth factors and cytokines suffer from problems with bioavailability and stability. Among the techniques for gene delivery to the wound site, particle-mediated bombardment with a device called the gene gun has become an important developmental tool. This instrument has been used in numerous examples of wound gene therapy with growth factors or their receptors in the last decade. Among the advantages of particle-mediated bombardment are ease and speed of preparation of the delivery vehicle, the stability of the DNA preparation, the absence of (viral) antigens, the ability to target the projectiles to different tissue depths and areas, and the rapid shedding of both particles and DNA if they are targeted to the epidermis. Clinical application of the technology remains limited by the relatively low efficiency of the method, the potential tissue damage created by impact of the particles, and the coverage area. The gene gun can also be used to facilitate the discovery and validation of gene products as wound healing agents.

Comparison and evaluation of gene therapy and epigenetic approaches for wound healing

During the past decade considerable evidence has mounted concerning the importance of growth factors in the wound healing process both for cell replication and for stimulating reparative cells to synthesize and secrete extracellular matrix components. During normal wound healing the growth factor concentration has to be maintained at a certain level. If the growth factor concentration is too low, normal healing fails to occur. Whereas if the growth factor concentration is too high due to either over-expression of the growth factor or too much growth factor being applied to the wound, aberrant wound healing will occur. One approach for controlling the amount of growth factor at the wound site during normal healing is through gene therapy and the titration of gene dosage. However if a narrow window exists between the beneficial therapeutic effect and toxic effects with increasing gene dosage, an agent may be necessary to give in combination with gene therapy to regulate the over-expression of growth factor. In addition to genetic approaches to regulate wound healing, epigenetic approaches also exist. Antisense oligodeoxynucleotides have been shown to regulate wound repair in certain model systems and to determine the protein(s) necessary for normal wound healing. A novel approach to regulate the activity of collagen genes, thereby affecting fibrosis, is to use a sense oligodeoxynucleotide having the same sequence of the cis element which regulates the promoter activity of a particular collagen gene. This exogenous oligodeoxynucleotide will compete with the cis element in the collagen gene for the trans-acting factor which regulates promoter activity. These epigenetic approaches afford the opportunity to regulate over-expression of growth factor and therefore preclude the potential toxic effects of gene therapy. Both genetic and epigenetic approaches for regulating the wound healing process, either normal or aberrant wound healing, have certain advantages and disadvantages which are discussed in the present article.

Matrix-enabled gene transfer for cutaneous wound repair

Several growth factor proteins have been evaluated as therapeutic agents for the treatment of chronic dermal wounds. Unfortunately, most have failed to produce significant improvements in wound healing, in part due to ineffective delivery and poor retention in the wound defect. It has been proposed that gene therapy might overcome the limitations of protein therapy via ongoing transcription and translation, thus prolonging the availability of the therapeutic protein. Reasoning that it would be of further benefit to ensure retention of the DNA vector as well as the therapeutic protein within the wound defect, we have evaluated matrix-enabled gene transfer for cutaneous wound repair (Gene Activated Matrix). Formulations consisting of bovine type I collagen mixed with adenoviral or plasmid gene vectors have been evaluated in 3 in vivo models. The therapeutic transgenes employed encode human platelet-derived growth factor-A or -B, proteins key to each phase of normal wound repair. Increased granulation tissue formation, vascularization, and reepithelialization have been shown compared to controls treated with collagen alone or collagen containing a reporter gene vector. Further enhancements of the tissue repair response have been achieved by combining matrix-enabled gene transfer with molecular targeting, in which the DNA vector is conjugated to a growth factor ligand (basic fibroblast growth factor). These promising results support the clinical evaluation of gene activated matrices for the treatment of chronic dermal wounds.

Gene therapy in wound repair and regeneration

The potential use of gene therapy to treat human disease increases with the development of various physical, chemical, and biological methods to deliver genes to mammalian cells, and with our rapidly expanding knowledge of the human genome. One area of therapeutic interest for gene therapy is the treatment of wound healing disorders. Most recently, recombinant human growth factor therapy has been examined as a means to treat problem wounds. However, this approach suffers from the difficulty in providing an accurate dose of growth factor and the expense of the recombinant proteins. Delivery of a gene that could be expressed within the wound is an attractive alternative to application of the protein. This review discusses several methods that have been used to deliver genes encoding growth factor proteins into wounds and the advantages/disadvantages of each approach. Novel methods to regulate the expression of the transgene are also presented, highlighting the ability of these unique vector systems to adjust gene dose as the wound heals. We expect that gene therapy will become a significant treatment modality for those wound healing pathologies refractory to other wound management approaches in the years ahead.

Tissue repair with a therapeutic transcription factor

The healing of tissue involves a wide range of molecular, cellular, and physiological events that are coordinated in a temporally specific manner. The cellular transcription factor early growth response factor 1 (Egr-1) is expressed minutes after acute injury and serves to stimulate the production of a class of growth factors whose role is to promote tissue repair. We have studied the effects of Egr-1 expression at the site of dermal wounding in rodents. We find that Egr-1 promotes angiogenesis in vitro and in vivo, increases collagen production, and accelerates wound closure. These results show that Egr-1 gene therapy accelerates the normal healing process and raises the potential use of this therapeutic transcription factor for any aspect of tissue repair.

Transcriptional targeting of adenoviral gene delivery into migrating wound keratinocytes using FiRE, a growth factor-inducible regulatory element

Impaired cutaneous wound healing is a common complication in diabetes, ischemia and venous insufficiency of lower extremities, and in long-term treatment with corticosteroids or other immunosuppressive agents. In development of gene therapy for wound repair, expression of therapeutic transgenes should be precisely targeted and controlled. Here, we describe a recombinant adenovirus RAdFiRE-EGFP, in which a growth factor inducible element (FiRE) of the murine syndecan-1 gene controls the expression of enhanced green fluorescent protein (EGFP) reporter gene. Treatment of RAdFiRE-EGFP-transduced murine epidermal keratinocytes in culture with FiRE-activating growth factor markedly enhanced the expression of EGFP. In ex vivo organ culture of wounded murine skin transduced with RAdFiRE-EGFP, the EGFP expression was specifically detected in wound margin keratinocytes, but not in intact skin. Activity of EGFP was first detected 2 days after a single application of RAdFiRE-EGFP and persisted up to 10 days. Similarly, FiRE-driven EGFP expression was detected specifically in epidermal keratinocytes in the edge of incisional wounds in murine skin transduced with RAdFiRE-EGFP. In contrast, adenovirus-mediated lacZ expression driven by CMV promoter was detected scattered in epidermal, dermal and subcutaneous layers in ex vivo and in vivo wounds, as well as in intact skin. These data demonstrate the feasibility of FiRE as a tool for transcriptional targeting of adenovirus-mediated transgene expression to cutaneous wound edge keratinocytes.

FGF2-Targeted adenovirus encoding platelet-derived growth factor-B enhances de novo tissue formation

Gene therapy has yet to achieve reproducible clinical efficacy, due to inadequate gene delivery, inadequate gene expression, or dose-limiting toxicity. We have developed a gene therapy technology for tissue repair and regeneration that employs a structural matrix for DNA delivery. The matrix holds the DNA vector at the treatment site and provides a scaffolding for in-growth and accumulation of repair cells and efficient DNA transfection. We now report, for the first time, matrix-mediated delivery of targeted DNA vectors for soft tissue repair. A collagen matrix was used to deliver an adenoviral vector encoding platelet-derived growth factor-B (AdPDGF-B), resulting in efficient transgene expression in vitro and in vivo. Increases in the overall levels of expression and in the relative amounts of secreted PDGF-BB were achieved when AdPDGF-B was conjugated to fibroblast growth factor (FGF2) such that the virus was targeted for cellular uptake via FGF receptors. Matrix-mediated delivery of AdPDGF-B enhanced wound healing responses in vivo, and FGF2 targeting generated effects comparable to nontargeted vectors at significantly lower doses. Therefore, matrix-mediated delivery in combination with FGF2 targeting overcomes some of the safety and efficacy limitations of current gene therapy strategies and is an attractive therapeutic approach for tissue repair and regeneration.

Gene therapy for spine fusion

Gene therapy is a novel therapeutic modality for repair and regeneration of musculoskeletal tissues, including the spine. Various methods for therapeutic gene transfer are presented in this article. Several studies in which gene transfer has been used specifically to enhance spine fusion in animal models are reviewed.

Boosting epidermal growth factor receptor expression by gene gun transfection stimulates epidermal growth in vivo

Expression constructs encoding a full-length cDNA encoding the human epidermal growth factor receptor, or reporter gene for green fluorescent protein or luciferase were coated onto gold particles and driven into porcine skin using a gene gun delivery system. Strategies for epidermal growth factor receptor boosting were tested in two types of wounds. For grafted wounds, intact porcine skin was pretreated by the introduction of the epidermal growth factor receptor expression construct 24 hours before its harvesting as a split-thickness skin graft. Partial-thickness excisional wound beds (donor sites) were transfected at the time of their creation. Wound healing parameters were subsequently tested in the presence or absence of excess epidermal growth factor ligand. Initial distributions of gene gun delivered gold particles as well as luciferase expression levels suggested that optimal skin penetrations and expression levels were achieved at 500 psi for intact epidermis and 300 psi for exposed wound beds. At 2 days after gene delivery, visualization of green fluorescent protein by fluorescence microscopy showed focal expression of green fluorescent protein at the advancing epithelial outgrowths found at wound edges or surviving epithelial remnants. Green fluorescent protein expression appeared transient since no green fluorescent protein was noted in specimens removed at 4 days after injury. Northern blot analysis on mRNA isolated from wounds 2 days after introduction of epidermal growth factor receptor coated gold particles by gene gun confirmed the expression of the human epidermal growth factor receptor transgene in both skin grafts and excisional wounds. Skin grafts showed subsequent biological responses to the introduction of excessive epidermal growth factor receptor as well as expression of the human epidermal growth factor receptor construct within healing epidermis. While control autografts (reporter gene treated, epidermal growth factor alone, placebo formula, no treatment) showed few 5'-bromodeoxyuridine-labeled cells, epidermal growth factor receptor autografts showed 5'-bromodeoxyuridine labeling of nearly every basal cell. Favorable wound healing outcomes were also shown within excisional wounds following in vivo boosting of epidermal growth factor receptor. Four days after receiving epidermal growth factor receptor particle growth factor receptor transgene. Application of topical epidermal growth factor ligand resulted in the highest percentage of resurfacing. Maximal re-epithelialization was noted in wound beds receiving both receptor boosting and excessive daily epidermal growth factor ligand. A modest increase in the thickness of the granulation tissue followed gene therapy with epidermal growth factor receptor. In summary these in vivo data suggest that it is possible to boost in vivo expression of a tyrosine kinase receptor during wound repair. Increased epidermal growth factor receptor expression has an integral impact on cell proliferation, rates of resurfacing and dermal components and merits consideration as a possible therapeutic agent.

Genetic modification of cultured skin substitutes by transduction of human keratinocytes and fibroblasts with platelet-derived growth factor-A

Gene therapy promises the potential for improved treatment of cutaneous wounds. This study evaluated whether genetically modified cultured skin substitutes can act as vehicles for gene therapy in an athymic mouse model of wound healing. Human keratinocytes and fibroblasts were genetically engineered by retroviral transduction to overexpress human platelet-derived growth factor-A chain. Three types of skin substitutes were prepared from collagen-glycosaminoglycan substrates populated with fibroblasts and keratinocytes: HF-/HK-, containing both unmodified fibroblasts and keratinocytes; HF-/HK+, containing unmodified fibroblasts and modified keratinocytes; and HF+/HK-, containing modified fibroblasts and unmodified keratinocytes. Skin substitutes were cultured for two weeks before grafting to full-thickness wounds on athymic mice. The modified skin substitutes secreted significantly elevated levels of platelet-derived growth factor throughout the culture period. Expression of retroviral platelet-derived growth factor-A mRNA was maintained after grafting to mice, and was detected in all HF-/HK+ grafts and one HF+/HK- graft at two weeks after surgery. Although no differences were seen between control and modified grafts, the results suggest that genetically modified cultured skin substitutes can be a feasible mechanism for cutaneous gene therapy. The cultured skin model used for these studies has advantages over other skin analogs containing only epidermal cells; because it contains both fibroblasts and keratinocytes, it therefore offers greater opportunities for genetic modification and potential modulation of wound healing.

Curcumin enhances wound healing in streptozotocin induced diabetic rats and genetically diabetic mice

Tissue repair and wound healing are complex processes that involve inflammation, granulation and tissue remodeling. Interactions of different cells, extracellular matrix proteins and their receptors are involved in wound healing, and are mediated by cytokines and growth factors. Previous studies from our laboratory have shown that curcumin (diferuloylmethane), a natural product obtained from the rhizomes of Curcuma longa, enhanced cutaneous wound healing in rats and guinea pigs. In this study, we have evaluated the efficacy of curcumin treatment by oral and topical applications on impaired wound healing in diabetic rats and genetically diabetic mice using a full thickness cutaneous punch wound model. Wounds of animals treated with curcumin showed earlier re-epithelialization, improved neovascularization, increased migration of various cells including dermal myofibroblasts, fibroblasts, and macrophages into the wound bed, and a higher collagen content. Immunohistochemical localization showed an increase in transforming growth factor-beta1 in curcumin-treated wounds compared to controls. Enhanced transforming growth factor-beta1 mRNA expression in treated wounds was confirmed by in situ hybridization, and laser scan cytometry. A delay in the apoptosis patterns was seen in diabetic wounds compared to curcumin treated wounds as shown by terminal deoxynucleotidyl transferase-mediated deoxyuridyl triphosphate nick end labeling analysis. Curcumin was effective both orally and topically. These results show that curcumin enhanced wound repair in diabetic impaired healing, and could be developed as a pharmacological agent in such clinical settings.