A hypomorphic mouse model of dystrophic epidermolysis bullosa reveals mechanisms of disease and response to fibroblast therapy

Medical cell technologies for treatment of patients suffering from recessive dystrophic epidermolysis bullosa. Method of intracutaneous administration of fibroblasts

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe inherited disease developing due to genetic abnormalities in the synthesis of Type VII collagen by fibroblasts. A low production rate of Type VII collagen and abnormalities related to the formation of anchoring fibrils weaken the epidermis and derma adhesion strength, which results in the formation of blisters or erosions in case of any mechanical injury. Fibroblasts and keratinocytes belong to the key sources of Type VII collagen in the skin. Application of allogeneic fibroblasts is a promising cell technique for treating RDEB patients. The therapeutic effect of fibroblasts intradermal administration is stipulated by high stability of newly synthesized Type VII collagen and its ability to form anchoring fibrils in the area of the dermoepidermal junction. According to experimental and clinical studies, it is possible to boost the content of Type VII collagen in the dermoepidermal junction area and heal long-term skin defects in RDEB patients by means of intradermal administration of allogeneic fibroblasts.

Epidermal Basement Membrane in Health and Disease

Skin, as the organ protecting the individual from environmental aggressions, constantly meets external insults and is dependent on mechanical toughness for its preserved function. Accordingly, the epidermal basement membrane (BM) zone has adapted to enforce tissue integrity. It harbors anchoring structures created through unique organization of common BM components and expression of proteins exclusive to the epidermal BM zone. Evidence for the importance of its correct assembly and the nonredundancy of its components for skin integrity is apparent from the multiple skin blistering disorders caused by mutations in genes coding for proteins associated with the epidermal BM and from autoimmune disorders in which autoantibodies target these molecules. However, it has become clear that these proteins not only provide mechanical support but are also critically involved in tissue homeostasis, repair, and regeneration. In this chapter, we provide an overview of the unique organization and components of the epidermal BM. A special focus will be given to its function during regeneration, and in inherited and acquired diseases.

High Local Concentrations of Intradermal MSCs Restore Skin Integrity and Facilitate Wound Healing in Dystrophic Epidermolysis Bullosa

Dystrophic epidermolysis bullosa (DEB) is an incurable skin fragility disorder caused by mutations in the COL7A1 gene, coding for the anchoring fibril protein collagen VII (C7). Life-long mechanosensitivity of skin and mucosal surfaces is associated with large body surface erosions, chronic wounds, and secondary fibrosis that severely impede functionality. Here, we present the first systematic long-term evaluation of the therapeutic potential of a mesenchymal stromal cell (MSC)-based therapy for DEB. Intradermal administration of MSCs in a DEB mouse model resulted in production and deposition of C7 at the dermal-epidermal junction, the physiological site of function. The effect was dose-dependent with MSCs being up to 10-fold more potent than dermal fibroblasts. MSCs promoted regeneration of DEB wounds via normalization of dermal and epidermal healing and improved skin integrity through de novo formation of functional immature anchoring fibrils. Additional benefits were gained by MSCs' anti-inflammatory effects, which led to decreased immune cell infiltration into injured DEB skin. In our setting, the clinical benefit of MSC injections lasted for more than 3 months. We conclude that MSCs are viable options for localized DEB therapy. Importantly, however, the cell number needed to achieve therapeutic efficacy excludes the use of systemic administration.

Innovative therapeutic strategies for recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is among the most serious rare skin diseases. It is also the rare skin disease for which most effort has been expended in developing advanced therapeutic interventions. RDEB is caused by collagen VII deficiency resulting from COL7A1 mutations. Therapeutic approaches seek to replenish collagen VII and thus restore dermal-epidermal adhesion. Therapeutic options under development include protein therapy and different cell-based and gene-based therapies. In addition to treating skin defects, some of these therapies may also target internal mucosa. In the coming years, these novel therapeutic approaches should substantially improve the quality of life of patients with RDEB.

The three-body problem of therapy with induced pluripotent stem cells

Regenerative medicine has a three-body problem: alignment of the dynamics of the genome, stem cell and patient. Focusing on the rare inherited fragile skin disorder epidermolysis bullosa, three recent innovative studies have used induced pluripotent stem cells and gene correction, revertant mosaicism or genome editing to advance the prospects of better cell-based therapeutics to restore skin structure and function for epidermolysis bullosa and potentially other inherited diseases.

Trans-splicing improvement by the combined application of antisense strategies

Spliceosome-mediated RNA trans-splicing has become an emergent tool for the repair of mutated pre-mRNAs in the treatment of genetic diseases. RNA trans-splicing molecules (RTMs) are designed to induce a specific trans-splicing reaction via a binding domain for a respective target pre-mRNA region. A previously established reporter-based screening system allows us to analyze the impact of various factors on the RTM trans-splicing efficiency in vitro. Using this system, we are further able to investigate the potential of antisense RNAs (AS RNAs), presuming to improve the trans-splicing efficiency of a selected RTM, specific for intron 102 of COL7A1. Mutations in the COL7A1 gene underlie the dystrophic subtype of the skin blistering disease epidermolysis bullosa (DEB). We have shown that co-transfections of the RTM and a selected AS RNA, interfering with competitive splicing elements on a COL7A1-minigene (COL7A1-MG), lead to a significant increase of the RNA trans-splicing efficiency. Thereby, accurate trans-splicing between the RTM and the COL7A1-MG is represented by the restoration of full-length green fluorescent protein GFP on mRNA and protein level. This mechanism can be crucial for the improvement of an RTM-mediated correction, especially in cases where a high trans-splicing efficiency is required.

Preconditioning of mesenchymal stem cells for improved transplantation efficacy in recessive dystrophic epidermolysis bullosa

Introduction

The use of hematopoietic cell transplantation (HCT) has previously been shown to ameliorate cutaneous blistering in pediatric patients with recessive dystrophic epidermolysis bullosa (RDEB), an inherited skin disorder that results from loss-of-function mutations in COL7A1 and manifests as deficient or absent type VII collagen protein (C7) within the epidermal basement membrane. Mesenchymal stem cells (MSCs) found within the HCT graft are believed to be partially responsible for this amelioration, in part due to their intrinsic immunomodulatory and trophic properties and also because they have been shown to restore C7 protein following intradermal injections in models of RDEB. However, MSCs have not yet been demonstrated to improve disease severity as a stand-alone systemic infusion therapy. Improving the efficacy and functional utility of MSCs via a pre-transplant conditioning regimen may bring systemic MSC infusions closer to clinical practice.

Methods

MSCs were isolated from 2- to 4-week-old mice and treated with varying concentrations of transforming growth factor-β (TGFβ; 5-20 ng/mL), tumor necrosis factor- α (TNFα; 10-40 ng/mL), and stromal cell-derived factor 1-α (SDF-1α; 30 ng/mL) for 24-72 hours.

Results

We demonstrate that treating murine MSCs with exogenous TGFβ (15 ng/mL) and TNFα (30 ng/mL) for 48 hours induces an 8-fold increase in Col7a1 expression and a significant increase in secretion of C7 protein, and that the effects of these cytokines are both time and concentration dependent. This cytokine treatment also promotes a 4-fold increase in Tsg-6 expression, a gene whose product is associated with improved wound-healing and immunosuppressive features. Finally, the addition of exogenous SDF-1α to this regimen induces a simultaneous upregulation of Col7a1, Tsg-6, and Cxcr4 expression.

Conclusions

These data suggest that preconditioning represents a feasible method for improving the functional utility of MSCs in the context of RDEB stem cell transplantation, and also highlight the applicability of preconditioning principles toward other cell-based therapies aimed at treating RDEB patients.

Dynamic involvement of ATG5 in cellular stress responses

Autophagy maintains cell and tissue homeostasis through catabolic degradation. To better delineate the in vivo function for autophagy in adaptive responses to tissue injury, we examined the impact of compromised autophagy in mouse submandibular glands (SMGs) subjected to main excretory duct ligation. Blocking outflow from exocrine glands causes glandular atrophy by increased ductal pressure. Atg5^f/−;Aqp5-Cre mice with salivary acinar-specific knockout (KO) of autophagy essential gene Atg5 were generated. While duct ligation induced autophagy and the expression of inflammatory mediators, SMGs in Atg5^f/−;Aqp5-Cre mice, before ligation, already expressed higher levels of proinflammatory cytokine and Cdkn1a/p21 messages. Extended ligation period resulted in the caspase-3 activation and acinar cell death, which was delayed by Atg5 knockout. Moreover, expression of a set of senescence-associated secretory phenotype (SASP) factors was elevated in the post-ligated glands. Dysregulation of cell-cycle inhibitor CDKN1A/p21 and activation of senescence-associated β-galactosidase were detected in the stressed SMG duct cells. These senescence markers peaked at day 3 after ligation and partially resolved by day 7 in post-ligated SMGs of wild-type (WT) mice, but not in KO mice. The role of autophagy-related 5 (ATG5)-dependent autophagy in regulating the tempo, duration and magnitude of cellular stress responses in vivo was corroborated by in vitro studies using MEFs lacking ATG5 or autophagy-related 7 (ATG7) and autophagy inhibitors. Collectively, our results highlight the role of ATG5 in the dynamic regulation of ligation-induced cellular senescence and apoptosis, and suggest the involvement of autophagy resolution in salivary repair.

Gene therapy: pursuing restoration of dermal adhesion in recessive dystrophic epidermolysis bullosa

The replacement of a defective gene with a fully functional copy is the goal of the most basic gene therapy. Recessive dystrophic epidermolysis bullosa (RDEB) is characterised by a lack of adhesion of the epidermis to the dermis. It is an ideal target for gene therapy as all variants of hereditary RDEB are caused by mutations in a single gene, COL7A1, coding for type VII collagen, a key component of anchoring fibrils that secure attachment of the epidermis to the dermis. RDEB is one of the most severe variants in the epidermolysis bullosa (EB) group of heritable skin diseases. Epidermolysis bullosa is defined by chronic fragility and blistering of the skin and mucous membranes due to mutations in the genes responsible for production of the basement membrane proteins. This condition has a high personal, medical and socio-economic impact. People with RDEB require a broad spectrum of medications and specialised care. Due to this being a systemic condition, most research focus is in the area of gene therapy. Recently, preclinical works have begun to show promise. They focus on the virally mediated ex vivo correction of autologous epithelium. These corrected cells are then to be expanded and grafted onto the patient following the lead of the first successful gene therapy in dermatology being a grafting of corrected tissue for junctional EB treatment. Current progress, outstanding challenges and future directions in translating these approaches in clinics are reviewed in this article.

Blister-inducing antibodies target multiple epitopes on collagen VII in mice

Epidermolysis bullosa acquisita (EBA) is an autoimmune subepidermal blistering disease of mucous membranes and the skin caused by autoantibodies against collagen VII. In silico and wet laboratory epitope mapping studies revealed numerous distinct epitopes recognized by EBA patients' autoantibodies within the non-collagenous (NC)1 and NC2 domains of collagen VII. However, the distribution of pathogenic epitopes on collagen VII has not yet been described. In this study, we therefore performed an in vivo functional epitope mapping of pathogenic autoantibodies in experimental EBA. Animals (n = 10/group) immunized against fragments of the NC1 and NC2 domains of collagen VII or injected with antibodies generated against the same fragments developed to different extent experimental EBA. Our results demonstrate that antibodies targeting multiple, distinct epitopes distributed over the entire NC1, but not NC2 domain of collagen VII induce blistering skin disease in vivo. Our present findings have crucial implications for the development of antigen-specific B- and T cell-targeted therapies in EBA.

Aminoglycosides restore full-length type VII collagen by overcoming premature termination codons: therapeutic implications for dystrophic epidermolysis bullosa

Patients with recessive dystrophic epidermolysis bullosa (RDEB) have severe, incurable skin fragility, blistering, and multiple skin wounds due to mutations in the gene encoding type VII collagen (C7), the major component of anchoring fibrils mediating epidermal-dermal adherence. Nearly 10-25% of RDEB patients carry nonsense mutations leading to premature stop codons (PTCs) that result in truncated C7. In this study, we evaluated the feasibility of using aminoglycosides to suppress PTCs and induce C7 expression in two RDEB keratinocyte cell lines (Q251X/Q251X and R578X/R906) and two primary RDEB fibroblasts (R578X/R578X and R163X/R1683X). Incubation of these cells with aminoglycosides (geneticin, gentamicin, and paromomycin) resulted in the synthesis and secretion of a full-length C7 in a dose-dependent and sustained manner. Importantly, aminoglycoside-induced C7 reversed the abnormal RDEB cell phenotype and incorporated into the dermal-epidermal junction of skin equivalents. We further demonstrated the general utility of aminoglycoside-mediated readthrough in 293 cells transiently transfected with expression vectors encoding 22 different RDEB nonsense mutations. This is the first study demonstrating that aminoglycosides can induce PTC readthrough and restore functional C7 in RDEB caused by nonsense mutations. Therefore, aminoglycosides may have therapeutic potential for RDEB patients and other inherited skin diseases caused by nonsense mutations.

Cell therapy in dermatology

Harnessing the regenerative capacity of keratinocytes and fibroblasts from human skin has created new opportunities to develop cell-based therapies for patients. Cultured cells and bioengineered skin products are being used to treat patients with inherited and acquired skin disorders associated with defective skin, and further clinical trials of new products are in progress. The capacity of extracutaneous sources of cells such as bone marrow is also being investigated for its plasticity in regenerating skin, and new strategies, such as the derivation of inducible pluripotent stem cells, also hold great promise for future cell therapies in dermatology. This article reviews some of the preclinical and clinical studies and future directions relating to cell therapy in dermatology, particularly for inherited skin diseases associated with fragile skin and poor wound healing.

Loss of collagen VII is associated with reduced transglutaminase 2 abundance and activity

Absence of collagen VII leads to widespread cellular and tissue phenotypes. However, the underlying molecular mechanisms are not well understood. To gain insights into cellular responses to loss of collagen VII, we undertook a quantitative disease proteomics approach. By using recessive dystrophic epidermolysis bullosa (RDEB), a skin blistering disease caused by collagen VII deficiency, as a genetic model, collagen VII-dependent differences in cellular protein abundances and protein-protein interactions were analyzed. Absence of collagen VII led to alterations of intracellular protein compositions and to perturbations in cell adhesion, protein trafficking, and the turnover pathway autophagy. A potential linker of the different cellular phenotypes is transglutaminase 2 (TGM2), a multifunctional enzyme important for protein cross-linking. TGM2 was identified as a stable interaction partner of collagen VII. In RDEB, both abundance and activity of TGM2 were reduced, accounting not only for diminished adhesion and perturbed autophagy but also for reduced cross-linking of the extracellular matrix and for decreased epidermal-dermal integrity in RDEB.

Inherited blistering skin diseases: underlying molecular mechanisms and emerging therapies

A key function of human skin is the formation of a structural barrier against the external environment. In part, this is achieved through the formation of a cornified cell envelope derived from a stratified squamous epithelium attached to an epithelial basement membrane. Resilient in health, the structural integrity of skin can become impaired or break down in a collection of inherited skin diseases, referred to as the blistering genodermatoses. These disorders arise from inherited gene mutations in a variety of structural and signalling proteins and manifest clinically as blisters or erosions following minor skin trauma. In some patients, blistering can be severe resulting in significant morbidity. Furthermore, a number of these conditions are associated with debilitating extra-cutaneous manifestations including gastro-intestinal, cardiac, and ocular complications. In recent years, an improved understanding of the molecular basis of the blistering genodermatoses has led to better disease classification and genetic counselling. For patients, this has also advanced translational research with the advent of new clinical trials of gene, protein, cell, drug, and small molecule therapies. Although curing inherited blistering skin diseases still remains elusive, significant improvements in patients' quality of life are already being achieved.

Treatment of hereditary epidermolysis bullosa: updates and future prospects

Epidermolysis bullosa (EB) represents a group of inherited blistering skin diseases, some forms of which are associated with considerable morbidity and increased mortality. Notably, in recessive dystrophic EB there can be extensive muco-cutaneous fragility and disease complications such as scars, contractures, anemia, malnutrition, and malignancy. Currently, there is no effective therapy or cure for EB. Over the last decade, however, a number of important advances have been made that are bringing new treatments closer to the clinic, including gene therapy, protein replacement therapy, cell therapies [allogeneic fibroblasts, mesenchymal stromal cells (MSCs), bone marrow stem cell transplantation, culturing/grafting revertant mosaic keratinocytes], gene editing/engineering, and clinical application of inducible pluripotent stem cells. Although a cure for EB still remains elusive, recent data on animal models and initial human clinical trials have raised the expectations of patients, clinicians, and researchers that disease modification and improved quality of life are feasible goals. Furthermore, the lessons learned in treating EB are likely to have significant implications for improving the management of other genetic diseases.

Patient-specific naturally gene-reverted induced pluripotent stem cells in recessive dystrophic epidermolysis bullosa

Spontaneous reversion of disease-causing mutations has been observed in some genetic disorders. In our clinical observations of severe generalized recessive dystrophic epidermolysis bullosa (RDEB), a currently incurable blistering genodermatosis caused by loss-of-function mutations in COL7A1 that results in a deficit of type VII collagen (C7), we have observed patches of healthy-appearing skin on some individuals. When biopsied, this skin revealed somatic mosaicism resulting from the self-correction of C7 deficiency. We believe this source of cells could represent an opportunity for translational “natural” gene therapy. We show that revertant RDEB keratinocytes expressing functional C7 can be reprogrammed into induced pluripotent stem cells (iPSCs) and that self-corrected RDEB iPSCs can be induced to differentiate into either epidermal or hematopoietic cell populations. Our results give proof in principle that an inexhaustible supply of functional patient-specific revertant cells can be obtained—potentially relevant to local wound therapy and systemic hematopoietic cell transplantation. This technology may also avoid some of the major limitations of other cell therapy strategies, e.g., immune rejection and insertional mutagenesis, which are associated with viral- and nonviral- mediated gene therapy. We believe this approach should be the starting point for autologous cellular therapies using “natural” gene therapy in RDEB and other diseases.

Superficial dermal fibroblasts enhance basement membrane and epidermal barrier formation in tissue-engineered skin: implications for treatment of skin basement membrane disorders

Basement membrane is a highly specialized structure that binds the dermis and the epidermis of the skin, and is mainly composed of laminins, nidogen, collagen types IV and VII, and the proteoglycans, collagen type XVIII and perlecan, all of which play critical roles in the function and resilience of skin. Both dermal fibroblasts and epidermal keratinocytes contribute to the development of the basement membrane, and in turn the basement membrane and underlying dermis influence the development and function of the epidermal barrier. Disruption of the basement membrane results in skin fragility, extensive painful blistering, and severe recurring wounds as seen in skin basement membrane disorders such as epidermolysis bullosa, a family of life-threatening congenital skin disorders. Currently, there are no successful strategies for treatment of these disorders; we propose the use of tissue-engineered skin as a promising approach for effective wound coverage and to enhance healing. Fibroblasts and keratinocytes isolated from superficial and deep dermis and epidermis, respectively, of tissue from abdominoplasty patients were independently cocultured on collagen-glycosaminoglycan matrices, and the resulting tissue-engineered skin was assessed for functional differences based on the underlying specific dermal fibroblast subpopulation. Tissue-engineered skin with superficial fibroblasts and keratinocytes formed a continuous epidermis with increased epidermal barrier function and expressed higher levels of epidermal proteins, keratin-5, and E-cadherin, compared to that with deep fibroblasts and keratinocytes, which had an intermittent epidermis. Further, tissue-engineered skin with superficial fibroblasts and keratinocytes formed better basement membrane, and produced more laminin-5, nidogen, collagen type VII, compared to that with deep fibroblasts and keratinocytes. Overall, our results demonstrate that tissue-engineered skin with superficial fibroblasts and keratinocytes forms significantly better basement membrane with higher expression of dermo-epidermal adhesive and anchoring proteins, and superior epidermis with enhanced barrier function compared to that with deep fibroblasts and keratinocytes, or with superficial fibroblasts, deep fibroblasts, and keratinocytes. The specific use of superficial fibroblasts in tissue-engineered skin may thus be more beneficial to promote adhesion of newly formed skin and wound healing, and is therefore promising for the treatment of patients with basement membrane disorders and other skin blistering diseases.

Generation of 3D skin equivalents fully reconstituted from human induced pluripotent stem cells (iPSCs)

Recent generation of patient-specific induced pluripotent stem cells (PS-iPSCs) provides significant advantages for cell- and gene-based therapy. Establishment of iPSC-based therapy for skin diseases requires efficient methodology for differentiating iPSCs into both keratinocytes and fibroblasts, the major cellular components of the skin, as well as the reconstruction of skin structures using these iPSC-derived skin components. We previously reported generation of keratinocytes from human iPSCs for use in the treatment of recessive dystrophic epidermolysis bullosa (RDEB) caused by mutations in the COL7A1 gene. Here, we developed a protocol for differentiating iPSCs into dermal fibroblasts, which also produce type VII collagen and therefore also have the potential to treat RDEB. Moreover, we generated in vitro 3D skin equivalents composed exclusively human iPSC-derived keratinocytes and fibroblasts for disease models and regenerative therapies for skin diseases, first demonstrating that iPSCs can provide the basis for modeling a human organ derived entirely from two different types of iPSC-derived cells.

Allogeneic blood and bone marrow cells for the treatment of severe epidermolysis bullosa: repair of the extracellular matrix

Contrary to the prevailing professional opinion of the past few decades, recent experimental and clinical data support the fact that protein replacement therapy by allogeneic blood and marrow transplantation is not limited to freely diffusible molecules such as enzymes, but also large structural proteins such as collagens. A prime example is the cross-correction of type VII collagen deficiency in generalised severe recessive dystrophic epidermolysis bullosa, in which blood and marrow transplantation can attenuate the mucocutaneous manifestations of the disease and improve patients' quality of life. Although allogeneic blood and marrow transplantation can improve the integrity of the skin and mucous membranes, today's accomplishments are only the first steps on the long pathway to cure. Future strategies will be built on the lessons learned from these first transplant studies.

One goal, different strategies--molecular and cellular approaches for the treatment of inherited skin fragility disorders

Epidermolysis bullosa (EB) is a heterogeneous group of inherited diseases characterized by the formation of blisters in the skin and mucosa. There is no cure or effective treatment for these potentially severe and fatal diseases. Over the past few years, several reports have proposed different molecular strategies as new therapeutic options for the management of EB. From classical vector-based gene therapy to cell-based strategies such as systemic application of bone marrow stem cells or local application of fibroblasts, a broad range of molecular approaches have been explored. This array also includes novel methods, such as protein replacement therapy, gene silencing and the use of induced pluripotent stem cells (iPCs). In this review, we summarize current concepts of how inherited blistering diseases might be treated in the future and discuss the opportunities, promises, concerns and risks of these innovative approaches.

Collagen VII plays a dual role in wound healing

Although a host of intracellular signals is known to contribute to wound healing, the role of the cell microenvironment in tissue repair remains elusive. Here we employed 2 different mouse models of genetic skin fragility to assess the role of the basement membrane protein collagen VII (COL7A1) in wound healing. COL7A1 secures the attachment of the epidermis to the dermis, and its mutations cause a human skin fragility disorder coined recessive dystrophic epidermolysis bullosa (RDEB) that is associated with a constant wound burden. We show that COL7A1 is instrumental for skin wound closure by 2 interconnected mechanisms. First, COL7A1 was required for re-epithelialization through organization of laminin-332 at the dermal-epidermal junction. Its loss perturbs laminin-332 organization during wound healing, which in turn abrogates strictly polarized expression of integrin α6β4 in basal keratinocytes and negatively impacts the laminin-332/integrin α6β4 signaling axis guiding keratinocyte migration. Second, COL7A1 supported dermal fibroblast migration and regulates their cytokine production in the granulation tissue. These findings, which were validated in human wounds, identify COL7A1 as a critical player in physiological wound healing in humans and mice and may facilitate development of therapeutic strategies not only for RDEB, but also for other chronic wounds.

Topical application of recombinant type VII collagen incorporates into the dermal-epidermal junction and promotes wound closure

Patients with recessive dystrophic epidermolysis bullosa (RDEB) have incurable skin fragility, blistering, and skin wounds due to mutations in the gene that codes for type VII collagen (C7) that mediates dermal-epidermal adherence in human skin. In this study, we evaluated if topically applied human recombinant C7 (rC7) could restore C7 at the dermal-epidermal junction (DEJ) and enhance wound healing. We found that rC7 applied topically onto murine skin wounds stably incorporated into the newly formed DEJ of healed wounds and accelerated wound closure by increasing re-epithelialization. Topical rC7 decreased the expression of fibrogenic transforming growth factor-β2 (TGF-β2) and increased the expression of anti-fibrogenic TGF-β3. These were accompanied by the reduced expression of connective tissue growth factor, fewer α smooth muscle actin (α-SMA)-positive myofibroblasts, and less deposition of collagen in the healed neodermis, consistent with less scar formation. In addition, using a mouse model in which skin from C7 knock out mice was grafted onto immunodeficient mice, we showed that applying rC7 onto RDEB grafts with wounds restored C7 and anchoring fibrils (AFs) at the DEJ of the grafts and corrected the dermal-epidermal separation. The topical application of rC7 may be useful for treating patients with RDEB and patients who have chronic skin wounds.

Cell- and protein-based therapy approaches for epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is a clinically heterogeneous heritable skin fragility disorder characterized by mechanically induced mucocutaneous blistering. On the molecular level DEB is caused by mutations leading to deficiency in collagen VII (CVII), a large extracellular protein building anchoring fibrils that attach the epidermis to the dermis. Severely affected patients suffer from wounds, which heal with excessive scarring causing mutilating deformities of hands and feet. The patients are also predisposed to development of aggressive squamous cell carcinomas at sites of chronic wounds. Currently no available therapies exist for this extremely disabling and stigmatizing disorder. We are developing and evaluating cell- and protein-based therapies for the management of DEB. Dermal fibroblasts are easy to propagate in vitro, they produce CVII, and they have immunomodulating capacities, which makes it possible to use allogeneic fibroblasts for therapy without risking major adverse effects from the host's immune system. Hence, fibroblasts, and fibroblast-like cells such as mesenchymal stromal cells, are prime candidates for cell-based DEB therapies. An alternative for management of disorders caused by defects in proteins with relatively low turnover rate is to introduce the protein de novo to the tissue by direct application of the protein. CVII is long-lived and expressed in moderate amounts in the skin; this makes injection of collagen VII protein a realistic approach for the treatment of DEB. Here we present methods and protocols that we are using for fibroblast- and recombinant CVII-based therapies of DEB in our model of this disease, the CVII hypomorphic mouse. These protocols are directed towards management of DEB but they can be easily adapted for the treatment of other skin fragility disorders.

Bone marrow transplantation in epidermolysis bullosa

Epidermolysis bullosa (EB) is a heterogeneous group of inherited blistering skin diseases. Severe forms of EB are associated with increased morbidity and mortality, and there is currently no effective treatment. To combat severe complications of EB, such as chronic erosions, scarring and malignancy, effective therapy needs to be given systemically and at an early age. One recent therapeutic advancement has been a clinical trial of whole bone marrow (BM) transplantation in children with the dystrophic form of EB. This led to correction of the inherent skin basement membrane defect and better skin integrity in some individuals. The challenge now is to precisely identify which BM cells contribute to skin recovery and what mechanisms are involved in tissue regeneration. An improved understanding of the key aspects of BM skin repair is likely to lead to significant health improvements for patients with EB and other skin diseases.

Rat model for dominant dystrophic epidermolysis bullosa: glycine substitution reduces collagen VII stability and shows gene-dosage effect

Dystrophic epidermolysis bullosa, a severely disabling hereditary skin fragility disorder, is caused by mutations in the gene coding for collagen VII, a specialized adhesion component of the dermal-epidermal junction zone. Both recessive and dominant forms are known; the latter account for about 40% of cases. Patients with dominant dystrophic epidermolysis bullosa exhibit a spectrum of symptoms ranging from mild localized to generalized skin manifestations. Individuals with the same mutation can display substantial phenotypic variance, emphasizing the role of modifying genes in this disorder. The etiology of dystrophic epidermolysis bullosa has been known for around two decades; however, important pathogenetic questions such as involvement of modifier genes remain unanswered and a causative therapy has yet to be developed. Much of the failure to make progress in these areas is due to the lack of suitable animal models that capture all aspects of this complex monogenetic disorder. Here, we report the first rat model of dominant dystrophic epidermolysis bullosa. Affected rats carry a spontaneous glycine to aspartic acid substitution, p.G1867D, within the main structural domain of collagen VII. This confers dominant-negative interference of protein folding and decreases the stability of mutant collagen VII molecules and their polymers, the anchoring fibrils. The phenotype comprises fragile and blister-prone skin, scarring and nail dystrophy. The model recapitulates all signs of the human disease with complete penetrance. Homozygous carriers of the mutation are more severely affected than heterozygous ones, demonstrating for the first time a gene-dosage effect of mutated alleles in dystrophic epidermolysis bullosa. This novel viable and workable animal model for dominant dystrophic epidermolysis bullosa will be valuable for addressing molecular disease mechanisms, effects of modifying genes, and development of novel molecular therapies for patients with dominantly transmitted skin disease.

Global remodelling of cellular microenvironment due to loss of collagen VII

Loss of collagen VII causes recessive dystrophic epidermolysis bullosa. Quantitative proteomics analysis of the extracellular matrix and secretome of human fibroblasts derived from pathologically altered skin reveals a global remodelling of the cellular microenvironment.

A global analysis of the microenvironment of human skin fibroblasts was carried out to reveal disease-related alterations in the extracellular proteome.

The loss of collagen VII causes a deregulation of the basement membrane and dermal matrix proteome.

Post-translational modifications of secreted proteins were altered in fibroblasts from recessive dystrophic epidermolysis bullosa samples.

Metalloproteases displayed reduced activity and turnover in collagen VII-deficient cells.

The mammalian cellular microenvironment is shaped by soluble factors and structural components, the extracellular matrix, providing physical support, regulating adhesion and signalling. A global, quantitative mass spectrometry strategy, combined with bioinformatics data processing, was developed to assess proteome differences in the microenvironment of primary human fibroblasts. We studied secreted proteins of fibroblasts from normal and pathologically altered skin and their post-translational modifications. The influence of collagen VII, an important structural component, which is lost in genetic skin fragility, was used as model. Loss of collagen VII had a global impact on the cellular microenvironment and was associated with proteome alterations highly relevant for disease pathogenesis including decrease in basement membrane components, increase in dermal matrix proteins, TGF-β and metalloproteases, but not higher protease activity. The definition of the proteome of fibroblast microenvironment and its plasticity in health and disease identified novel disease mechanisms and potential targets of intervention.

Animal models of skin disease for drug discovery

INTRODUCTION

Discovery of novel drugs, treatments, and testing of consumer products in the field of dermatology is a multi-billion dollar business. Due to the distressing nature of many dermatological diseases, and the enormous consumer demand for products to reverse the effects of skin photodamage, aging, and hair loss, this is a very active field.

AREAS COVERED

In this paper, we will cover the use of animal models that have been reported to recapitulate to a greater or lesser extent the features of human dermatological disease. There has been a remarkable increase in the number and variety of transgenic mouse models in recent years, and the basic strategy for constructing them is outlined.

EXPERT OPINION

Inflammatory and autoimmune skin diseases are all represented by a range of mouse models both transgenic and normal. Skin cancer is mainly studied in mice and fish. Wound healing is studied in a wider range of animal species, and skin infections such as acne and leprosy also have been studied in animal models. Moving to the more consumer-oriented area of dermatology, there are models for studying the harmful effect of sunlight on the skin, and testing of sunscreens, and several different animal models of hair loss or alopecia.

Management of severe epidermolysis bullosa by haematopoietic transplant: principles, perspectives and pitfalls

People with severe forms of epidermolysis bullosa (EB) develop widespread blistering and progressively debilitating multisystem complications that may result in a shortened lifespan. As some wounds in EB individuals are difficult or impossible to access with topical therapy, we examined the potential of systemic therapy with normal haematopoietic stem cells. In both animal models and children with EB, healthy donor cells from the haematopoietic graft migrated to the injured skin; simultaneously, there was an increase in the production of skin-specific structural proteins deficient in EB, increased skin integrity and reduced tendency to blister formation. Even though the majority of evaluable individuals have had a positive response in skin healing, frequently changing their quality of life, the improvement in lifestyle has been varied and the overall clinical response incomplete. To change the current amelioration of disease into a full cure, we propose to (i) increase safety as well as efficacy of haematopoietic cell transplant (HCT) using co-infusion of mesenchymal stromal/stem cells with haematopoietic stem cells and non-myeloablative conditioning for transplant; (ii) optimize homing of donor cells into the skin erosions in animal models of EB; and (iii) discover and test new drugs for EB therapy using patient-specific induced pluripotent stem cells. We conclude that although HCT has always been a risky treatment restricted to those with serious life-threatening or debilitating diseases, by most benchmarks, the results of HCT in EB have shown that HCT has the potential of being a durable, systemic therapy for people with severe forms of EB.

A COL7A1 mutation causes dystrophic epidermolysis bullosa in Rotes Höhenvieh cattle

We identified a congenital mechanobullous skin disorder in six calves on a single farm of an endangered German cattle breed in 2010. The condition presented as a large loss of skin distal to the fetlocks and at the mucosa of the muzzle. All affected calves were euthanized on humane grounds due to the severity, extent and progression of the skin and oral lesions. Examination of skin samples under light microscopy revealed detachment of the epidermis from the dermis at the level of the dermo epidermal junction, leading to the diagnosis of a subepidermal bullous dermatosis such as epidermolysis bullosa. The pedigree was consistent with monogenic autosomal recessive inheritance. We localized the causative mutation to an 18 Mb interval on chromosome 22 by homozygosity mapping. The COL7A1 gene encoding collagen type VII alpha 1 is located within this interval and COL7A1 mutations have been shown to cause inherited dystrophic epidermolysis bullosa (DEB) in humans. A SNP in the bovine COL7A1 exon 49 (c.4756C>T) was perfectly associated with the observed disease. The homozygous mutant T/T genotype was exclusively present in affected calves and their parents were heterozygous C/T confirming the assumed recessive mode of inheritance. All known cases and genotyped carriers were related to a single cow, which is supposed to be the founder animal. The mutant T allele was absent in 63 animals from 24 cattle breeds. The identified mutation causes a premature stop codon which leads to a truncated protein representing a complete loss of COL7A1 function (p.R1586*). We thus have identified a candidate causative mutation for this genetic disease using only three cases to unravel its molecular basis. Selection against this mutation can now be used to eliminate the mutant allele from the Rotes Höhenvieh breed.

Antibodies to pathogenic epitopes on type XVII collagen cause skin fragility in a complement-dependent and -independent manner

In bullous pemphigoid (BP), the most prevalent autoimmune blistering disease, type XVII collagen (COL17) is targeted by circulating autoantibodies. BP is thought to be an autoantibody-mediated complement-fixing blistering disease, and a juxtamembranous noncollagenous 16A (NC16A) domain spanning Glu(490) to Arg(566) was proved to be the main pathogenic region on COL17, although precise pathogenic epitopes within NC16A have not been elucidated. In this study, we showed that injection of rabbit IgG Abs targeting Asp(522) to Gln(545) induced skin fragility associated with in vivo deposition of IgG and complement in neonatal COL17-humanized mice. Notably, immunoadsorption of rabbit anti-NC16A IgG Ab with this epitope (Asp(522) to Gln(545)) or the anti-NC16A IgG administered together with the peptides of this epitope as a decoy ameliorated skin fragility in the injected neonatal COL17-humanized mice compared with the anti-NC16A IgG alone even though all of the mice showed both IgG and complement deposition. These results led us to investigate an additional, complement-independent mechanism of skin fragility in the mice injected with anti-COL17 Abs. The rabbit anti-NC16A IgG depleted the expression of COL17 in cultured normal human keratinocytes, whereas immunoadsorption of the same IgG with this epitope significantly suppressed the depletion effect. Moreover, passive transfer of F(ab')(2) fragments of the human BP or rabbit IgG Abs against COL17 demonstrated skin fragility in neonatal COL17-humanized mice. In summary, this study reveals the importance of Abs directed against distinct epitopes on COL17, which induce skin fragility in complement-dependent as well as complement-independent ways.

Novel molecular therapies for heritable skin disorders

Tremendous progress has been made in the past two decades in molecular genetics of heritable skin diseases, and pathogenic mutations have been identified in as many as 500 distinct human genes. This progress has resulted in improved diagnosis with prognostic implications, has refined genetic counseling, and has formed the basis for prenatal and presymptomatic testing and preimplantation genetic diagnosis. However, there has been relatively little progress in developing effective and specific treatments for these often devastating diseases. However, very recently, a number of novel molecular strategies, including gene therapy, cell-based approaches, and protein replacement therapy, have been explored for the treatment of these conditions. This overview will focus on the prototypic heritable blistering disorders, epidermolysis bullosa, and related keratinopathies, in which significant progress has been made recently toward treatment, and it will illustrate how some of the translational research therapies have already entered the clinical arena.

Targeted proteolysis of plectin isoform 1a accounts for hemidesmosome dysfunction in mice mimicking the dominant skin blistering disease EBS-Ogna

Autosomal recessive mutations in the cytolinker protein plectin account for the multisystem disorders epidermolysis bullosa simplex (EBS) associated with muscular dystrophy (EBS-MD), pyloric atresia (EBS-PA), and congenital myasthenia (EBS-CMS). In contrast, a dominant missense mutation leads to the disease EBS-Ogna, manifesting exclusively as skin fragility. We have exploited this trait to study the molecular basis of hemidesmosome failure in EBS-Ogna and to reveal the contribution of plectin to hemidesmosome homeostasis. We generated EBS-Ogna knock-in mice mimicking the human phenotype and show that blistering reflects insufficient protein levels of the hemidesmosome-associated plectin isoform 1a. We found that plectin 1a, in contrast to plectin 1c, the major isoform expressed in epidermal keratinocytes, is proteolytically degraded, supporting the notion that degradation of hemidesmosome-anchored plectin is spatially controlled. Using recombinant proteins, we show that the mutation renders plectin's 190-nm-long coiled-coil rod domain more vulnerable to cleavage by calpains and other proteases activated in the epidermis but not in skeletal muscle. Accordingly, treatment of cultured EBS-Ogna keratinocytes as well as of EBS-Ogna mouse skin with calpain inhibitors resulted in increased plectin 1a protein expression levels. Moreover, we report that plectin's rod domain forms dimeric structures that can further associate laterally into remarkably stable (paracrystalline) polymers. We propose focal self-association of plectin molecules as a novel mechanism contributing to hemidesmosome homeostasis and stabilization.

Hemidesmosomes are specialized protein complexes that promote anchorage of the basal keratinocyte cell layer of the epidermis to the underlying dermis. They provide tissue integrity and resistance to mechanical forces. When hemidesmosomes do not function properly, skin blistering ensues in response to mechanical trauma. Plectin is an essential component of hemidesmosomes. Humans carrying recessive mutations in the plectin gene most frequently develop multisystem disorders, where in addition to skin other tissues are also affected. However, there is a unique dominant plectin mutation, which leads to the disease epidermolysis bullosa simplex Ogna (EBS-Ogna), affecting skin exclusively. Because of that, EBS-Ogna is an exceptional system to study the contribution of plectin to hemidesmosome function. We have generated an EBS-Ogna mouse model that mimics the human disease. Using this model, we have learned that selective degradation of hemidesmosome-associated plectin isoform 1a by proteases activated specifically in keratinocytes results in reduced numbers and dysfunction of hemidesmosomes. In contrast, plectin-1c, another plectin isoform expressed in keratinocytes, is not degraded. Moreover, we find that plectin dimers can oligomerize via their long coiled-coil rod domain, a process likely to be instrumental in maintenance of hemidesmosome integrity. These findings highlight the importance of plectin-1a for hemidesmosome function.

Concise review: Transplantation of human hematopoietic cells for extracellular matrix protein deficiency in epidermolysis bullosa

The skin is constantly exposed to environmental insults and requires effective repair processes to maintain its protective function. Wound healing is severely compromised in people with congenital absence of structural proteins of the skin, such as in dystrophic epidermolysis bullosa, a severe congenital mechanobullous disorder caused by mutations in collagen type VII. Remarkably, stem cell transplantation can ameliorate deficiency of this skin-specific structural protein in both animal models and in children with the disorder. Healthy donor cells from the hematopoietic graft migrate to the injured skin; simultaneously, there is an increase in the production of collagen type VII, increased skin integrity, and reduced tendency to blister formation. How hematogenous stem cells from bone marrow and cord blood can alter skin architecture and wound healing in a robust, clinically meaningful way is unclear. We review the data and the resulting hypotheses that have a potential to illuminate the mechanisms for these effects. Further modifications in the use of stem cell transplantation as a durable source of extracellular matrix proteins may make this regenerative medicine approach effective in other cutaneous and extracutaneous conditions.

Correction of dog dystrophic epidermolysis bullosa by transplantation of genetically modified epidermal autografts

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe skin blistering condition caused by mutations in the gene coding for collagen type VII. Genetically engineered RDEB dog keratinocytes were used to generate autologous epidermal sheets subsequently grafted on two RDEB dogs carrying a homozygous missense mutation in the col7a1 gene and expressing baseline amounts of the aberrant protein. Transplanted cells regenerated a differentiated and vascularized auto-renewing epidermis progressively repopulated by dendritic cells and melanocytes. No adverse immune reaction was detected in either dog. In dog 1, the grafted epidermis firmly adhered to the dermis throughout the 24-month follow-up, which correlated with efficient transduction (100%) of highly clonogenic epithelial cells and sustained transgene expression. In dog 2, less efficient (65%) transduction of primary keratinocytes resulted in a loss of the transplanted epidermis and graft blistering 5 months after transplantation. These data provide the proof of principle for ex vivo gene therapy of RDEB patients with missense mutations in collagen type VII by engraftment of the reconstructed epidermis, and demonstrate that highly efficient transduction of epidermal stem cells is crucial for successful gene therapy of inherited skin diseases in which correction of the genetic defect confers no major selective advantage in cell culture.

Cell-extracellular matrix interactions in normal and diseased skin

Mammalian skin comprises a multi-layered epithelium, the epidermis, and an underlying connective tissue, the dermis. The epidermal extracellular matrix is a basement membrane, whereas the dermal ECM comprises fibrillar collagens and associated proteins. There is considerable heterogeneity in ECM composition within both epidermis and dermis. The functional significance of this extends beyond cell adhesion to a range of cell autonomous and nonautonomous processes, including control of epidermal stem cell fate. In skin, cell-ECM interactions influence normal homeostasis, aging, wound healing, and disease. Disturbed integrin and ECM signaling contributes to both tumor formation and fibrosis. Strategies for manipulating cell-ECM interactions to repair skin defects and intervene in a variety of skin diseases hold promise for the future.

Genetic analyses of integrin signaling

The development of multicellular organisms, as well as maintenance of organ architecture and function, requires robust regulation of cell fates. This is in part achieved by conserved signaling pathways through which cells process extracellular information and translate this information into changes in proliferation, differentiation, migration, and cell shape. Gene deletion studies in higher eukaryotes have assigned critical roles for components of the extracellular matrix (ECM) and their cellular receptors in a vast number of developmental processes, indicating that a large proportion of this signaling is regulated by cell-ECM interactions. In addition, genetic alterations in components of this signaling axis play causative roles in several human diseases. This review will discuss what genetic analyses in mice and lower organisms have taught us about adhesion signaling in development and disease.

Autoimmunity against type VII collagen in inflammatory bowel disease

Autoimmunity against type VII collagen, an adhesion molecule of the extracellular matrix in epithelial basement membranes, is causing the rare organ-specific epidermolysis bullosa acquisita (EBA). An intriguing association between EBA and inflammatory bowel disease (IBD) has been extensively documented over the last decades, but, because of the very low incidence of EBA, received little attention from physicians involved in the care of patients with IBD. More recently, autoantibodies against type VII collagen have been detected in up to 68% of IBD patients. Although these findings suggest that chronic intestinal inflammation in IBD predisposes for autoimmunity against type VII collagen, their relevance for the pathogenesis of both IBD and EBA is still unclear. In this review article, the main features of the association between IBD and EBA are presented and pathomechanistic hypotheses as well as future lines of investigation in this area are discussed. Future research should provide new pathomechanistic insights and will likely facilitate the development of more specific and effective immunotherapeutic strategies for both conditions.