Keratins and the skin

Blood-group-related carbohydrates are expressed in organotypic cultures of human skin and oral mucosa

Cellular maturation and migration are usually associated with changes in cell-surface carbohydrates, but the relationship between these changes and cell behaviour is at present largely unknown. To investigate whether an organotypic culture system can be used as an in vitro model to study the function of cell-surface carbohydrates, we established organotypic cultures of skin and buccal mucosa. In these cultures, keratinocytes are grown at the air-liquid interface on a supporting matrix consisting of homologous fibroblasts embedded in a collagen type I gel. We examined the expression of blood-group-related carbohydrate structures, including Lewis x, sialylated Lewis x, Lewis y, Lewis a, and Lewis b, on the surface of epithelial cells in the cultures. We compared the results with the expression of more well-established markers, including cytokeratins, integrins, bullous pemphigoid antigen and laminin, in the same cultures. The organotypic skin and oral mucosa cultures showed a histological differentiation pattern analogous to that of normal skin and buccal mucosa, and a tissue-specific expression of carbohydrate structures and cytokeratins. However, both types of organotypic cultures also expressed markers which are normally seen during wound healing, including Lewis y, cytokeratin 16, and cytokeratin 19. We conclude that the organotypic cultures of oral mucosa and skin are suitable models for future studies of the function of cell-surface carbohydrates, although the expression of wound healing markers has to be taken into consideration.

Klf4 is a transcription factor required for establishing the barrier function of the skin

Located at the interface between body and environment, the epidermis must protect the body against toxic agents and dehydration, and protect itself against physical and mechanical stresses. Acquired just before birth and at the last stage of epidermal differentiation, the skin's proteinaceous/lipid barrier creates a surface seal essential for protecting animals against microbial infections and dehydration. We show here that Kruppel-like factor 4 (Klf4, encoded by the gene Klf4), highly expressed in the differentiating layers of epidermis, is both vital to and selective for barrier acquisition. Klf4-/- mice die shortly after birth due to loss of skin barrier function, as measured by penetration of external dyes and rapid loss of body fluids. The defect was not corrected by grafting of Klf4-/- skin onto nude mice. Loss of the barrier occurs without morphological and biochemical alterations to the well-known structural features of epidermis that are essential for mechanical integrity. Instead, late-stage differentiation structures are selectively perturbed, including the cornified envelope, a likely scaffold for lipid organization. Using suppressive subtractive hybridization, we identified three transcripts encoding cornified envelope proteins with altered expression in the absence of Klf4. Sprr2a is one, and is the only epidermal gene whose promoter is known to possess a functional Klf4 binding site. Our studies provide new insights into transcriptional governance of barrier function, and pave the way for unravelling the molecular events that orchestrate this essential process.

Gene therapy and dermatology: more than just skin deep

BACKGROUND

Recent advances in the molecular characterization of dermatologic disease have substantively augmented the understanding of the pathogenetic processes underlying disorders of the skin. This new knowledge coupled with progress in gene delivery technologies has paved the way for introducing cutaneous gene therapy into the dermatologic therapeutic armamentorium.

OBJECTIVE

This review article includes an overview of the current strategies for delivery of gene therapy with an emphasis on the potential role of cutaneous gene delivery in the treatment of skin and systemic diseases.

CONCLUSIONS

Accessibility for gene delivery, clinical evaluation, and topical modulation of gene expression render the skin a very attractive tissue for therapeutic gene delivery. However, there are several key hurdles to be overcome before cutaneous gene therapy becomes a viable clinical option. These include difficulties in inducing sustained expression of the desired gene in vivo, the challenge of targeting genes to long-lived stem cells, and the difficulty in achieving specific and uniform transfer to different compartments of the skin. However, these problems are not insurmountable and will likely be resolved in conjunction with ongoing advances in delineating gene expression profiles and other molecular properties of the skin, strategies for stem cell isolation, and improved approaches to regulating gene delivery and expression. These advances should create the framework for translating the enormous potential of cutaneous gene therapy into the clinical arena and, thereby, substantively improving the management of both cutaneous and systemic disease.

Characterization of a new tissue-engineered human skin equivalent with hair

We designed a new tissue-engineered skin equivalent in which complete pilosebaceous units were integrated. This model was produced exclusively from human fibroblasts and keratinocytes and did not contain any synthetic material. Fibroblasts were cultured for 35 d with ascorbic acid and formed a thick fibrous sheet in the culture dish. The dermal equivalent was composed of stacked fibroblast sheets and exhibited some ultrastructural organization found in normal connective tissues. Keratinocytes seeded on this tissue formed a stratified and cornified epidermis and expressed typical markers of differentiation (keratin 10, filaggrin, and transglutaminase). After 4 wk of culture, a continuous and ultrastructurally organized basement membrane was observed and associated with the expression of laminin and collagen IV and VII. Complete pilosebaceous units were obtained by thermolysin digestion and inserted in this skin equivalent in order to assess the role of the transfollicular route in percutaneous absorption. The presence of hair follicles abolished the lag-time observed during hydrocortisone diffusion and increased significantly its rate of penetration in comparison to the control (skin equivalent with sham hair insertion). Therefore, this new hairy human skin equivalent model allowed an experimental design in which the only variable was the presence of pilosebaceous units and provided new data confirming the importance of hair follicles in percutaneous absorption.

HPV16 E6 oncoprotein inhibits apoptosis induced during serum-calcium differentiation of foreskin human keratinocytes

Transfection of human papillomavirus (HPV) 16 E6 oncogene into foreskin primary human keratinocytes (PHKs) causes the formation of colonies of viable cells resistant to serum-calcium differentiation. To define the stage of keratinocyte differentiation inhibited by E6, we examined the response of PHKs to serum and calcium with respect to parameters of both growth and differentiation. The effect of HPV16 E6 was evaluated by infection with recombinant retroviruses encoding the E6 protein. Results of these studies indicated that terminal differentiation of cultured foreskin keratinocytes, triggered by serum and calcium, is a progressive process (2-3 weeks) that ends with cell death with characteristics of apoptosis. Human keratinocyte terminal differentiation was accompanied by time-related changes in the expression of cellular proteins involved in the control pathways of apoptosis, including downregulation of Bcl-2 and p53 and upregulation of Bax, which coincided with the appearance of morphological signs of apoptosis. E6 expression did not override the differentiation-associated G1 arrest or prevent the induction of squamous differentiation-specific markers, transglutaminase 1 and involucrin. E6 expression led, however, to a significant reduction in cell stratification and cell death by apoptosis, which correlated with prolonged expression of Bcl-2 and reduced elevation of Bax levels that occurred concomitant with a complete loss of p53. The data argue that E6 inhibits terminal differentiation of foreskin PHKs through inhibition of their differentiation-induced apoptotic program.

Application of a platinum replica method to the study of the cytoskeleton of isolated hair cells, supporting cells and whole mounts of the organ of Corti

We adapted a method of platinum replica to study the cytoskeleton of isolated cells of the guinea pig organ of Corti. This technique combined high image resolution with the ability to visualize the three-dimensional organization of the cytoskeleton of a whole cell. The procedure includes: isolation of hair cells and supporting cells using collagenase digestion, attachment of the cells to a coverslip, detergent extraction, chemical fixation, critical point drying, platinum/carbon coating, and transmission electron microscopy analysis. By using the method of platinum replica, we confirmed the existence of structural domains in the cortical lattice of outer hair cells. Based on the analysis of the partly destroyed cortical lattice, we propose that circumferential filaments are underlined with a thin flexible network. In addition, we established that the base of each stereocilium had a cone-like expansion of actin filaments and was surrounded by a thin bundle of filaments. We also produced replicas of the protrusion of the cuticular plate into the cytoplasm (infracuticular network) and the reticular lamina cytoskeleton. Our data indicated that the platinum replica method is useful for studying structural interactions among different cytoskeletal elements in the reticular lamina, as well as the cortex of outer hair cells and the cytoskeleton of supporting cells.

Evidence for a novel glutamate-mediated signaling pathway in keratinocytes

Phenotypic alterations in keratinocyte behavior are essential for maintaining epidermal integrity during growth and wound repair and rely on co-ordinated cell signaling events. Numerous growth factors and cytokines have been shown to be instrumental in guiding such changes in keratinocyte activity; here we provide evidence which proposes a novel epidermal signaling pathway mediated by the excitatory amino acid glutamate. Glutamate is the major excitatory neurotransmitter at synaptic junctions within the central nervous system; however, we have identified expression in vivo of several regulatory molecules associated with glutamate signaling in keratinocytes. In resting rat skin epidermis, different classes of glutamate receptors, transporters, and a recently described clustering protein were shown to display distinct distribution patterns, supportive of a multifunctional cellular communication pathway. Immunoreactive N-methyl-D-aspartate-type, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type, and metabotropic-type glutamate receptors were colocalized with the specific glutamate transporter EAAC1 in basal layer keratinocytes, and GLT-1, a related transporter, was expressed suprabasally. In full-thickness rat skin wounds, marked modifications in the distribution of N-methyl-D-aspartate receptors and EAAC1 were observed during re-epithelialization, and alterations in N-methyl-D-aspartate receptor expression accompanied embryonic epidermal development, implicating glutamate signaling in these important biologic events. Furthermore, we provide evidence that these receptors are functional in vitro. These data provide strong evidence to support a role for glutamate in the control of epidermal renewal, and therefore suggest potentially novel therapeutic targets for the treatment of skin disease and enhancement of wound healing.

180-kD bullous pemphigoid antigen/type XVII collagen: tissue-specific expression and molecular interactions with keratin 18

The 180-kD bullous pemphigoid antigen (BPAG2) is a hemidesmosomal transmembrane protein, also known as type XVII collagen. In this study, potential interactions of BPAG2 with other proteins expressed in epidermal keratinocytes were explored by yeast two-hybrid system using the amino-terminal intracellular domain of BPAG2 as a bait. Several independent interacting clones encoding keratin 18 (K18) were identified when the keratinocyte cDNA library, cloned into the yeast two-hybrid activation domain vector, was screened. The peptide sequence responsible for the interaction of BPAG2 was restricted to amino acids 15-25, and substitution of a valine residue in the middle of this sequence by a proline (V23P) by site-directed mutagenesis abolished the interaction. Further examination of the K18 sequences by restricted cDNA constructs in yeast two-hybrid system identified a carboxyl-terminal segment corresponding to helix 2B domain as critical for BPAG2 binding. The interaction of BPAG2/K18 was confirmed by an in vitro protein-protein interaction assay, which also confirmed that normal human keratinocytes express K18 in culture. The tissue specific expression of BPAG2 was first examined using a multi-tissue RNA blot. Human multiple tissue cDNA panels representing a variety of adult and fetal tissues as well as tumor cells were used as PCR-templates to study the expression patterns of both BPAG2 and K18. The results demonstrated significant level of expression of BPAG2, besides in epidermal keratinocytes, also in a variety of tissues with predominant epithelial component, such as mammary, salivary and thyroid glands, colon, prostate, testis, placenta, and adult and fetal thymus, as well as in colon, pancreatic and prostatic adenocarcinoma cell lines, and an ovarian carcinoma. As expected, K18 transcript is present in liver, pancreas, colon, placenta, and in fetal kidney. Collectively, the results suggest that BPAG2 has a relatively broad tissue distribution including specialized and simple epithelia, and that within the tissues such as colon and placenta, BPAG2 may have direct interactions with K18, a keratin characteristically expressed in a simple epithelia.

Upregulation and redistribution of E-MAP-115 (epithelial microtubule-associated protein of 115 kDa) in terminally differentiating keratinocytes is coincident with the formation of intercellular contacts

Microtubules are involved in the positioning and movement of organelles and vesicles and therefore play fundamental roles in cell polarization and differentiation. Their organization and properties are cell-type specific and are controlled by microtubule-associated proteins (MAP). E-MAP-115 (epithelial microtubule-associated protein of 115 kDa) has been identified as a microtubule-stabilizing protein predominantly expressed in epithelial cells. We have used human skin and primary keratinocytes as a model to assess a putative function of E-MAP-115 in stabilizing and reorganizing the microtubule network during epithelial cell differentiation. Immunolabeling of skin sections indicated that E-MAP-115 is predominantly expressed in the suprabasal layers of the normal epidermis and, in agreement with this observation, is relatively abundant in squamous cell carcinomas but barely detectable in basal cell carcinomas. In primary keratinocytes whose terminal differentiation was induced by increasing the Ca2+ concentration of the medium, E-MAP-115 expression significantly increased during the first day, as observed by northern and western blot analysis. Parallel immunofluorescence studies showed an early redistribution of E-MAP-115 from microtubules with a paranuclear localization to cortical microtubules organized in spike-like bundles facing intercellular contacts. This phenomenon is transient and can be reversed by Ca2+ depletion. Treatment of cells with cytoskeleton-active drugs after raising the Ca2+ concentration indicated that E-MAP-115 is associated with a subset of stable microtubules and that the cortical localization of these microtubules is dependent on other microtubules but not on strong interactions with the actin cytoskeleton or the plasma membrane. The mechanism whereby E-MAP-115 would redistribute to and stabilize cortical microtubules used for the polarized transport of vesicles towards the plasma membrane, where important reorganizations take place upon stratification, is discussed.

Cancer from the outside, aging from the inside: mouse models to study the consequences of defective nucleotide excision repair

In recent years, mouse models have been generated to study the syndromes associated with a defect in nucleotide excision repair (NER). Thus, via conventional knockout gene targeting or by mimicking patient-specific alleles, mouse models for xeroderma pigmentosum (XP), Cockayne syndrome (CS) and photosensitive trichothiodystrophy (TTD) have been obtained. The generation of this series of mouse mutants allows in vivo investigation of some intriguing questions that have puzzled the field, such as the paradoxical absence of cancer development in TTD and CS despite their NER deficiencies, and the role of the ERCC1 gene in mitotic recombination and cross-link repair. Other interesting issues include the pathophysiology of the non-NER related clinical symptoms in TTD and CS patients and the proposed involvement of NER and transcription in the process of aging. This review will focus on data obtained thus far and discuss further utilization of the mouse mutants for unraveling some of the fascinating and medically relevant aspects associated with defects in NER and related processes.

Altered taurine release following hypotonic stress in astrocytes from mice deficient for GFAP and vimentin

Astrocytes maintain their volume in response to changes in osmotic pressure in their environment by an afflux/influx of ions and organic osmoequivalents. The initial swelling of an astrocyte transferred to a hypoosmotic medium is thus reversed within minutes. The mechanisms which trigger this process as well as the sensors for cell volume are largely unknown, however, the cytoskeleton appears to be involved. We have addressed the role of one component of the cytoskeleton, the intermediate filaments, in the maintenance of astrocytic cell volume. Astrocytes from wild type mice were compared with cells from mice deficient for either glial fibrillary acidic protein (GFAP-/-) or vimentin (vimentin-/-) and with astrocytes from mice deficient for both proteins (GFAP-/-vim-/-). Whereas GFAP-/- and vimentin-/- cultured or reactive astrocytes retain intermediate filaments, the GFAP-/-vim-/- astrocytes are completely devoid of these structures. The rate of efflux of the preloaded osmoequivalent 3H-taurine from primary and passaged cultures of astrocytes was monitored. A reduction of NaCl (25 mM) in the perfusion medium led to a 400-900% increase of 3H-taurine afflux in astrocytes from wild type mice. The stimulated efflux was not significantly affected in astrocytes from GFAP-/- or vimentin-/- mice. However, the efflux from astrocytes from GFAP-/-vim-/- mice was 25-46% lower than the wild type levels. The results strengthen the role of the cytoskeleton in astrocyte volume regulation and suggest an involvement of intermediate filaments in the process.

The ovo gene required for cuticle formation and oogenesis in flies is involved in hair formation and spermatogenesis in mice

The Drosophila svb/ovo gene gives rise to differentially expressed transcripts encoding a zinc finger protein. svb/ovo has two distinct genetic functions: shavenbaby (svb) is required for proper formation of extracellular projections that are produced by certain epidermal cells in late-stage differentiation; ovo is required for survival and differentiation of female germ cells. We cloned a mouse gene, movo1 encoding a nuclear transcription factor that is highly similar to its fly counterpart in its zinc-finger sequences. In mice, the gene is expressed in skin, where it localizes to the differentiating cells of epidermis and hair follicles, and in testes, where it is present in spermatocytes and spermatids. Using gene targeting, we show that movo1 is required for proper development of both hair and sperm. movo1(-/-) mice are small, produce aberrant hairs, and display hypogenitalism, with a reduced ability to reproduce. These mice also develop abnormalities in kidney, where movo1 is also expressed. Our findings reveal remarkable parallels between mice and flies in epidermal appendage formation and in germ-cell maturation. Furthermore, they uncover a phenotype similar to that of Bardet-Biedl syndrome, a human disorder that maps to the same locus as human ovo1.

BDNF overexpression induces differential increases among subsets of sympathetic innervation in murine back skin

Besides their recognized dependence on nerve growth factor (NGF) during development, the dependence of mature sympathetic ganglion neurons on other neurotrophins is still unclear. Here, we have investigated the sympathetic innervation of back skin in mice overexpressing brain-derived neurotrophic factor (BDNF) under the alpha-myosin heavy-chain promoter, as well as in BDNF knockout (-/-) mice. Compared with wild-type controls, the dorsal skin of BDNF overexpressing mice displayed a significantly enhanced number of adrenergic, tyrosine hydroxylase-immunoreactive (IR) nerve fibres, while cholinergic or peptidergic sensory nerve fibres appeared unaltered. The adrenergic hyperinnervation in dorsal skin of BDNF overexpressing mice was most pronounced in the arrector pili muscle of hair follicles, while no increase of tyrosine hydroxylase-or neuropeptide Y-IR fibres associated with subcutaneous blood vessels was found. Instead, back skin of BDNF knockout (-/-) mice contained significantly fewer tyrosine hydroxylase-IR dermal nerve fibres than wild-type animals. This suggests that BDNF plays an important role in the control of different subsets of adrenergic innervation in murine back skin, and indicates that paravertebral sympathetic ganglia display a previously unrecognized differential BDNF-dependence in vivo.

Sonic hedgehog signaling is essential for hair development

BACKGROUND

The skin is responsible for forming a variety of epidermal structures that differ amongst vertebrates. In each case the specific structure (for example scale, feather or hair) arises from an epidermal placode as a result of epithelial-mesenchymal interactions with the underlying dermal mesenchyme. Expression of members of the Wnt, Hedgehog and bone morphogenetic protein families (Wnt10b, Sonic hedgehog (Shh) and Bmp2/Bmp4, respectively) in the epidermis correlates with the initiation of hair follicle formation. Further, their expression continues into either the epidermally derived hair matrix which forms the hair itself, or the dermal papilla which is responsible for induction of the hair matrix. To address the role of Shh in the hair follicle, we have examined Shh null mutant mice.

RESULTS

We found that follicle development in the Shh mutant embryo arrested after the initial epidermal-dermal interactions that lead to the formation of a dermal papilla anlage and ingrowth of the epidermis. Wnt10b, Bmp2 and Bmp4 continued to be expressed at this time, however. When grafted to nude mice (which lack T cells), Shh mutant skin gave rise to large abnormal follicles containing a small dermal papilla. Although these follicles showed high rates of proliferation and some differentiation of hair matrix cells into hair-shaft-like material, no hair was formed.

CONCLUSIONS

Shh signaling is not required for initiating hair follicle development. Shh signaling is essential, however, for controlling ingrowth and morphogenesis of the hair follicle.

Expression of epidermal keratins and the cornified envelope protein involucrin is influenced by permeability barrier disruption

In previous studies we have shown that experimental permeability barrier disruption leads to an increase in epidermal lipid and DNA synthesis. Here we investigate whether barrier disruption also influences keratins and cornified envelope proteins as major structural keratinocyte proteins. Cutaneous barrier disruption was achieved in hairless mouse skin by treatments with acetone +/- occlusion, sodium dodecyl sulfate, or tape-stripping. As a chronic model for barrier disruption, we used essential fatty acid deficient mice. Epidermal keratins were determined by one- and two-dimensional gel electrophoresis, immunoblots, and anti-keratin antibodies in biopsy samples. In addition, the expression of the cornified envelope proteins loricrin and involucrin after barrier disruption was determined by specific antibodies in human skin. Acute as well as chronic barrier disruption resulted in the induction of the expression of keratins K6, K16, and K17. Occlusion after acute disruption led to a slight reduction of keratin K6 and K16 expression. Expression of basal keratins K5 and K14 was reduced after both methods of barrier disruption. Suprabasal keratin K10 expression was increased after acute barrier disruption and K1 as well as K10 expression was increased after chronic barrier disruption. Loricrin expression in mouse and in human skin was unchanged after barrier disruption. In contrast, involucrin expression, which was restricted to the granular and upper spinous layers in normal human skin, showed an extension to the lower spinous layers 24 h after acetone treatment. In summary, our results document that acute or chronic barrier disruption leads to expression of keratins K6, K16, and K17 and to a premature expression of involucrin. We suggest that the coordinated regulation of lipid, DNA, keratin, and involucrin synthesis is critical for epidermal permeability barrier function.

Cytokeratins of the equine hoof wall, chestnut and skin: bio- and immunohisto-chemistry

The equine skin and its appendages (chestnut, hoof capsule, ergot, sebaceous glands, sweat glands and hair) consist mainly of keratinocytes. The intermediate filament cytoskeleton of these cells in involved in specialised functions, such as mechanical co-ordination of the cytoskeleton of the cell or tissue. In this study, 7 monoclonal antibodies, one polyclonal antibody and immunoblot analysis were used to characterise cytokeratins (separated by 1- and 2-dimensional gel electrophoresis) from the hoof wall and chestnut. The tissue distribution of these cytokeratins was studied by immunohistochemical staining of sections from the skin, chestnut, periople, matrix of the stratum medium of the hoof wall, and the stratum internum of the hoof wall. The results of analysis revealed a difference in the content of cytokeratins between: 1) cell layers within the same tissue, 2) skin and more proliferative epidermal tissues, i.e., chestnut and periople, 3) tissues containing a stratum granulosum (skin, chestnut and periople) and tissues without stratum granulosum (stratum medium and internum of the hoof wall), 4) stratum medium and stratum internum of the hoof wall and 5) stratum internum at the most proximal level and at the mid level of the hoof wall.

Isolation and characterization of a putative keratin-associated protein gene expressed in embryonic skin of mice

Embryonic mouse skin undergoes substantial morphologic changes from 13 days post-coitus (dpc) to 16 dpc, i.e., from simple layers of epithelial cells and periderm at 13.5 dpc to almost fully differentiated stratified epithelium with the rudiments of hair follicles at 16.5 dpc. Using RNA differential display, we isolated a gene involved in the development of mouse epidermis. This gene, tentatively designated as 4C32, encodes 197 amino acids containing six direct repeats of 10 amino acids with the CQ motif. The repetitive structure with the CQ motif is seen in most keratin-associated protein families, which are known to be specifically expressed in hair follicles. 4C32 is expressed in the outermost layer of the embryonic epidermis at 15.5 and 16.5 dpc, and abruptly disappears at 17.5 dpc, suggesting that 4C32 is expressed in the periderm. The periderm is a superficial layer of embryonic epidermis, and is known to disappear at 17 dpc in mouse embryos. The 4C32 transcripts were also detected in the developing and matured tongue tissues and in the tail scale, but not at any stage in hair follicles.

A mouse model for the basal transcription/DNA repair syndrome trichothiodystrophy

The sun-sensitive form of the severe neurodevelopmental, brittle hair disorder trichothiodystrophy (TTD) is caused by point mutations in the essential XPB and XPD helicase subunits of the dual functional DNA repair/basal transcription factor TFIIH. The phenotype is hypothesized to be in part derived from a nucleotide excision repair defect and in part from a subtle basal transcription deficiency accounting for the nonrepair TTD features. Using a novel gene-targeting strategy, we have mimicked the causative XPD point mutation of a TTD patient in the mouse. TTD mice reflect to a remarkable extent the human disorder, including brittle hair, developmental abnormalities, reduced life span, UV sensitivity, and skin abnormalities. The cutaneous symptoms are associated with reduced transcription of a skin-specific gene strongly supporting the concept of TTD as a human disease due to inborn defects in basal transcription and DNA repair.

Activation of papillomavirus late gene transcription and genome amplification upon differentiation in semisolid medium is coincident with expression of involucrin and transglutaminase but not keratin-10

The life cycle of the papillomaviruses is closely linked to host cell differentiation, as demonstrated by the fact that amplification of viral DNA and transcription of late genes occur only in the suprabasal cells of a differentiated epithelium. Previous studies examining the pathogenesis of papillomavirus infections have relied on the use of organotypic raft cultures or lesions from patients to examine these differentiation-dependent viral activities. In this study, we used a simple system for epithelial differentiation to study human papillomavirus (HPV) late functions. We demonstrate that the suspension of HPV-infected keratinocytes in semisolid medium containing 1.6% methylcellulose for 24 h was sufficient for the activation of the late promoter, transcription of late genes, and amplification of viral DNA. These activities were shown to be linked to and coincide with cellular differentiation. Expression of the late protein E1(wedge)E4 and amplification of viral DNA were detected in the identical set of cells after suspension in methylcellulose. This technique was also used to analyze the differentiation properties of the cells which expressed the late protein E1(wedge)E4. While induction of the spinous layer markers involucrin and transglutaminase was compatible with late promoter induction, expression of the differentiation-specific keratin-10 was shown not to be required for HPV late functions. Interestingly, while the majority of normal human keratinocytes induced filaggrin expression by 24 h, this marker of the granular layer was induced in a smaller subset of HPV type 31 (HPV-31)-positive cells at this time point. The HPV-31-positive cells which expressed filaggrin did not induce the late protein E1(wedge)E4. Use of the methylcellulose system to induce epithelial differentiation coupled with the ability to perform a genetic analysis of HPV functions by using transfection of cloned viral DNA will facilitate the study of the regulation of the papillomavirus life cycle.

Analysis of sequences controlling tissue-specific and hyperproliferation-related keratin 6 gene expression in transgenic mice

Keratin 6 (K6) is an intermediate filament protein found in hair follicles and in several internal stratified epithelia. This keratin has been the focus of special attention because it is also strongly induced in epidermal interfollicular keratinocytes in hyperproliferative situations and in certain conditions leading to abnormal differentiation. To localize and identify the sequences controlling this complex expression pattern, and because of their potential use in transgenic mouse models and gene therapy strategies for epidermal hyperproliferative disorders, we have thoroughly analyzed a 9 kbp region of this gene previously shown to direct proper tissue-specific and inducible expression in transgenic mice. To reproduce the K6 constitutive expression pattern, cooperation is necessary between elements located in at least two different regions, one distal between -9 and -4 kbp and one proximal between -830 and -125 bp with respect to the CAP site. The ability to induce expression under hyperproliferative conditions resides in the 2.4 kbp fragment preceding the transcription start site. When this DNA fragment was analyzed in more detail, we found that all subfragments tested contained regulatory elements necessary for inducible expression. Thus, a complex organization of K6 regulatory elements emerges, as both the constitutive and the inducible expressions of this gene are under the control of multiple elements dispersed throughout relatively large 5' flanking DNA fragments. These findings will allow the expression of cloned genes in transgenic mouse skin in response to pathological or applied hyperproliferative stimuli, avoiding the effects of their constitutive expression in other epithelia.

Hoxc13 mutant mice lack external hair

Hox genes are usually expressed temporally and spatially in a colinear manner with respect to their positions in the Hox complex. Consistent with the expected pattern for a paralogous group 13 member, early embryonic Hoxc13 expression is found in the nails and tail. Hoxc13 is also expressed in vibrissae, in the filiform papillae of the tongue, and in hair follicles throughout the body; a pattern that apparently violates spatial colinearity. Mice carrying mutant alleles of Hoxc13 have been generated by gene targeting. Homozygotes have defects in every region in which gene expression is seen. The most striking defect is brittle hair resulting in alopecia (hairless mice). One explanation for this novel role is that Hoxc13 has been recruited for a function common to hair, nail, and filiform papilla development.

Organization of cell junctions and cytoskeleton in the reticular lamina in normal and ototoxically damaged organ of Corti

The reticular lamina creates an ion barrier, withstands mechanical stress in the organ of Corti and is able to maintain its integrity during and after severe hair cell loss. Tight junctions maintain the ionic gradient whereas adherens junctions and the cytoskeleton are responsible for the integrity and mechanical resistance of tissues. In this study we used immunofluorescence and electron microscopy to examine the distribution of proteins of tight junctions (cingulin), adherens junctions (E-cadherin, alpha- and beta-catenin) and the cytoskeleton (actin, cytokeratin and tubulin) in whole-mounts of the normal and ototoxically damaged organ of Corti. In normal ears the proteins of adherens junctions were found in all cell types of the reticular lamina. We now demonstrate that all cells forming the reticular lamina partially overlap each other organizing extensive cell contacts with a complex three-dimensional shape. During scar formation, the tight junctions as well as adherens junctions between hair and supporting cells appeared in two distinct focal planes, which could help to preserve the ionic barrier and tissue integrity during hair cell degeneration. During scar formation all cytoskeletal structures in the reticular lamina were reorganized in a specific spatio-temporal pattern. We present a three-dimensional model of cell contact organization in the reticular lamina of normal ears and during scar formation.

Integrators of epidermal growth and differentiation: distinct functions for beta 1 and beta 4 integrins

Mammalian epithelia are critically dependent on interactions with components in the underlying basal lamina for proper morphogenesis and function. Substratum attachment is essential for survival, proliferation, movement, and differentiation; detachment compromises the cell's ability to perform these functions, often resulting in human disease. Interactions with the extracellular matrix are mediated through transmembrane integrin receptors that transmit signals to the cytoskeleton and to signaling molecules within the proliferating cells of the epithelium. In the past year, novel insights have emerged regarding the specific role of integrins in their attachment to extracellular matrix and in their signal transduction pathways within the epidermis.

Delays in malignant tumor development in transgenic mice by forced epidermal keratin 10 expression in mouse skin carcinomas

The keratin cytoskeleton is formed in different epidermal compartments by distinct polypeptides. Basal, proliferative keratinocytes express keratin (K) 5 and K14, whereas, suprabasal, post-mitotic keratinocytes express K1 and K10. Changes in this keratin pattern have been found to occur in hyperproliferative skin disorders and, in particular, throughout mouse epidermal carcinogenesis. Whereas some keratins not found in normal epidermis (K6, K16, K13, and K8) are induced at different stages of tumor development, K1 and K10 expression is lost. To determine whether K1 and K10 loss is just a consequence of the altered differentiation program or an event required for tumor progression, we generated transgenic mice carrying the human keratin 10 gene (hK10) under the control of a bovine keratin 6 gene regulatory region, which is silent in normal skin but is induced and drives transgene expression in hyperproliferative skin keratinocytes and, therefore, in skin tumors. Transgenic animals subjected to a complete carcinogenesis protocol developed tumors that contained various amounts of transgenic hK10. Although no significant difference was found in tumor number or malignancy, tumor onset was significantly delayed in transgenic mice, indicating that the presence of K10 actually impairs tumor development.

Characterization of Skn-1a/i POU domain factors and linkage to papillomavirus gene expression

Tissue-restricted POU domain transcription factors, which bind octamer or octamer-like gene sequences, play roles in cellular differentiation and the development of several organs. We have previously identified a POU domain gene, Skn-1a/i, expressed primarily in epidermis, that encodes at least two products through alternative splicing. One of these, Skn-1a, acts as a transcriptional activator, and the other, Skn-1i, contains an inhibitory domain in the NH2 terminus, which prevents DNA-binding in vitro and transcriptional activation in vivo. We now demonstrate that when Skn-1i is expressed in eukaryotic cells it can bind to an octamer site, suggesting that in vivo cellular factors modulate the activity of the inhibitory domain to permit DNA-binding. Yet the inhibitory domain does not allow transactivation by Skn-1i or by a heterologous transactivator containing this domain in cis. Furthermore, we demonstrate that Skn-1a, Tst-1, and Oct-1 are the major octamer-binding proteins in epidermis. Since Skn-1a is primarily expressed in suprabasal cells of the epidermis, we have tested its possible role in the regulation of epidermal papillomaviruses. In transient transfection assays, Skn-1a and Tst-1 can activate the long control region of the epidermis-specific human papillomavirus 1A (HPV-1A). Consistent with these in vivo transcription data, in vitro DNA binding studies identify three octamer-like sites, which are capable of binding Skn-1a, in the HPV-1A long control region. Mutations of all three octamer-like sites prevent transactivation by Skn-1a in transient transfection assays. Taken together, these results provide evidence that Skn-1a and Tst-1 may provide a molecular link between HPV gene expression and epidermal differentiation.

Cloning and expression analysis of mouse Cclp1, a new gene encoding a coiled-coil-like protein

Here we describe the nucleotide sequence and expression pattern of a novel gene termed Coiled-coil-like protein 1 (Cclp1). A 2646bp open reading frame encodes a 882 amino acid protein with a predicted coiled-coil domain at the amino terminus. Cclp1 is expressed in a variety of adult tissues and during different stages of embryogenesis. The broad expression pattern suggests a general cellular function of CCLP1.

Keith R. Porter Lecture, 1996. Of mice and men: genetic disorders of the cytoskeleton

Since the time when I was a postdoctoral fellow under the supervision of Dr. Howard Green, then at the Massachusetts Institute of Technology, I have been interested in understanding the molecular mechanisms underlying growth, differentiation, and development in the mammalian ectoderm. The ectoderm gives rise to epidermal keratinocytes and to neurons, which are the only two cell types of the body that devote most of their protein-synthesizing machinery to developing an elaborate cytoskeletal architecture composed of 10-nm intermediate filaments (IFs). Our interest is in understanding the architecture of the cytoskeleton in keratinocytes and in neurons, and in elucidating how perturbations in this architecture can lead to degenerative diseases of the skin and the nervous system. I will concentrate on the intermediate filament network of the skin and its associated genetic disorders, since this has been a long-standing interest of my laboratory at the University of Chicago.

The distribution of the desmosomal protein, plakophilin 1, in human skin and skin tumors

Desmosomes are predominant among the types of plaque-bearing adhering junctions found in human skin. These structures contain a set of desmosomal cadherins and cytoplasmic plaque proteins, the synthesis of which is differentiation dependent. As plakophilin 1, a member of the armadillo gene family, is an important accessory desmosomal plaque protein, we raised several monoclonal antibodies specific for this protein and applied immunohistochemical and immunoblotting procedures to study the distribution of plakophilin 1 in desmosomes in adult and fetal skin, psoriatic epidermis, various epithelial skin tumors, and keratinocyte sheets grown in culture. In epidermis, the spinous layers were prominently immunostained by plakophilin 1 antibodies, whereas the basal cell layer was only weakly stained and the stratum corneum was entirely unstained. The staining observed in psoriatic epidermis was somewhat heterogeneous. In hair follicles, the outer root sheath (ORS) was delineated in its suprabasal cell layers, with variable staining in its upper and lower parts. All basal cells of the ORS remained unstained, as did upper inner root sheath (IRS) and matrix cells of lower bulb. In eccrine sweat glands, the reaction was confined to inner dermal ductal cells, with the acini remaining unstained. The desmosomal immunostaining observed in basal cell carcinomas (BCCs) and squamous cell carcinomas (SCCs) was very heterogeneous: In general, junctions in well-differentiated stratified tumor regions were more intensely stained than sections of poorly differentiated and invasively growing BCCs and SCCs. Plakophilin 1 was also prominent in the desmosomes of keratinocyte sheets grown in culture. The cell type-specific, i.e., differentiation-dependent, distribution of desmosomal plakophilin 1 is discussed in relation both to the stratification of the cutaneous epithelia and to tumor differentiation and growth.

Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins

Modification of Ser and Thr residues by attachment of O-linked N-acetylglucos-amine [Ser(Thr)-O-GlcNAcylation] to eukaryotic nuclear and cytosolic proteins is as dynamic and possibly as abundant as Ser(Thr) phosphorylation. Known O-GlcNAcylated proteins include cytoskeletal proteins and their regulatory proteins; viral proteins; nuclear-pore, heat-shock, tumor-suppressor, and nuclearoncogene proteins; RNA polymerase II catalytic subunit; and a multitude of transcription factors. Although functionally diverse, all of these proteins are also phosphoproteins. Most O-GlcNAcylated proteins form highly regulated multimeric associations that are dependent upon their posttranslational modifications. Evidence is mounting that O-GlcNAcylation is an important regulatory modification that may have a reciprocal relationship with O-phosphorylation and may modulate many biological processes in eukaryotes.

Sequence variants in human neurofilament proteins: absence of linkage to familial amyotrophic lateral sclerosis

Neurofilaments, assembled from NF-L (68 kd), NF-M (95 kd), and NF-H (115 kd), are the most abundant structural components in large myelinated axons, particularly those of motor neurons. Aberrant neurofilament accumulation in cell bodies and axons of motor neurons is a prominent pathological feature of several motor neuron diseases, including sporadic and familial amyotrophic lateral sclerosis (ALS). Transgenic methods have proved in mice that mutation in or increased expression of neurofilament subunits can be primary causes of motor neuron disease that mimics the neurofilamentous pathology often reported in human disease. To examine whether mutation in neurofilament subunits causes or predisposes to ALS, we used single-strand conformation polymorphism coupled with DNA sequencing to search for mutations in the entirety of the human NF-L, NF-M, and NF-H genes from 100 familial ALS patients known not to carry mutations in superoxide dismutase 1 (SOD1), as well as from 75 sporadic ALS patients. Six polypeptide sequence variants were identified in rod and tail domains of NF-L, NF-M, or NF-H. However, all were found at comparable frequency in DNAs from normal individuals and no variant cosegregated with familial disease. Two deletions found previously in NF-H genes of sporadic ALS patients were not seen in this group of familial or sporadic ALS patients.