Targeting gene expression to the epidermis of transgenic mice: potential applications to genetic skin disorders

Desmosomal cadherin misexpression alters beta-catenin stability and epidermal differentiation

Desmosomal adhesion is important for the integrity and protective barrier function of the epidermis and is disregulated during carcinogenesis. Strong adhesion between keratinocytes is conferred by the desmosomal cadherins, desmocollin (Dsc) and desmoglein. These constitute two gene families, members of which are differentially expressed in epidermal strata. It has been suggested that this stratum-specific expression regulates keratinocyte differentiation. We tested this hypothesis by misdirecting the expression of the basally abundant desmosomal cadherins Dsc3a and Dsc3b to suprabasal differentiating keratinocytes in transgenic mice. No phenotype was apparent until adulthood, when mice developed variable ventral alopecia and had altered keratinocyte differentiation within affected areas. The follicular changes were reminiscent of changes in transgenic mice with an altered beta-catenin stability. Stabilized beta-catenin and increased beta-catenin transcriptional activity were demonstrated in transgenic mice prior to the phenotypic change and in transgenic keratinocytes as a consequence of transgene expression. Hence, a link between desmosomal cadherins and beta-catenin stability and signaling was demonstrated, and it was shown that desmocollin cadherin expression can affect keratinocyte differentiation. Furthermore, the first function for a "b-type" desmocollin cadherin was demonstrated.

Suprabasal desmoglein 3 expression in the epidermis of transgenic mice results in hyperproliferation and abnormal differentiation

The desmoglein 1 (Dsg1) and desmocollin 1 (Dsc1) isoforms of the desmosomal cadherins are expressed in the suprabasal layers of epidermis, whereas Dsg3 and Dsc3 are more strongly expressed basally. This differential expression may have a function in epidermal morphogenesis and/or may regulate the proliferation and differentiation of keratinocytes. To test this hypothesis, we changed the expression pattern by overexpressing human Dsg3 under the control of the keratin 1 (K1) promoter in the suprabasal epidermis of transgenic mice. From around 12 weeks of age, the mice exhibited flaking of the skin accompanied by epidermal pustules and thinning of the hair. Histological analysis of affected areas revealed acanthosis, hypergranulosis, hyperkeratosis, localized parakeratosis, and abnormal hair follicles. This phenotype has some features in common with human ichthyosiform diseases. Electron microscopy revealed a mild epidermal spongiosis. Suprabasally, desmosomes showed incorporation of the exogenous protein by immunogold labeling but were normal in structure. The epidermis was hyperproliferative, and differentiation was abnormal, demonstrated by expression of K14 in the suprabasal layer, restriction of K1, and strong induction of K6 and K16. The changes resembled those found in previous studies in which growth factors, cytokines, and integrins had been overexpressed in epidermis. Thus our data strongly support the view that Dsg3 contributes to the regulation of epidermal differentiation. Our results contrast markedly with those recently obtained by expressing Dsg3 in epidermis under the involucrin promoter. Possible reasons for this difference are considered in this paper.

Overexpression of a constitutively active form of c-src in skin epidermis increases sensitivity to tumor promotion by 12-O-tetradecanoylphorbol-13-acetate

Transgenic mice were developed to study the role of c-src in epithelial tumorigenesis through targeted expression of a constitutively active form of murine c-src (src(529)). Src(529) was targeted to the interfollicular epidermis with the human keratin 1 (HK1) promoter. The skin phenotype of these mice was characterized by exaggerated epidermal hyperplasia and hyperkeratosis within the first week after birth. The severity of this phenotype correlated with overall src kinase activity, both of which subsided with age. Treatment of adult HK1.src(529) transgenic mice with the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate resulted in an increase in epidermal hyperplasia and labeling index significantly greater than that seen in nontransgenic littermates. In addition, HK1.src(529) transgenic mice developed papillomas earlier and in significantly greater numbers compared with nontransgenic littermates in a standard initiation-promotion experiment. The data support the hypothesis that activation of c-src kinase plays a role in skin tumor promotion.

Analysis of mouse keratin 6a regulatory sequences in transgenic mice reveals constitutive, tissue-specific expression by a keratin 6a minigene

The analysis of keratin 6 expression is complicated by the presence of multiple isoforms that are expressed constitutively in a number of internal stratified epithelia, in palmoplantar epidermis, and in the companion cell layer of the hair follicle. In addition, keratin 6 expression is inducible in interfollicular epidermis and the outer root sheath of the follicle, in response to wounding stimuli, phorbol esters, or retinoic acid. In order to establish the critical regions involved in the regulation of keratin 6a (the dominant isoform in mice), we generated transgenic mice with two different-sized mouse keratin 6a constructs containing either 1.3 kb or 0.12 kb of 5' flanking sequence linked to the lacZ reporter gene. Both constructs also contained the first intron and the 3' flanking sequence of mouse keratin 6a. Ectopic expression of either transgene was not observed. Double-label immunofluorescence analyses demonstrated expression of the reporter gene in keratin 6 expressing tissues, including the hair follicle, tongue, footpad, and nail bed, showing that both transgenes retained keratinocyte-specific expression. Quantitative analysis of beta-galactosidase activity verified that both the 1.3 and 0.12 kb keratin 6a promoter constructs produced similar levels of the reporter. Notably, both constructs were constitutively expressed in the outer root sheath and interfollicular epidermis in the absence of any activating stimulus, suggesting that they lack the regulatory elements that normally silence transcription in these cells. This study has revealed that a keratin 6a minigene contains critical cis elements that mediate tissue-specific expression and that the elements regulating keratin 6 induction lie distal to the 1.3 kb promoter region.

Enhancement of susceptibility to diverse skin tumor promoters by activation of the insulin-like growth factor-1 receptor in the epidermis of transgenic mice

Insulin-like growth factor-1 (IGF-1) and its receptor are believed to play an important role in mitogenesis and neoplastic transformation. The purpose of this study was to further examine the role of IGF-1 during tumor promotion in mouse skin. HK1.IGF1 transgenic mice, which overexpress IGF-1 in epidermis via the human keratin 1 promoter, were previously shown to be hypersensitive to skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate (TPA). We examined these mice for their sensitivity to diverse classes of tumor-promoting agents. HK1.IGF-1 transgenic mice initiated with 7,12-dimethylbenz[a]anthracene were more sensitive to treatment with a wide variety of tumor promoters, including chrysarobin, okadaic acid, and benzoyl peroxide, which resulted in more rapid development of tumors and a dramatic increase in the number of tumors per mouse compared with corresponding non-transgenic mice treated with the same compounds. Histological analyses of skin from HK1.IGF-1 mice treated with various tumor promoters revealed that these mice were also more sensitive to the induction of epidermal hyperplasia and cell proliferation. Analysis of the IGF-1 receptor (IGF-1r) and epidermal growth factor (EGFr) in the epidermis of TPA-treated HK1.IGF-1 transgenic and non-transgenic mice revealed that both receptors were activated (hyperphosphorylated on tyrosine residues), and the level of activation was higher in transgenic mice. The mechanism for the increased sensitivity of HK1.IGF-1 mice to tumor promoters may involve cooperation between the IGF-1r and EGFr signaling pathways. Our data suggest that IGF-1r signaling may play an important role in the process of tumor promotion by diverse classes of tumor promoters.

Cutaneous gene therapy

Gene therapy efforts in a variety of tissues have foundered on fundamental technologic barriers, such as difficulties in achieving high-efficiency gene transfer to diseased tissues and in sustaining delivered transgene production. The skin offers an attractive tissue for development of approaches to therapeutic gene delivery by virtue of its accessibility for regulation by topical agents, the ease of gene transfer into cutaneous tissues, and the ready ability to monitor the impact of somatic gene transfer. With the ability of the skin to deliver therapeutic polypeptides to the systemic circulation and the recent molecular characterization of monogenic skin diseases, efforts to target genes to the skin are expected to accelerate. The current status of gene therapy efforts is reviewed, with a special focus on the skin.

The expression and localization of bcl-2 protein in normal skin and in non-melanoma skin cancers

Non-melanoma skin cancers are the most commonly diagnosed malignancies and are typically indolent in their clinical behavior. Although predisposing factors leading to the development of these cancers, such as ultraviolet irradiation, are well described, the molecular events involved in their pathogenesis are incompletely understood. The localization of bcl-2 expression within the skin was determined using immunohistochemical methodologies and an anti-bcl-2 monoclonal antibody. The cytoarchitectural distribution of bcl-2 protein in normal skin included basal keratinocytes, the dermal papillae of the hair follicle, the keratinized Huxley's and Henle's layers, and the keratinized outer root sheath cells of the isthmus and infundibulum of the hair follicle. In addition, intense immunoreactivity was noted in the secretory coil of eccrine sweat glands. The distribution of bcl-2 protein within normal skin did not correlate with the known histologic localization of stem cell compartments. Basal cell carcinomas expressed high levels of bcl-2 protein. In contrast, squamous cell carcinomas typically exhibited no immunohistochemically detectable bcl-2 protein. The findings suggest a potential contribution of bcl-2 gene deregulation to the pathogenesis of some types of non-melanoma skin cancer.

Keratin and keratinization

A flood of new knowledge and discoveries in the basic science of keratins and keratinization has appeared in the past several years. This review summarizes this recent information with a focus on the epithelial keratin polypeptides, keratin intermediate filaments, keratohyaline granule proteins, cell envelope formation and cell envelope proteins, "soft" keratinization, true disorders of keratinization (i.e., epidermolysis bullosa simplex and epidermolytic hyperkeratosis), and disease and drug effects on keratinization.