Delayed wound healing in keratin 6a knockout mice

Wound-healing studies in transgenic and knockout mice

Injury to the skin initiates a cascade of events including inflammation, new tissue formation, and tissue remodeling, that finally lead to at least partial reconstruction of the original tissue. Historically, animal models of repair have taught us much about how this repair process is orchestrated and, over recent years, the use of genetically modified mice has helped define the roles of many key molecules. Aside from conventional knockout technology, many ingenious approaches have been adopted, allowing researchers to circumvent such problems as embryonic lethality, or to affect gene function in a tissue- or temporal-specific manner. Together, these studies provide us with a growing source of information describing, to date, the in vivo function of nearly 100 proteins in the context of wound repair. This article focuses on the studies in which genetically modified mouse models have helped elucidate the roles that many soluble mediators play during wound repair, encompassing the fibroblast growth factor (FGF) and transforming growth factor-beta (TGF-beta) families and also data on cytokines and chemokines. Finally, we include a table summarizing all of the currently published data in this rapidly growing field. For a regularly updated web archive of studies, we have constructed a Compendium of Published Wound Healing Studies on Genetically Modified Mice which is avaialble at http://icbxs.ethz.ch/members/grose/woundtransgenic/home.html.

From an Enhanceosome to a Repressosome: Molecular Antagonism between Glucocorticoids and EGF Leads to Inhibition of Wound Healing

Wound healing in its complexity depends on the concerted activity of many signaling pathways. Here, we analyzed how the simultaneous presence of glucocorticoids (GC), retinoic acid (RA) and epidermal growth factor (EGF) affect wound healing at the molecular, cellular and tissue levels. We found that GC inhibit wound healing by inhibiting keratinocyte migration, whereas RA does not. Furthermore, GC block EGF-mediated migration, whereas RA does not. On the molecular level, these compounds target expression of one of the earliest markers of wound healing, cytoskeletal components, keratins K6 and K16. Both GC and RA repress their transcription, whereas EGF induces it. Interestingly, the GC inhibition is mediated by a repressosome complex consisting of four monomers of the GC receptor, beta-catenin and coactivator-associated-arginine-methyltransferase-1. GC are dominant, EGF cannot rescue GC-mediated inhibition. Pre-treatment of keratinocytes with GC shifts the balance towards the repressosome, allowing for dominant inhibition of K6 even in the presence of EGF or c-fos/c-jun. Although RA receptor gamma and glucocorticoid receptor bind to the same response element repressing transcription of keratins K6/K16, RA receptor interacts with the components of the EGF-enhanceosome (co-activators: glucocorticoid-receptor-interactive protein-1(GRIP-1)/steroid-receptors coactivator-1 (SRC-1)) without breaking it. Consequently, RA has a co-dominant effect with EGF: when present simultaneously, their effects balance each other. When keratinocytes are pre-treated with mitogen-activated protein kinase (MAPK) inhibitor, thus blocking EGF, the balance is shifted towards the RA repression. Similar to clinical findings, pre-treatment of keratinocytes with RA blocks GC-mediated inhibition. In summary, our results identify complex molecular mechanisms through which RA alleviates GC-mediated inhibition of wound healing.

Overcoming functional redundancy to elicit pachyonychia congenita-like nail lesions in transgenic mice

Mutations affecting the coding sequence of intermediate filament (IF) proteins account for >30 disorders, including numerous skin bullous diseases, myopathies, neuropathies, and even progeria. The manipulation of IF genes in mice has been widely successful for modeling key features of such clinically distinct disorders. A notable exception is pachyonychia congenita (PC), a disorder in which the nail and other epithelial appendages are profoundly aberrant. Most cases of PC are due to mutations in one of the following keratin-encoding genes: K6, K16, and K17. Yet null alleles obliterating the function of both K6 genes (K6alpha and K6beta) or the K17 gene, as well as the targeted expression of a dominant-negative K6alpha mutant, elicit only a subset of PC-specific epithelial lesions (excluding that of the nail in mice). We show that newborn mice null for K6alpha, K6beta, and K17 exhibit severe lysis restricted to the nail bed epithelium, where all three genes are robustly expressed, providing strong evidence that this region of the nail unit is initially targeted in PC. Our findings point to significant redundancy among the multiple keratins expressed in hair and nail, which can be related to the common ancestry, clustered organization, and sequence relatedness of specific keratin genes.

Loss of keratin 10 leads to mitogen-activated protein kinase (MAPK) activation, increased keratinocyte turnover, and decreased tumor formation in mice

Keratin 10 (K10) is the major protein in the upper epidermis where it maintains keratinocyte integrity. Others have reported that K10 may act as a tumor suppressor upon ectopic expression in mice. Although K10(-/-) mice show significant epidermal hyperproliferation, accompanied by an activation of the mitogen-activated protein kinase (MAPK) pathway, they formed no spontaneous tumors. Here, we report that K10(-/-) mice treated with 7,12-dimethylbenz[a]anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) developed far less papillomas than wild-type mice. BrdU(5-bromo-2'-deoxyuridine)-labeling revealed a strongly accelerated keratinocyte turnover in K10(-/-) epidermis suggesting an increased elimination of initiated keratinocytes at early stages of developing tumors. This is further supported by the absence of label-retaining cells 18 d after the pulse whereas in wild-type mice label-retaining cells were still present. The concomitant increase in K6, K16, and K17 in K10 null epidermis and the increased motility of keratinocytes is in agreement with the pliability versus resilience hypothesis, stating that K10 and K1 render cells more stable and static. The K10(-/-) knockout represent the first mouse model showing that loss of a keratin, a cytoskeletal protein, reduces tumor formation. This is probably caused by an accelerated turnover of keratinocytes, possibly mediated by activation of MAPK pathways.

Real-time monitoring of keratin 5 expression during burn re-epithelialization1

BACKGROUND

Keratin is a major protein produced during epithelialization following burn injury and is a useful marker for assessing wound healing. Transgenic mice expressing enhanced green fluorescent protein (EGFP) driven by the keratin 5 (K5) promoter (K5GFP mice) were used to monitor keratin expression, and thus, re-epithelialization of burn wounds.

MATERIALS AND METHODS

K5GFP transgenic mice were created using conventional techniques, with PCR and Southern blot confirmation of transgene incorporation, followed by selection of the line with the most intense and consistent basal epithelial EGFP expression. Epi-fluorescent microscopy of 24 K5GFP mouse flanks and 10 negative littermate controls was used to characterize EGFP intensity, before wounding and serially for 30 days after administration of a standardized burn wound and excision. Biopsy sections of K5GFP and negative control mice were stained with K5 antibody and imaged with confocal microscopy to characterize the distribution of EGFP and K5 at baseline and after injury and to examine the correlation between K5 expression and EGFP expression during healing.

RESULTS

Green fluorescence intensity increased at the advancing wound margin of burned K5GFP mice, reaching a maximum between days 12 and 15 post-burn and then decreasing as healing completed. K5 and EGFP expression increased in parallel in burned K5GFP mice as demonstrated by confocal microscopy.

CONCLUSION

EGFP expression correlated with K5 expression during wound healing and therefore serves as a good marker of re-epithelialization. This transgenic model allows noninvasive, real-time assessment of in vivo K5 expression and will be useful in the study of wound healing.

A role for thyroid hormone in wound healing through keratin gene expression

The importance of thyroid hormone (TH) in wound healing is not well understood. To gain insight, we evaluated the impact of TH deficiency on wound-healing genes in cultured keratinocytes. By RT-PCR, keratin 6a (K6a) and 16 (K16) gene expression in TH replete cells was 3.8- (P < 0.005) and 1.9-fold (P < 0.05) greater, respectively, than expression in TH-deficient cells. By real-time PCR, TH replete cell expression of K6a, K16, and K17 was greater than in deficient cells: 18- (P < 0.001), 10- (P < 0.001), and 4-fold (P < 0.005), respectively. To examine TH requirement for optimal wound healing, we contrasted TH-deficient vs. ip T(3)-treated mice. Four days after wounding, ip T(3)-treated mice had twice the degree of wound closure as hypothyroid mice (P < 0.001). By RT-PCR, K6a and K17 gene expression from control mouse skin was greater than from hypothyroid mouse skin: 5- (P < 0.001) and 1.7-fold (P < 0.05), respectively. T(3) is necessary for the keratinocyte proliferation required for optimal wound healing. T(3) exerts influence by stimulating expression of the wound-healing keratin genes. Thus, for hypothyroid patients undergoing surgery that cannot be delayed until euthyroidism is achieved, our data support T(3) treatment for the perioperative period.

Matrix metalloproteinase-19 expression in normal and diseased skin: dysregulation by epidermal proliferation

Most of the matrix metalloproteinases (MMP) are not expressed in normal intact skin but they are upregulated in inflamed or diseased skin. The recently cloned MMP-19 is one of the few MMP members that are also expressed in healthy epidermis. In this study, we found that MMP-19 is generally coexpressed with cytokeratin 14 that is confined to keratinocytes of the stratum basale. MMP-19 was also detected in hair follicles, sebaceous glands, and eccrine sweat glands. Its expression, however, changed in cutaneous diseases exhibiting increased alternations of epidermal proliferation, such as psoriasis, eczema, and tinea. In the affected area, MMP-19 was also found in suprabasal and spinous epidermal layers. We also studied the regulation of MMP-19 expression at the protein level, as well as by using a promoter assay. The constitutive expression of MMP-19 was upregulated with phorbol myristate acetate and downregulated with retinoic acid and dexamethasone. Tumor necrosis factor-alpha, interleukin (IL)-6, TGF-beta, IL-15, IL-8, and RANTES as well as the bacterial compounds lipopolysaccharide and lipoteichoic acid did not show any profound effect in HaCaT cells. In contrast, type IV and type I collagens upregulated MMP-19 significantly. The dysregulation of MMP-19 expression in epidermis suggests its possible involvement in the perpetuation of cutaneous infections and proliferative disorders such as psoriasis.

Assessment of splice variant-specific functions of desmocollin 1 in the skin

Desmocollin 1 (Dsc1) is part of a desmosomal cell adhesion receptor formed in terminally differentiating keratinocytes of stratified epithelia. The dsc1 gene encodes two proteins (Dsc1a and Dsc1b) that differ only with respect to their COOH-terminal cytoplasmic amino acid sequences. On the basis of in vitro experiments, it is thought that the Dsc1a variant is essential for assembly of the desmosomal plaque, a structure that connects desmosomes to the intermediate filament cytoskeleton of epithelial cells. We have generated mice that synthesize a truncated Dsc1 receptor that lacks both the Dsc1a- and Dsc1b-specific COOH-terminal domains. This mutant transmembrane receptor, which does not bind the common desmosomal plaque proteins plakoglobin and plakophilin 1, is integrated into functional desmosomes. Interestingly, our mutant mice did not show the epidermal fragility previously observed in dsc1-null mice. This suggests that neither the Dsc1a- nor the Dsc1b-specific COOH-terminal cytoplasmic domain is required for establishing and maintaining desmosomal adhesion. However, a comparison of our mutants with dsc1-null mice suggests that the Dsc1 extracellular domain is necessary to maintain structural integrity of the skin.

Loss of keratin 6 (K6) proteins reveals a function for intermediate filaments during wound repair

The ability to heal wounds is vital to all organisms. In mammalian tissues, alterations in intermediate filament (IF) gene expression represent an early reaction of cells surviving injury. We investigated the role of keratin IFs during the epithelialization of skin wounds using a keratin 6α and 6β (K6α/K6β)-null mouse model. In skin explant culture, null keratinocytes exhibit an enhanced epithelialization potential due to increased migration. The extent of the phenotype is strain dependent, and is accompanied by alterations in keratin IF and F-actin organization. However, in wounded skin in vivo, null keratinocytes rupture as they attempt to migrate under the blood clot. Fragility of the K6α/K6β-null epidermis is confirmed when applying trauma to chemically treated skin. We propose that the alterations in IF gene expression after tissue injury foster a compromise between the need to display the cellular pliability necessary for timely migration and the requirement for resilience sufficient to withstand the rigors of a wound site.

Keap1-null mutation leads to postnatal lethality due to constitutive Nrf2 activation

Transcription factor Nrf2 (encoded by Nfe2l2) regulates a battery of detoxifying and antioxidant genes, and Keap1 represses Nrf2 function. When we ablated Keap1, Keap1-deficient mice died postnatally, probably from malnutrition resulting from hyperkeratosis in the esophagus and forestomach. Nrf2 activity affects the expression levels of several squamous epithelial genes. Biochemical data show that, without Keap1, Nrf2 constitutively accumulates in the nucleus to stimulate transcription of cytoprotective genes. Breeding to Nrf2-deficient mice reversed the phenotypic Keap1 deficiencies. These experiments show that Keap1 acts upstream of Nrf2 in the cellular response to oxidative and xenobiotic stress.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Phenotypes, genotypes and their contribution to understanding keratin function

A large number of mutations in keratin genes underlie inherited tissue fragility disorders of epithelia. The genotype-phenotype correlations emerging from these studies provide a rich source of information about the function of keratins that would have taken decades to achieve by a purely transgenic approach. Human disease studies are being supplemented by engineered mouse mutant studies, which give access to the effects of genetic alterations unlikely to occur naturally. Evidence is emerging that the great diversity of keratins might be required to enable cells to adapt their structure in response to different signalling pathways.

Role for keratins 6 and 17 during wound closure in embryonic mouse skin

Injury to adult skin triggers a response designed to restore its vital barrier function. A conserved aspect of this response is a rapid switch in gene expression whereby the type II keratin 6 (K6) and type I keratins 16 and 17 (K16, K17) are induced in epithelial cells at the wound edge. This induction occurs at the expense of the keratins normally expressed during terminal differentiation and correlates with the activation of epithelial cells at the wound edge, ahead of their migration into the wound site. Here, we show that the capacity to enact this switch is already acquired in E11.5 stage mouse embryos. Such early timing is well ahead of the onset of differentiation-specific gene expression (approximately E13.5) and the acquisition of barrier formation by developing epidermis (approximately E16.5). Induction of K6, K16, and K17 correlates with changes in the morphology of epithelial cells at the wound edge. The closure of embryonic wounds is significantly delayed in K17 null embryos, but not embryos null for K6. These observations significantly extend the correlation between K6, K16, and K17 expression and epithelial wound closure, and provide direct evidence that expression of these keratins, K17 in particular, is important for the timeliness of this process.

Defects in keratinocyte activation during wound healing in the syndecan-1-deficient mouse

Mice lacking syndecan-1 are viable, fertile and have morphologically normal skin, hair and ocular surface epithelia. While studying the response of these mice to corneal epithelial and skin wounding, we identified defects in epithelial cell proliferation and regulation of integrin expression. mRNA profiling of corneal epithelial tissues obtained from wild-type and syndecan-1(-/-) mice suggest that these defects result from differences in overall gene transcription. In the cornea, syndecan-1(-/-) epithelial cells migrate more slowly, show reduced localization of alpha9 integrin during closure of wounds and fail to increase their proliferation rate 24 hours after wounding. In the skin, we did not document a migration defect after full thickness wounds but did observe cell proliferation delays and reduced localization of alpha9 integrin in the syndecan-1(-/-) epidermis after dermabrasion. Despite increased cell proliferation rates in the uninjured syndecan-1(-/-) epidermis and the corneal epithelium, morphologically normal epithelial thickness is maintained prior to injury; however, wounding is accompanied by prolonged hypoplasia in both tissues. Analyses of integrin protein levels in extracts from full thickness skin, revealed increased levels of alpha3 and alpha9 integrins both prior to injury and after hair removal in syndecan-1(-/-) mice but no increase 2 days after dermabrasion. These data for the first time show involvement of alpha9 integrin in skin wound healing and demonstrate essential roles for syndecan-1 in mediating cell proliferation and regulation of integrin expression in normal and wounded epithelial tissues.

Disruption of steroid and prolactin receptor patterning in the mammary gland correlates with a block in lobuloalveolar development

Targeted deletion of the bZIP transcription factor, CCAAT/enhancer binding protein-beta (C/EBPbeta), was shown previously to result in aberrant ductal morphogenesis and decreased lobuloalveolar development, accompanied by an altered pattern of progesterone receptor (PR) expression. Here, similar changes in the level and pattern of prolactin receptor (PrlR) expression were observed while screening for differentially expressed genes in C/EBPbeta(null) mice. PR patterning was also altered in PrlR(null) mice, as well as in mammary tissue transplants from both PrlR(null) and signal transducer and activator of transcription (Stat) 5a/b-deficient mice, with concomitant defects in hormone-induced proliferation. Down-regulation of PR and activation of Stat5 phosphorylation were seen after estrogen and progesterone treatment in both C/EBPbeta(null) and wild-type mice, indicating that these signaling pathways were functional, despite the failure of steroid hormones to induce proliferation. IGF binding protein-5, IGF-II, and insulin receptor substrate-1 all displayed altered patterns and levels of expression in C/EBPbeta(null) mice, suggestive of a change in the IGF signaling axis. In addition, small proline-rich protein (SPRR2A), a marker of epidermal differentiation, and keratin 6 were misexpressed in the mammary epithelium of C/EBPbeta(null) mice. Together, these data suggest that C/EBPbeta is a master regulator of mammary epithelial cell fate and that the correct spatial pattern of PR and PrlR expression is a critical determinant of hormone-regulated cell proliferation.

Hague (Hag). A new mouse hair mutation with an unstable semidominant allele

A spontaneous mouse hair mutation was identified in a C3H/HeN colony. The mode of inheritance of the mutation was semidominant, with incomplete penetrance when heterozygous. The trait is controlled by a single locus hague (Hag), which was mapped to the telomeric region of chromosome 15. This mutation was shown to be unstable, since its transmission could be switched from semidominant to recessive. To identify the causative gene and the nature of the mutation, hague was introduced into a high-resolution and high-density molecular genetic map. Over 2000 meioses were analyzed and the mutation was mapped to the keratin 2 complex genes. A YAC and BAC physical map of the critical region was then constructed and the gene involved was located in a 600- to 800-kb-long segment. Fourteen genes were mapped to this region; of these, 11 were expressed in the skin (5 epidermic cytokeratin and 6 hard keratin genes), but none were mutated in hague mice.

Regenerative biology: the emerging field of tissue repair and restoration

Regenerative biology has now been recognized as a new field with certain aims and goals. One direction of this new field is to understand the basic mechanisms by which tissues can be repaired and restored. The other direction examines the possibility of using this basic knowledge to apply it to medicine with the goal to clinically repair damaged tissues. Regeneration of tissues can occur by the differentiation of stem cells (local or non-local) or by the transdifferentiation of local terminally differentiated cells. While the transdifferentiation aspects are old, during the past few years many data have accumulated regarding the existence of stem cells and their participation in tissue renewal. This review will present an overview of the potential of all vertebrate organs to regenerate and of the basic mechanisms involved.

The molecular basis of hereditary palmoplantar keratodermas

In recent years, the gene defects causing many types of hereditary palmoplantar keratoderma have been discovered. These genes encode a variety of proteins involved in the terminal differentiation of keratinocytes and the formation of the cornified cell envelope. In this article, we review the molecular defects underlying various palmoplantar keratodermas with particular attention to the role of these molecules in the terminal differentiation of palmoplantar epidermis. Of the proteins involved in keratodermas, loricrin, keratins, and desmosomal proteins provide the protein structure of the cornified cell envelope. Connexins form intercellular gap junctions, which regulate ionic calcium signals necessary for the expression of the proteins that form the cornified cell envelope. Cathepsins likely mediate enzymatic processes necessary for the formation and dissolution of the cornified cell envelope. The clinical phenotypes produced by various mutations affecting these proteins are discussed vis-à-vis data from genetic, cellular, and molecular experiments.

Hyperproliferation, induction of c-Myc and 14-3-3σ, but no cell fragility in keratin-10-null mice

In the past, keratins have been established as structural proteins. Indeed,mutations in keratin 10 (K10) and other epidermal keratins lead to severe skin fragility syndromes. Here, we present adult K10-/- mice, which reveal a novel connection between the regulation of cell proliferation and K10. Unlike most keratin mutant mice, the epidermis of adult K10-/-mice showed no cytolysis but displayed hyperproliferation of basal keratinocytes and an increased cell size. BrdU labelling revealed a shortened transition time for keratinocytes migrating outwards and DAPI staining of epidermal sheets uncovered an impaired organization of epidermal proliferation units. These remarkable changes were accompanied by the induction of c-Myc,cyclin D1, 14-3-3σ and of wound healing keratins K6 and K16. The phosphorylation of Rb remained unaltered. In line with the downregulation of K10 in squamous cell carcinomas and its absence in proliferating cells in vivo, our data suggest that the tissue-restricted expression of some members of the keratin gene family not only serves structural functions. Our results imply that the altered composition of the suprabasal cytoskeleton is able to alter the proliferation state of basal cells through the induction of c-Myc. A previous model based on transfection of K10 in immortalized human keratinocytes suggested a direct involvement of K10 in cell cycle control. While those experiments were performed in human cultured keratinocytes, our data establish, that in vivo, K10 acts by an indirect control mechanism in trans.

'Hard' and 'soft' principles defining the structure, function and regulation of keratin intermediate filaments

Keratins make up the largest subgroup of intermediate filament proteins and represent the most abundant proteins in epithelial cells. They exist as highly dynamic networks of cytoplasmic 10-12 nm filaments that are obligate heteropolymers involving type I and type II keratins. The primary function of keratins is to protect epithelial cells from mechanical and nonmechanical stresses that result in cell death. Other emerging functions include roles in cell signaling, the stress response and apoptosis, as well as unique roles that are keratin specific and tissue specific. The role of keratins in a number of human skin, hair, ocular, oral and liver diseases is now established and meshes well with the evidence gathered from transgenic mouse models. The phenotypes associated with defects in keratin proteins are subject to significant modulation by functional redundancy within the family and modifier genes as well. Keratin filaments undergo complex regulation involving post-translational modifications and interactions with self and with various classes of associated proteins.

p53 mutant mice that display early ageing-associated phenotypes

The p53 tumour suppressor is activated by numerous stressors to induce apoptosis, cell cycle arrest, or senescence. To study the biological effects of altered p53 function, we generated mice with a deletion mutation in the first six exons of the p53 gene that express a truncated RNA capable of encoding a carboxy-terminal p53 fragment. This mutation confers phenotypes consistent with activated p53 rather than inactivated p53. Mutant (p53+/m) mice exhibit enhanced resistance to spontaneous tumours compared with wild-type (p53+/+) littermates. As p53+/m mice age, they display an early onset of phenotypes associated with ageing. These include reduced longevity, osteoporosis, generalized organ atrophy and a diminished stress tolerance. A second line of transgenic mice containing a temperature-sensitive mutant allele of p53 also exhibits early ageing phenotypes. These data suggest that p53 has a role in regulating organismal ageing.

Tissue regeneration: hair follicle as a model

The Cre-loxP strategy has allowed us to generate the mice whose keratinocytes are devoid of Stat3, which play a pivotal role in the signal transduction following the stimulation with various growth factors/cytokines, such as EGF, HGF, or IL-6. Although keratinocyte-specific Stat3-disrupted mice were born normal with intact skin and the first hair cycle, they exhibited retardation of wound healing and absence of the second hair cycle onward, leading to development of spontaneous skin ulcers and alopecia as they aged. Thus, analyses of these mice reveal that Stat3 in keratinocytes contributes to the regeneration of epidermis and hair cycle process.

Increased levels of keratin 16 alter epithelialization potential of mouse skin keratinocytes in vivo and ex vivo

The process of wound repair in adult skin is complex, involving dermal contraction and epithelial migration to repair the lesion and restore the skin's barrier properties. At the wound edge, keratinocytes undergo many changes that engender an epithelialization behavior. The type II keratin 6 and type I keratins 16 and 17 are induced well before cell migration begins, but the role of these proteins is not understood. Forced expression of human K16 in skin epithelia of transgenic mice has been shown to cause dose-dependent skin lesions concomitant with alterations in keratin filament organization and in cell adhesion. Here we show, with the use of a quantitative assay, that these transgenic mice show a delay in the closure of full-thickness skin wounds in situ compared with wild-type and low-expressing K16 transgenic mice. We adapted and validated an ex vivo skin explant culture system to better assess epithelialization in a wound-like environment. Transgenic K16 explants exhibit a significant reduction of keratinocyte outgrowth in this setting. This delay is transgene dose-dependent, and is more severe when K16 is expressed in mitotic compared with post-mitotic keratinocytes. Various lines of evidence suggest that the mechanism(s) involved is complex and not strictly cell autonomous. These findings have important implications for the function of K16 in vivo.

Discovery of a novel murine keratin 6 (K6) isoform explains the absence of hair and nail defects in mice deficient for K6a and K6b

The murine genome is known to have two keratin 6 (K6) genes, mouse K6 (MK6)a and MK6b. These genes display a complex expression pattern with constitutive expression in the epithelia of oral mucosa, hair follicles, and nail beds. We generated mice deficient for both genes through embryonic stem cell technology. The majority of MK6a/b-/- mice die of starvation within the first two weeks of life. This is due to a localized disintegration of the dorsal tongue epithelium, which results in the build up of a plaque of cell debris that severely impairs feeding. However, approximately 25% of MK6a/b-/- mice survive to adulthood. Remarkably, the surviving MK6a/b-/- mice have normal hair and nails. To our surprise, we discovered MK6 staining both in the hair follicle and the nail bed of MK6a/b-/- mice, indicating the presence of a third MK6 gene. We cloned this previously unknown murine keratin gene and found it to be highly homologous to human K6hf, which is expressed in hair follicles. We therefore termed this gene MK6 hair follicle (MK6hf). The presence of MK6hf in the MK6a/b-/- follicles and nails offers an explanation for the absence of hair and nail defects in MK6a/b-/- animals.

An inducible mouse model for epidermolysis bullosa simplex: implications for gene therapy

The Dowling-Meara variant of epidermolysis bullosa simplex (EBS-DM) is a severe blistering disease inherited in an autosomal-dominant fashion. Here we report the generation of a mouse model that allows focal activation of a mutant keratin 14 allele in epidermal stem cells upon topical administration of an inducer, resulting in EBS phenotypes in treated areas. Using laser capture microdissection, we show that induced blisters healed by migration of surrounding nonphenotypic stem cells into the wound bed. This observation provides an explanation for the lack of mosaic forms of EBS-DM. In addition, we show that decreased mutant keratin 14 expression resulted in normal morphology and functions of the skin. Our results have important implications for gene therapy of EBS and other dominantly inherited diseases.

Focal activation of a mutant allele defines the role of stem cells in mosaic skin disorders

Stem cells are crucial for the formation and maintenance of tissues and organs. The role of stem cells in the pathogenesis of mosaic skin disorders remains unclear. To study the molecular and cellular basis of mosaicism, we established a mouse model for the autosomal-dominant skin blistering disorder, epidermolytic hyperkeratosis (MIM 113800), which is caused by mutations in either keratin K1 or K10. This genetic model allows activation of a somatic K10 mutation in epidermal stem cells in a spatially and temporally controlled manner using an inducible Cre recombinase. Our results indicate that lack of selective pressure against certain mutations in epidermal stem cells leads to mosaic phenotypes. This finding has important implications for the development of new strategies for somatic gene therapy of dominant genodermatoses.

A reporter transgene based on a human keratin 6 gene promoter is specifically expressed in the periderm of mouse embryos

We report the developmental regulation of a lacZ reporter transgene fused to the promoter region of the human keratin 6a gene. In mouse embryos, the transgene is expressed in the periderm (the outermost layer of embryonic epidermis), as are the endogenous keratin 6 alpha and beta genes. A subset of periderm cells, localized to temporary epithelial fusions, is known to contain keratin 6 protein, and we find that these cells also harbor LacZ enzymatic activity.

Biochemical control of melanogenesis and melanosomal organization

Current knowledge on the regulation of mammalian pigmentation at the genetic and biochemical level, and constituents that participate in melanosomal organization, is summarized. Approximately 25% of the more than 80 genes known to regulate pigmentation in mammals have been cloned and characterized to date. Almost half of those encode proteins that localize, either specifically or nonspecifically, to melanosomes; mutations in those genes generally lead to phenotypic changes in pigmentation as well as in other pleiotropic changes. The expression and function of these proteins not only affects phenotypic appearance, but also the properties of melanins, especially their photoprotective characteristics. Because many of those melanosomal proteins also serve as melanoma-specific targets, regulation of their expression has dramatic implications for immune targeting of malignant melanoma.

DNA photodamage stimulates melanogenesis and other photoprotective responses

Ultraviolet (UV) irradiation is a major source of environmental damage to skin. Melanin pigmentation protects against this damage by absorbing UV photons and UV-generated free radicals before they can react with DNA and other critical cellular components; and UV-induced melanogenesis or tanning is widely recognized as exposed skin's major defense against further UV damage. This article reviews extensive data suggesting DNA damage or DNA repair intermediates directly triggers tanning and other photoprotective responses. Evidence includes the observations that tanning is enhanced in cultured pigment cells by accelerating repair of UV-induced cyclobutane pyrimidine dimers or by treating the cells with UV-mimetic DNA-damaging chemicals. Moreover, small single stranded DNA fragments such as thymidine dinucleotides (pTpT), the substrate for almost all DNA photoproducts, also stimulates tanning when added to cultured pigment cells or applied topically to intact skin. In bacteria, single stranded DNA generated by DNA damage or its repair activates a protease that in turn derepresses over 20 genes whose protein products enhance DNA repair and otherwise promote cell survival, a phenomenon termed the SOS response. Interestingly, pTpT also enhances repair of UV-induced DNA damage in human cells and animal skin, at least in part by activating the tumor suppressor protein and transcription factor p53 and thus upregulating a variety of gene products involved in DNA repair and cell cycle regulation. Together, these data suggest that human cells have an evolutionarily conserved SOS-like response in which UV-induced DNA damage serves as signal to induce photoprotective responses such as tanning and increased DNA repair capacity. The responses can also be triggered in the absence of DNA damage by addition of small single-stranded DNA fragments such as pTpT.