Isolation, sequence, and expression of a human keratin K5 gene: transcriptional regulation of keratins and insights into pairwise control

Biodegradable Gelatin Microcarriers Facilitate Re-Epithelialization of Human Cutaneous Wounds - An In Vitro Study in Human Skin

The possibility to use a suspended tridimensional matrix as scaffolding for re-epithelialization of in vitro cutaneous wounds was investigated with the aid of a human in vitro wound healing model based on viable full thickness skin. Macroporous gelatin microcarriers, CultiSpher-S, were applied to in vitro wounds and cultured for 21 days. Tissue sections showed incorporation of wound edge keratinocytes into the microcarriers and thicker neoepidermis in wounds treated with microcarriers. Thickness of the neoepidermis was measured digitally, using immunohistochemical staining of keratins as epithelial demarcation. Air-lifting of wounds enhanced stratification in control wounds as well as wounds with CultiSpher-S. Immunohistochemical staining revealed expression of keratin 5, keratin 10, and laminin 5 in the neoepidermal component. We conclude that the CultiSpher-S microcarriers can function as tissue guiding scaffold for re-epithelialization of cutaneous wounds.

Keratin 5 expression in squamocellular carcinoma of the head and neck

Keratin 5 (K5) is present in the basal layer of a stratified squamous keratinized and non-keratinized epithelium. K5 and K14 have been demonstrated in the mucosa and tumors of the oral cavity, oropharynx, hypopharynx and larynx, and in the mitotic active basal cells of a stratified squamous epithelium. The aim of the present study was to assess K5 expression in squamocellular carcinoma with various localizations in the head and neck. A total of 13 biopsy fragments were included from patients diagnosed with squamocellular carcinoma of the larynx area (n=2), pharynx (n=2), hard palate (n=1), tongue (n=2), submandibular (n=1), lip (n=1), gingival sulcus (n=1), nasal pyramid (n=1), maxilla (n=1) and zygomatic (n=1). The immunohistochemical staining for K5 was evaluated according to the following score criteria: 0 (0% positive cells); 1 (<10% positive cells); 2 (10-30% positive cells); and 3 (>30% positive cells). K5 expression was observed in all squamocellular carcinomas included in the present study with scores between 1 and 3. For well- and moderately-differentiated histopathological types, a maximum score of 3 was recorded for all of the cases, not including the laryngeal area, which presented a score of 2. The following scores were identified in the regions of the poorly differentiated carcinomas: Jaw, 3; gingival sulcus, 2; and tongue and submandibular area, 1. These observations may aid with an improved stratification of head and neck squamocellular carcinoma, thus improving the diagnosis and treatment strategies for this type of cancer.

Cell surface markers CD44 and CD166 localized specific populations of salivary acinar cells

OBJECTIVE

Experimental approaches tested to date for functional restoration of salivary glands (SGs) are tissue engineering, gene transfer, and cell therapy. To further develop these therapies, identifying specific cell surface markers for the isolation of salivary acinar cells is needed. To test a panel of cell surface markers [used in the isolation of mesenchymal stem cells, (MSCs)] for the localization of salivary acinar cells.

MATERIALS

Human submandibular and parotid glands were immunostained with a panel of MSC markers and co-localized with salivary acinar cell differentiation markers [α-amylase, Na-K-2Cl cotransporter-1, aquaporin-5 (AQP5)]. Additional cell markers were also used, such as α-smooth muscle actin (to identify myoepithelial cells), cytokeratin-5 (basal ductal cells), and c-Kit (progenitor cells).

RESULTS

CD44 identified serous acini, while CD166 identified mucous acini. Cytokeratin-5 identified basal duct cells and 50% of myoepithelial cells. None of the remaining cell surface markers (Stro-1, CD90, CD106, CD105, CD146, CD19, CD45, and c-Kit) were expressed in any human salivary cell.

CONCLUSIONS

CD44 and CD166 localized human salivary serous and mucous acinar cells, respectively. These two cell surface markers will be useful in the isolation of specific populations of salivary acinar cells.

Constitutive expression of human keratin 14 gene in mouse lung induces premalignant lesions and squamous differentiation

Squamous cell carcinoma accounts for 20% of all human lung cancers and is strongly linked to cigarette smoking. It develops through premalignant changes that are characterized by high levels of keratin 14 (K14) expression in the airway epithelium and evolve through basal cell hyperplasia, squamous metaplasia and dysplasia to carcinoma in situ and invasive carcinoma. In order to explore the impact of K14 in the pulmonary epithelium that normally lacks both squamous differentiation and K14 expression, human keratin 14 gene hK14 was constitutively expressed in mouse airway progenitor cells using a mouse Clara cell specific 10 kDa protein (CC10) promoter. While the lungs of CC10-hK14 transgenic mice developed normally, we detected increased expression of K14 and the molecular markers of squamous differentiation program such as involucrin, loricrin, small proline-rich protein 1A, transglutaminase 1 and cholesterol sulfotransferase 2B1. In contrast, wild-type lungs were negative. Aging CC10-hK14 mice revealed multifocal airway cell hyperplasia, occasional squamous metaplasia and their lung tumors displayed evidence for multidirectional differentiation. We conclude that constitutive expression of hK14 initiates squamous differentiation program in the mouse lung, but fails to promote squamous maturation. Our study provides a novel model for assessing the mechanisms of premalignant lesions in vivo by modifying differentiation and proliferation of airway progenitor cells.

Intermediate filaments: a historical perspective

Intracellular protein filaments intermediate in size between actin microfilaments and microtubules are composed of a surprising variety of tissue specific proteins commonly interconnected with other filamentous systems for mechanical stability and decorated by a variety of proteins that provide specialized functions. The sequence conservation of the coiled-coil, alpha-helical structure responsible for polymerization into individual 10 nm filaments defines the classification of intermediate filament proteins into a large gene family. Individual filaments further assemble into bundles and branched cytoskeletons visible in the light microscope. However, it is the diversity of the variable terminal domains that likely contributes most to different functions. The search for the functions of intermediate filament proteins has led to discoveries of roles in diseases of the skin, heart, muscle, liver, brain, adipose tissues and even premature aging. The diversity of uses of intermediate filaments as structural elements and scaffolds for organizing the distribution of decorating molecules contrasts with other cytoskeletal elements. This review is an attempt to provide some recollection of how such a diverse field emerged and changed over about 30 years.

A small surface hydrophobic stripe in the coiled-coil domain of type I keratins mediates tetramer stability

Intermediate filaments (IFs) are fibrous polymers encoded by a large family of differentially expressed genes that provide crucial structural support in the cytoplasm and nucleus in higher eukaryotes. The mechanisms involved in bringing together ∼16 elongated coiled-coil dimers to form an IF are poorly defined. Available evidence suggests that tetramer subunits play a key role during IF assembly and regulation. Through molecular modeling and site-directed mutagenesis, we document a hitherto unnoticed hydrophobic stripe exposed at the surface of coiled-coil keratin heterodimers that contributes to the extraordinary stability of heterotetramers. The inability of K16 to form urea-stable tetramers in vitro correlates with an increase in its turnover rate in vivo. The data presented support a specific conformation for the assembly competent IF tetramer, provide a molecular basis for their differential stability in vitro, and point to the physiological relevance associated with this property in vivo.

Defining the properties of the nonhelical tail domain in type II keratin 5: insight from a bullous disease-causing mutation

Inherited mutations in the intermediate filament (IF) proteins keratin 5 (K5) or keratin 14 (K14) cause epidermolysis bullosa simplex (EBS), in which basal layer keratinocytes rupture upon trauma to the epidermis. Most mutations are missense alleles affecting amino acids located in the central alpha-helical rod domain of K5 and K14. Here, we study the properties of an unusual EBS-causing mutation in which a nucleotide deletion (1649delG) alters the last 41 amino acids and adds 35 residues to the C terminus of K5. Relative to wild type, filaments coassembled in vitro from purified K5-1649delG and K14 proteins are shorter and exhibit weak viscoelastic properties when placed under strain. Loss of the C-terminal 41 residues contributes to these alterations. When transfected in cultured epithelial cells, K5-1649delG incorporates into preexisting keratin IFs and also forms multiple small aggregates that often colocalize with hsp70 in the cytoplasm. Aggregation is purely a function of the K5-1649delG tail domain; in contrast, the cloned 109 residue-long tail domain from wild type K5 is distributed throughout the cytoplasm and colocalizes partly with keratin IFs. These data provide a mechanistic basis for the cell fragility seen in individuals bearing the K5-1649delG allele, and point to the role of the C-terminal 41 residues in determining K5's assembly properties.

In vivo and in vitro neurogenesis in human olfactory epithelium

The birth and differentiation of neurons have been extensively studied in the olfactory epithelium (OE) of rodents but not in humans. The goal of this study was to characterize cellular composition and molecular expression of human OE in vivo and in vitro. In rodent OE, there are horizontal basal cells and globose basal cells that are morphologically and functionally distinct. In human OE, however, there appears to be no morphological distinction among basal cells, with almost all cells having round cell bodies similar to rodent globose basal cells. Unlike the case in rodents, human basal cells, including putative neuronal precursors, express p75NGFR, suggesting a distinctive role for p75NGFR in human OE neurogenesis. Molecular expression of neuronal cells during differentiation in human OE grossly follows that in rodents. However, the topographical organization of immature and mature ORNs in human OE differs from that of rodents, in that immature and mature ORNs in humans are dispersed throughout the OE, whereas rodent counterparts have a highly laminar organization. These observations together suggest that the birth and differentiation of neuronal cells in human OE differ from those in rodents. In OE explant culture, neuronal cells derived from human OE biopsy express markers for immature and mature neurons, grossly recapitulating neuronal differentiation of olfactory neurons in vivo. Furthermore, small numbers of cells are doubly label for bromodeoxyuridine and olfactory marker protein, indicating that neuronal cells born in vitro reach maturity. These data highlight species-related differences in OE development and demonstrate the utility of explant culture for experimental studies of human neuronal development.

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.

Characterization of early assembly intermediates of recombinant human keratins

The intermediate filaments (IFs) form major structural elements of the cytoskeleton. In vitro analyses of these fibrous proteins reveal very different assembly properties for the nuclear and cytoplasmic IF proteins. However, keratins in particular, the largest and most heterogenous group of cytoplasmic IF proteins, have been difficult to analyze due to their rapid assembly dynamics under the near-physiological conditions used for other IF proteins. We show here that keratins, like other cytoplasmic IF proteins, go through a stage of assembling into full-width soluble complexes, i.e., "unit-length filaments" (ULFs). In contrast to other IF proteins, however, longitudinal annealing of keratin ULFs into long filaments quasi-coincides with their formation. In vitro assembly of IF proteins into filaments can be initiated by an increase of the ionic strength and/or lowering of the pH of the assembly buffer. We now document that 23-mer peptides from the head domains of various IF proteins can induce filament formation even under conditions of low salt and high pH. This suggests that the "heads" are involved in the formation and longitudinal association of the ULFs. Using a Tris-buffering protocol that causes formation of soluble oligomers at pH 9, the epidermal keratins K5/14 form less regular filaments and less efficiently than the simple epithelial keratins K8/18. In sodium phosphate buffers (pH 7.5), however, K5/14 were able to form long partially unraveled filaments which compacted into extended, regular filaments upon addition of 20 mM KCl. Applying the same assembly regimen to mutant K14 R125H demonstrated that mutations causing a severe disease phenotype and morphological filament abnormalities can form long, regular filaments with surprising efficiency in vitro.

A role for the 1A and L1 rod domain segments in head domain organization and function of intermediate filaments: structural analysis of trichocyte keratin

A dynamic model is proposed to explain how the 1A and linker L1 segments of the rod domain in intermediate filament (IF) proteins affect the head domain organization and vice versa. We have shown in oxidized trichocyte IF that the head domain sequences fold back over and interact with the rod domain. This phenomenon may occur widely in reduced IF as well. Its function may be to stabilize the 1A segments into a parallel two-stranded coiled coil or something closely similar. Under differing reversible conditions, such as altered states of IF assembly, or posttranslational modifications, such as phosphorylation etc., the head domains may no longer associate with the 1A segment. This could destabilize segment 1A and cause the two alpha-helical strands to separate. Linker L1 would thus act as a hinge and allow the heads to function over a wide lateral range. This model has been explored using the amino acid sequences of the head (N-terminal) domains of Type I and Type II trichocyte keratin intermediate filament chains. This has allowed several quasi-repeats to be identified. The secondary structure corresponding to these repeats has been predicted and a model has been produced for key elements of the Type II head domain. Extant disulfide cross-link data have been used as structural constraints. A model for the head domain structure predicts that a twisted beta-sheet region may wrap around the 1A segment and this may reversibly stabilize a coiled-coil conformation for 1A. The evidence in favor of the swinging head model for IF is discussed.

Formation of a normal epidermis supported by increased stability of keratins 5 and 14 in keratin 10 null mice

The expression of distinct keratin pairs during epidermal differentiation is assumed to fulfill specific and essential cytoskeletal functions. This is supported by a great variety of genodermatoses exhibiting tissue fragility because of keratin mutations. Here, we show that the loss of K10, the most prominent epidermal protein, allowed the formation of a normal epidermis in neonatal mice without signs of fragility or wound-healing response. However, there were profound changes in the composition of suprabasal keratin filaments. K5/14 persisted suprabasally at elevated protein levels, whereas their mRNAs remained restricted to the basal keratinocytes. This indicated a novel mechanism regulating keratin turnover. Moreover, the amount of K1 was reduced. In the absence of its natural partner we observed the formation of a minor amount of novel K1/14/15 filaments as revealed by immunogold electron microscopy. We suggest that these changes maintained epidermal integrity. Furthermore, suprabasal keratinocytes contained larger keratohyalin granules similar to our previous K10T mice. A comparison of profilaggrin processing in K10T and K10(-/-) mice revealed an accumulation of filaggrin precursors in the former but not in the latter, suggesting a requirement of intact keratin filaments for the processing. The mild phenotype of K10(-/-) mice suggests that there is a considerable redundancy in the keratin gene family.

Complete cytolysis and neonatal lethality in keratin 5 knockout mice reveal its fundamental role in skin integrity and in epidermolysis bullosa simplex

In human patients, a wide range of mutations in keratin (K) 5 or K14 lead to the blistering skin disorder epidermolysis bullosa simplex. Given that K14 deficiency does not lead to the ablation of a basal cell cytoskeleton because of a compensatory role of K15, we have investigated the requirement for the keratin cytoskeleton in basal cells by inactivating the K5 gene in mice. We report that the K5(-/-) mice die shortly after birth, lack keratin filaments in the basal epidermis, and are more severely affected than K14(-/-) mice. In contrast to the K14(-/-) mice, we detected a strong induction of the wound-healing keratin K6 in the suprabasal epidermis of cytolyzed areas of postnatal K5(-/-) mice. In addition, K5 and K14 mice differed with respect to tongue lesions. Moreover, we show that in the absence of K5 and other type II keratins, residual K14 and K15 aggregated along hemidesmosomes, demonstrating that individual keratins without a partner are stable in vivo. Our data indicate that K5 may be the natural partner of K15 and K17. We suggest that K5 null mutations may be lethal in human epidermolysis bullosa simplex patients.

Molecular confirmation of the unique phenotype of epidermolysis bullosa simplex with mottled pigmentation

BACKGROUND

A distinctive subtype of epidermolysis bullosa simplex, with the additional feature of mottled pigmentation (EBS-MP), was initially characterized in a Swedish family in 1979, and seven further families have been reported. Features of EBS-MP that are observed in most affected patients include acral blistering early in childhood, mottled pigmentation distributed in a number of sites, focal punctate hyperkeratoses of the palms and soles, and dystrophic, thickened nails. The genetic basis of EBS-MP has been ascribed in five unrelated families to a heterozygous point mutation, P25L, in the non-helical V1 domain of K5.

OBJECTIVES

We report a clinical, ultrastructural and molecular study of two of the earliest families to be clinically characterized as EBS-MP.

METHODS

The P25L mutation was identified in all affected members of each of these families, bringing the total number of EBS-MP families with this mutation to seven.

RESULTS

This unusual recurrent mutation may uniquely cause EBS-MP.

CONCLUSIONS

While the exact molecular mechanisms by which this mutation causes epidermolysis, palmoplantar keratoderma and pigmentation remain elusive, we suggest possible molecular mechanisms through which the P25L substitution could cause this unusual phenotype.

Introducing a null mutation in the mouse K6alpha and K6beta genes reveals their essential structural role in the oral mucosa

Mammalian genomes feature multiple genes encoding highly related keratin 6 (K6) isoforms. These type II keratins show a complex regulation with constitutive and inducible components in several stratified epithelia, including the oral mucosa and skin. Two functional genes, K6alpha and K6beta, exist in a head-to-tail tandem array in mouse genomes. We inactivated these two genes simultaneously via targeting and homologous recombination. K6 null mice are viable and initially indistinguishable from their littermates. Starting at two to three days after birth, they show a growth delay associated with reduced milk intake and the presence of white plaques in the posterior region of dorsal tongue and upper palate. These regions are subjected to greater mechanical stress during suckling. Morphological analyses implicate the filiform papillae as being particularly sensitive to trauma in K6alpha/K6beta null mice, and establish the complete absence of keratin filaments in their anterior compartment. All null mice die about a week after birth. These studies demonstrate an essential structural role for K6 isoforms in the oral mucosa, and implicate filiform papillae as being the major stress bearing structures in dorsal tongue epithelium.

Partial rescue of epithelial phenotype in integrin beta4 null mice by a keratin-5 promoter driven human integrin beta4 transgene

Integrin beta4 null mice exhibit extensive epidermal detachment, reminiscent of the human skin blistering disease junctional epidermolysis bullosa associated with pyloric atresia. Hemidesmosomes, the stable adhesion structures of squamous epithelia, are not formed in the absence of alpha6beta4. Null mutant mice die shortly after birth, but apart from their striking epithelial phenotype, no obvious developmental defects have been observed. To elucidate the cause of death in these mice, we generated transgenic mice with a heterologous construct consisting of the squamous epithelial-specific keratin-5 promoter and a human integrin beta4 subunit cDNA. The transgene was not expressed in the presence of endogenous beta4, probably as a result of competition for a limited pool of alpha6 subunits. In a beta4 null background, however, the transgene was expressed, and its expression pattern followed that of squamous epithelial-specific keratins. These rescued pups appeared healthy and ultrastructural analysis revealed that the interspecies heterodimer alpha6(mouse)/beta4(human) was sufficient to trigger the assembly of hemidesmosomes. After a variable period of up to 48 hours after birth these animals began to exhibit haemorrhages at the plantar and palmar areas. We observed the formation of small blisters and found that the transgene was not detectably expressed in this region, which is devoid of hair follicles. The rescued neonates became increasingly cyanotic and died soon after the onset of this phenomenon. We performed a developmental study of the expression of beta4 in the complete respiratory tract, but we found no correlation between the spatiotemporal distribution of beta4 and the onset of the respiratory insufficiency. It became clear, however, that there was a gradual detachment of squamous epithelia in the oral and nasal cavities which led to obstruction of the respiratory tract, suggesting that in beta4 null and rescued mice, neonatal death was a direct consequence of decreased adhesion properties of hairless squamous epithelia, rather than a developmental defect of the lungs.

The functional diversity of epidermal keratins revealed by the partial rescue of the keratin 14 null phenotype by keratin 16

The type I epidermal keratins K14 and K16 are remarkably similar at the primary sequence level. While a structural function has been clearly defined for K14, we have proposed that a function of K16 may be to play a role in the process of keratinocyte activation that occurs after acute injury to stratified epithelia. To compare directly the functions of the two keratins we have targeted the expression of the human K16 cDNA to the progenitor basal layer of the epidermis of K14 null mice. Mice null for K14 blister extensively and die approximately 2 d after birth (Lloyd, C., Q.C. Yu, J. Cheng, K. Turksen, L. Degenstein, E. Hutton, and E. Fuchs. 1995. J. Cell Biol. 129:1329-1344). The skin of mice expressing K16 in the absence of K14 developed normally without evidence of blistering. However, as the mice aged they featured extensive alopecia, chronic epidermal ulcers in areas of frequent physical contact, and alterations in other stratified epithelia. Mice expressing a control K16-C14 cDNA also rescue the blistering phenotype of the K14 null mice with only a small percentage exhibiting minor alopecia. While K16 is capable of rescuing the blistering, phenotypic complementation in the resulting skin is incomplete due to the multiple age dependent anomalies. Despite their high sequence similarity, K16 and K14 are not functionally equivalent in the epidermis and other stratified epithelia and it is primarily the carboxy-terminal approximately 105 amino acids of K16 that define these differences.

The type I keratin 19 possesses distinct and context-dependent assembly properties

Keratins (K), the cytoplasmic intermediate filament (IF) proteins of epithelial cells, are encoded by a multigene family and expressed in a tissue- and differentiation-specific manner. In human skin, keratinocytes of the basal layer of epidermis and the outer root sheath of hair follicles express K5 and K14 as their main keratins. A small subpopulation of basal cells exhibiting stem-cell like characteristics express, in addition, K19. At 40 kDa, this keratin is the smallest IF protein due to an exceptionally short carboxyl-terminal domain. We examined the assembly properties of K19 and contrasted them to K14 in vitro and in vivo. Relative to K5-K14, we find that K5-K19 form less stable tetramers that polymerize into shorter and narrower IFs in vitro. When transiently co-expressed in cultured baby hamster kidney cells, the K5 and K19 combination fails to form a filamentous array, whereas the K5-K14 and K8-K19 ones readily do so. Transient expression of K19 in the epithelial cell lines T51B-Ni and A431 results in its integration into the endogenous keratin network with minimal if any perturbation. Collectively, these results indicate that K19 possesses assembly properties that are distinct from those of K14 and suggest that it may impart unique properties to the basal cells expressing it in skin epithelia.

Functional differences between keratins of stratified and simple epithelia

Dividing populations of stratified and simple epithelial tissues express keratins 5 and 14, and keratins 8 and 18, respectively. It has been suggested that these keratins form a mechanical framework important to cellular integrity, since their absence gives rise to a blistering skin disorder in neonatal epidermis, and hemorrhaging within the embryonic liver. An unresolved fundamental issue is whether different keratins perform unique functions in epithelia. We now address this question using transgenic technology to express a K16-14 hybrid epidermal keratin transgene and a K18 simple epithelial keratin transgene in the epidermis of mice null for K14. Under conditions where the hybrid epidermal keratin restored a wild-type phenotype to newborn epidermis, K18 partially but not fully rescued. The explanation does not appear to reside in an inability of K18 to form 10-nm filaments with K5, which it does in vitro and in vivo. Rather, it appears that the keratin network formed between K5 and K18 is deficient in withstanding mechanical stress, leading to perturbations in the keratin network in regions of the skin that are subjected either to natural or to mechanically induced trauma. Taken together, these findings suggest that the loss of a type I epidermal keratin cannot be fully compensated by its counterpart of simple epithelial cells, and that in vivo, all keratins are not equivalent.

Mouse keratin 4 is necessary for internal epithelial integrity

Keratins are intermediate filaments of epithelial cells. Mutations in keratin genes expressed in skin lead to human disorders, including epidermolysis bullosa simplex and epidermolytic hyperkeratosis. We examined the role of keratin 4 (K4) in maintaining the integrity of internal epithelial linings by using gene targeting to generate mice containing a null mutation in the epithelial K4 gene. Homozygous mice that do not express K4 develop a spectrum of phenotypes that affect several organs which express K4 including the esophagus, tongue, and cornea. The cellular phenotypes include basal hyperplasia, lack of maturation, hyperkeratosis, atypical nuclei, perinuclear clearing, and cell degeneration. These results are consistent with the notion that K4 is required for internal epithelial cell integrity. As mutations in K4 in humans lead to a disorder called white sponge nevus, the K4-deficient mice may serve as models for white sponge nevus and for understanding the role of K4 in cellular proliferation and differentiation.

Directed expression of keratin 16 to the progenitor basal cells of transgenic mouse skin delays skin maturation

We previously hypothesized that the type I keratin 16 (K16) plays a role in the process of keratinocyte activation that occurs in response to skin injury (Paladini, R.D., K. Takahashi, N.S. Bravo, and P.A. Coulombe. 1996. J. Cell Biol. 132:381-397). To further examine its properties in vivo, the human K16 cDNA was constitutively expressed in the progenitor basal layer of transgenic mouse skin using the K14 gene promoter. Mice that express approximately as much K16 protein as endogenous K14 display a dramatic postnatal phenotype that consists of skin that is hyperkeratotic, scaly, and essentially devoid of fur. Histologically, the epidermis is thickened because of hyperproliferation of transgenic basal cells, whereas the hair follicles are decreased in number, poorly developed, and hypoproliferative. Microscopically, the transgenic keratinocytes are hypertrophic and feature an altered keratin filament network and decreased cell-cell adhesion. The phenotype normalizes at approximately 5 wk after birth. In contrast, control mice expressing a K16-K14 chimeric protein to comparable levels are normal. The character and temporal evolution of the phenotype in the K16 transgenic mice are reminiscent of the activated EGF receptor- mediated signaling pathway in skin. In fact, tyrosine phosphorylation of the EGF receptor is increased in the newborn skin of K16 transgenic mice. We conclude that expression of K16 can significantly alter the response of skin keratinocytes to signaling cues, a distinctive property likely resulting from its unique COOH-terminal tail domain.

Defining the interactions between intermediate filaments and desmosomes

Desmoplakin (DP), plakoglobin (PG), and plakophilin 1 (PP1) are desmosomal components lacking a transmembrane domain, thus making them candidate linker proteins for connecting intermediate filaments and desmosomes. Using deletion and site-directed mutagenesis, we show that remarkably, removal of approximately 1% of DP's sequence obliterates its ability to associate with desmosomes. Conversely, when linked to a foreign protein, as few as 86 NH2-terminal DP residues are sufficient to target to desmosomes efficiently. In in vitro overlay assays, the DP head specifically associates with itself and with desmocollin 1a (Dsc1a). In similar overlay assays, PP1 binds to DP and Dsc1a, and to a lesser extent, desmoglein 1 (Dsg1), while PG binds to Dsg1 and more weakly to Dsc1a and DP. Interestingly, like DP, PG and PP1 associate with epidermal keratins, although PG is considerably weaker in its ability to do so. As judged by overlay assays, the amino terminal head domain of type II keratins appears to have a special importance in establishing these connections. Taken together, our findings provide new insights into the complexities of the links between desmosomes and intermediate filaments (IFs). Our results suggest a model whereby at desmosome sites within dividing epidermal cells, DP and PG anchor to desmosomal cadherins and to each other, forming an ordered array of nontransmembrane proteins that then bind to keratin IFs. As epidermal cells differentiate, PP1 is added as a molecular reinforcement to the plaque, enhancing anchorage to IFs and accounting at least partially for the increase in numbers and stability of desmosomes in suprabasal cells.

A proline residue in the alpha-helical rod domain of type I keratin 16 destabilizes keratin heterotetramers

The type I keratins 14 (K14) and 16 (K16) are distinct in their assembly properties and their expression pattern despite a high degree of sequence identity. Understanding K16 function and regulation is of interest, given its strong induction in keratinocytes located at the wound edge after injury to stratified epithelia. We reported previously that, compared with K14, K16 forms unstable heterotetramers with either K5 or K6 as the type II keratin pairing partner (Paladini, R. D., Takahashi, K., Bravo, N. S., and Coulombe, P. A. (1996) J. Cell Biol. 132, 381-397). We show here that yet another related type I keratin, K17, forms stable heterotetramers with a variety of type II keratins, further accentuating the unique nature of K16. Analysis of chimeric K14-K16 proteins in a heterotetramer formation assay indicated that the instability determinant resides in a 220-amino acid segment within the alpha-helical rod domain of K16. Site-directed mutagenesis revealed that Pro188, an amino acid residue located in subdomain 1B of the rod, accounts quantitatively for the instability of K16-containing heterotetramers under denaturing conditions. In vitro polymerization studies suggest that the presence of Pro188 correlates with a reduction in assembly efficiency. In addition to their implications for the stable conformation of the keratin heterotetramers, these findings suggest that the tetramer-forming properties of K16 may influence its partitioning between the soluble and polymer pools, and hence contribute to its regulation in epithelial cells under resting and wound repair conditions.

Stress, apoptosis, and mitosis induce phosphorylation of human keratin 8 at Ser-73 in tissues and cultured cells

Simple epithelia express keratins 8 (K8) and 18 (K18) as their major intermediate filament proteins. We previously showed that several types of cell stress such as heat and virus infection result in a distinct hyperphosphorylated form of K8 (termed HK8). To better characterize K8/18 phosphorylation, we generated monoclonal antibodies by immunizing mice with hyperphosphorylated keratins that were purified from colonic cultured human HT29 cells pretreated with okadaic acid. One antibody specifically recognized HK8, and the epitope was identified as 71LLpSPL which corresponds to K8 phosphorylation at Ser-73. Generation of HK8 occurs in mitotic HT29 cells, basal crypt mitotic cells in normal mouse intestine, and in regenerating mouse hepatocytes after partial hepatectomy. Prominent levels of HK8 were also generated in HT29 cells that were induced to undergo apoptosis using anisomycin or etoposide. In addition, mouse hepatotoxicity that is induced by chronic feeding with griseofulvin resulted in HK8 formation in the liver. Our results demonstrate that a "reverse immunological" approach, coupled with enhancing in vivo phosphorylation using phosphatase inhibitors, can result in the identification of physiologic phosphorylation states. As such, K8 Ser-73 phosphorylation generates a distinct HK8 species under a variety of in vivo conditions including mitosis, apoptosis, and cell stress. The low steady state levels of HK8 during mitosis, in contrast to stress and apoptosis, suggest that accumulation of HK8 may represent a physiologic stress marker for simple epithelia.

Defining a region of the human keratin 6a gene that confers inducible expression in stratified epithelia of transgenic mice

Injury to the epidermis and other stratified epithelia triggers a repair response involving the rapid induction of several genes, including keratin 6 (K6). The signaling pathways and mechanisms presiding over this induction in keratinocytes at the wound edge remain to be defined. We reported previously that of the multiple genes encoding K6 isoforms in human, K6a is dominant in skin epithelia (Takahashi, K., Paladini, R., Coulombe, P. A. (1995) J. Biol. Chem. 270, 18581-18592). Using bacterial LacZ as a reporter gene in transgenic mice, we show that the proximal 5.2 kilobases of 5'-upstream sequence from the K6a gene fails to direct sustained expression in any adult tissue, including those where K6 is constitutively expressed (e.g. hair follicle, nail, oral mucosa, tongue, esophagus, forestomach). In contrast, the proximal 960 base pairs of 5'-upstream sequence suffice to mediate an induction of beta-galactosidase expression in a near-correct spatial and temporal fashion after injury to epidermis and other stratified epithelia. Transgene expression also occurs following topical application of phorbol esters, all-trans-retinoic acid, or 2-4-dinitro-1-fluorobenzene, all known to induce K6 expression in skin. Our data show that critical regulatory sequences for this inducibility are located between -960 and -550 bp in the 5'-upstream sequence of K6a and that their activity is influenced by enhancer element(s) located between -2500 and -5200 base pairs. These findings have important implications for the control of gene expression after injury to stratified epithelia.

Phosphorylation of human keratin 8 in vivo at conserved head domain serine 23 and at epidermal growth factor-stimulated tail domain serine 431

Dynamic phosphorylation is one mechanism that regulates the more than 20 keratin type I and II intermediate filament proteins in epithelial cells. The major type II keratin in "simple type" glandular epithelia is keratin 8 (K8). We used biochemical and mutational approaches to localize two major in vivo phosphorylation sites of human K8 to the head (Ser-23) and tail (Ser-431) domains. Since Ser-23 of K8 is highly conserved among all type II keratins, we also examined if the corresponding Ser-59 in stratified epithelial keratin 6e is phosphorylated. Mutation of K6e Ser-59 abolished its phosphorylation in 32PO4-labeled baby hamster kidney cell transfectants. With regard to K8 phosphorylation at Ser-431, it increases dramatically upon stimulation of cells with epidermal growth factor (EGF) or after mitotic arrest and is the major K8 phosphorylated residue after incubating K8 immunoprecipitates with mitogen-activated protein or cdc2 kinases. A monoclonal antibody that specifically recognizes phosphoserine 431-K8 manifests increased reactivity with K8 and recognizes reorganized K8/18 filaments after EGF stimulation. Our results suggest that in vivo serine phosphorylation of K8 and K6e within the conserved head domain motif is likely to reflect a conserved phosphorylation site of most if not all type II keratins. Furthermore, K8 Ser-431 phosphorylation occurs after EGF stimulation and during mitotic arrest and is likely to be mediated by mitogen-activated protein and cdc2 kinases, respectively.

Activator protein 1 activity is involved in the regulation of the cell type-specific expression from the proximal promoter of the human profilaggrin gene

The human profilaggrin gene is expressed in the granular layer during the late stages of terminal differentiation of the epidermis. In in vitro transcription experiments we show that the abundance of the mRNA and the specificity of the expression are regulated primarily at the level of transcription. We found that the 5'-flanking sequences control the transcription in a keratinocyte-specific mode and that as little as 116 base pairs preceding the mRNA initiation site is sufficient to restrict the transcription to epidermal cells in vitro. This specificity depends critically on the presence of an activator protein 1 (AP1) motif at position -77. Binding of c-jun/c-fos heterodimers to this sequence confers high levels of expression to the reporter constructs in cultured epidermal keratinocytes, while having little effect in HeLa cells. The transactivating properties of c-jun are essential in this process. On the other hand, junB and junD, which are involved in transactivating the transcription of earlier epidermal differentiation markers, control profilaggrin expression through a pathway which does not depend on a direct binding at the AP1 site and is not cell-type specific. These data indicate that AP1 factors are involved in a complex, multipathway regulation of the profilaggrin gene expression.

An essential cytoskeletal linker protein connecting actin microfilaments to intermediate filaments

Typified by rapid degeneration of sensory neurons, dystonia musculorum mice have a defective BPAG1 gene, known to be expressed in epidermis. We report a neuronal splice form, BPAG1n, which localizes to sensory axons. Both isoforms have a coiled-coil rod, followed by a carboxy domain that associates with intermediate filaments. However, the amino terminus of BPAG1n differs from BPAG1e in that it contains a functional actin-binding domain. In transfected cells, BPAG1n coaligns neurofilaments and microfilaments, establishing this as a cytoskeletal protein interconnecting actin and intermediate filament cytoskeletons. In BPAG1 null mice, axonal architecture is markedly perturbed, consistent with a failure to tether neurofilaments to the actin cytoskeleton and underscoring the physiological relevance of this protein.

Genetic analysis of a severe case of Dowling-Meara epidermolysis bullosa simplex

The epidermis serves an important protective function, which it manifests by producing an extensive cytoskeletal architecture, the unique feature of which are keratin filaments. Through studies that began with epidermolysis bullosa simplex (EBS) and now extend to a group of autosomal dominant human blistering skin disorders it was discovered that defects in the keratin genes lead to cell fragility and degeneration upon mechanical trauma. In most cases of EBS, point mutations occur in the keratin 5 (K5) and K14 genes expressed in the basal layer of the epidermis. The precise location of the mutation and the degree to which it causes perturbations in filament assembly correlate with disease severity. In the present study, we examine a case of EBS, which clinically lies at the severe end of the spectrum of Dowling-Meara EBS and which shows keratin filament clumping in suprabasal as well as basal cells. We show that one of the two K14 alleles has a single point substitution, giving rise to a Y129D mutation. This mutation resides 4 residues internal to the R125C/H hotspot known to account for the majority of Dowling-Meara cases. We provide functional and structural evidence to suggest why the Y129D mutation may be capable of creating such a severe form of EBS.

Overexpression of human keratin 16 produces a distinct skin phenotype in transgenic mouse skin

Human cytokeratin 16 (K16; 48 kDa) is constitutively expressed in postmitotic keratinocytes in a variety of stratified epithelial tissues, but it is best known for the marked enhancement of its expression in stratified squamous epithelia showing hyperproliferation or abnormal differentiation. Of particular interest to us, K16 is strongly induced at the wound edge after injury to the epidermis, and its accumulation correlates spatially and temporally with the onset of reepithelialization. To examine the properties of K16 in its natural cellular context, we introduced a wild-type human K16 gene into the germ line of transgenic mice. Several transgenic lines were established and characterized. Under most conditions, the human K16 transgene is regulated tissue specifically in the skin of transgenic mice. Animals that feature low levels of transgene expression are indistinguishable from controls during the first 6-8 months of life. In contrast, transgenic animals expressing the transgene at higher levels develop skin lesions at 1 week after birth, coinciding with the emergence of fur. At a cellular level, alterations begin with the reorganization of keratin filaments and are first seen at the level of the hair follicle outer root sheath (ORS), where K16 expression is known to occur constitutively. The lesions then progressively spread to involve the proximal epidermis, with which the ORS is contiguous. Elevated transgene expression is associated with a marked thickening of these two epithelia, along with altered keratinocyte cytoarchitecture and aberrant keratinization but no keratinocyte lysis. The implications of this phenotype for epithelial differentiation, human genodermatoses, and wound healing in skin are discussed.

Cloning and characterization of multiple human genes and cDNAs encoding highly related type II keratin 6 isoforms

The human type II keratin 6 (K6; 56 kDa) is expressed in a heterogeneous array of epithelial tissues under normal conditions, but is better known for its strong induction in stratified epithelia that feature an enhanced cell proliferation rate or abnormal differentiation. Previous work has established the existence of two functional genes encoding K6 protein isoforms in the human genome, although only a partial cDNA clone is available for K6a, the dominant human K6 isoform in skin epithelial tissues (Tyner, A., and Fuchs, E. (1986) J. Cell Biol. 103, 1945-1955). We screened human genomic and skin cDNA libraries with probes derived from the K6b gene, and isolated clones containing the full-length gene and cDNA predicted to encode K6a. A thorough characterization of a large number of genomic (57) as well as cDNA (64) clones further revealed the existence of as many as six different human K6 protein isoforms that are highly related at the gene structure, nucleotide sequence, and predicted amino acid sequence levels. Based on the information accumulated to date we propose an evolutionary model in which the multiplicity of human K6 genes is explained by successive gene duplication events. We further demonstrate that K6a is clearly the dominant K6 isoform in skin tissue samples and cultured epithelial cell lines and that the various isoforms are differentially regulated within and between epithelial tissue types. Our findings have direct implications for an understanding of the regulation and function of K6 during hyperproliferation in stratified epithelia and the search for disease-causing mutations in K6 sequences in the human population.

The A/B domain of truncated retinoic acid receptors can block differentiation and promote features of malignancy

Recently, we discovered that stable introduction of a carboxyl-terminally truncated retinoic acid receptor gamma (tRAR gamma) into an epidermal keratinocyte line blocked the ability of these cells to differentiate, as judged by their failure to express late markers of squamous differentiation. We now demonstrate a correlation between the level of residual endogenous RAR activity of tRAR gamma-expressing keratinocyte lines and degree of terminal differentiation. Mutagenesis studies localize the effects to the A/B subdomain of the truncated receptor. Despite tRAR gamma's capacity to interfere with RAR-mediated transactivation of retinoic acid response elements (RAREs) in keratinocytes, the effects of the truncated receptor are independent of its ability to bind DNA and directly interact with endogenous RARs. tRAR alpha also inhibits RARE-mediated gene expression in keratinocytes, even though its full-length counterpart enhances RARE activity in these cells. Intriguingly, both tRAR gamma and RAR gamma suppress keratin promoter activity in epidermal cells, although for tRAR gamma, the effect is mediated through the A/B domain whereas for RAR gamma, the effects require DNA binding. Taken together, these findings suggest that the truncation allows for new and aberrant interactions with transcriptional proteins/cofactors that participate in governing RARE activity. This discovery may have relevance in tumorigenesis, where genetic lesions can result in mutant RARs or in loss of receptor expression.

Expression of cytokeratin mRNAs in normal human esophageal epithelium

BACKGROUND

The cytokeratin (CK) pattern is accepted to be characteristic of a given epithelial cell or tissue. Specific changes in the CK pattern or in the expression of individual CKs may be characteristic in the early development of particular epithelial pathologies. Up to now no systematic hybridohistochemical study on the expression of CKs in normal human esophageal epithelium has been performed. Nevertheless, this knowledge may be of great importance for further research concerning the understanding of the structure and differentiation of normal esophageal epithelium and of the development of non-neoplastic and neoplastic esophageal malignancies. Therefore, we investigated the expression and distribution of nine different CK mRNAs throughout the normal human esophageal mucosa.

METHODS

A non-radioactive in situ hydridization protocol was used to study the expression of CK mRNAs in fixed and paraffin-embedded human esophageal mucosa. Digoxigenin-labelled cRNA probes were produced by in vitro transcription of cDNA clones, coding for human CKs.

RESULTS

In situ hybridization and immunodetection of the hybrids revealed a distinct but different distribution pattern for each specific CK mRNA. The described signal pattern was consistently found at all levels of the esophagus. We observed differences in the expression of some CK mRNAs between the interpapillar and papillar compartment of the esophageal epithelium. Mainly in the papillar regions some mRNAs are already expressed in more basally located cells in comparison with the interpapillar regions. Our results substantiate the hypothesis concerning the formation of papillae in the esophageal mucosa. We have also described some observations on the expression of CK mRNAs in fortuitous sections through excretory ducts of esophageal submucosal glands.

CONCLUSIONS

The distinct, characteristic, and reproducible distribution pattern observed for each specific CK mRNA indicates that the expression of the genes encoding CKs in the esophageal epithelium as well depends on the cell proliferation, on vertical cell migration and differentiation, and on detachment from the basal lamina. The results presented should be considered as complementary to the already existing immunohistochemical results concerning the distribution of esophageal CK proteins.

Sequences 5' of the bovine keratin 5 gene direct tissue- and cell-type-specific expression of a lacZ gene in the adult and during development

Expression of keratin K5 (and K14) in multilayered epithelia occurs predominantly in the basal layer of proliferating keratinocytes. When a keratinocyte becomes committed to terminal differentiation, it moves out of the basal layer towards the epithelial surface. As part of this program of terminal differentiation, the expression of K5 (and K14) is downregulated in suprabasal cells, and new pairs of differentiation-specific keratins are expressed. To define the cis-acting DNA sequences required for K5 cell-type- and differentiation-specific expression, chimeric gene fusions between portions of the bovine keratin K5 locus and the Escherichia coli lacZ gene were used to generate transgenic mice. In the genomic fragment consisting of 5.3 kb of 5' flanking sequences, 6.1 kb corresponding to the body of the gene and 4.5 kb of 3' flanking sequences, the subfragment extending from -5300 bp to +138 bp was the smaller region that directed lacZ expression to stratified epithelia in a manner analogous to the endogenous keratin K5. Proximal sequences from -1300 bp to +138 bp were inactive. We also determined the expression pattern of keratin K5 during mouse development using an antiserum specific for mouse keratin K5. Expression was first detected in ectodermal cells of 11.5 days postcoitum embryos, and from day 13.5 postcoitum onwards K5 was detected in the precursors of most epithelia and organs which express K5 at adult stages. This pattern was reproduced, with few differences, by the construct with sequences from -5300 bp to +138 bp fused to the lacZ gene. These findings identify sequences between -5.3 kb and -1.3 kb of the bovine K5 gene as being important for cell-type- and differentiation-specific gene expression both during mouse development and in the adult.

A human keratin 14 "knockout": the absence of K14 leads to severe epidermolysis bullosa simplex and a function for an intermediate filament protein

Since their discovery, the function of intermediate filaments (IFs) has remained obscure. In skin, epidermal cells have extensive cytoskeletal architectures of IFs, composed of type I and type II keratin heterodimers. Clues to possible functions of these proteins have come from recent studies showing that several autosomal-dominant, blistering skin disorders are caused by defects in genes that encode epidermal keratins. These diseases all exhibit cell degeneration and keratin network perturbations in cells that express the particular mutant keratin gene. However, it is not clear from these studies whether cytolysis arises from the presence of large insoluble keratin aggregates that compromise cellular physiology or from the absence of an extensive keratin filament network, which jeopardizes mechanical integrity. We report here the analysis of an extremely rare case of severe recessive epidermolysis bullosa simplex (EBS), where the patient lacks a discernible keratin filament network in basal epidermal cells. Genetic analyses revealed a homozygous point mutation that yielded a premature termination codon in the major basal type I keratin gene and caused complete ablation of K14. The consanguineous parents were normal, each harboring one copy of the null K14 mutation. Analysis of cultured keratinocytes enabled us to document that the loss of K14 is not compensated for by the up-regulation of any other type I keratin. When taken together with the in vivo studies showing the presence of cell fragility generated from the lack of an extensive basal keratin network, these findings provide the first clear demonstration of loss of function associated with the absence of an IF protein in vivo.

Activation of the silent human cytokeratin 17 pseudogene-promoter region by cryptic enhancer elements of the cytokeratin 17 gene

We have previously described the three loci CK-CA, CK-CB and CK-CC in the human genome that contain clustered type-I cytokeratin genes and reported the complete nucleic acid sequences of the functional cytokeratin 17 gene located in CK-CA and two closely related pseudogenes present in CK-CB and CK-CC [Troyanovsky, S.M., Leube, R.E. & Franke, W.W. (1992) Eur. J. Cell Biol. 59, 127-137]. By nucleic acid sequence analysis, we now show that extensive similarities between the functional gene and the pseudogenes exist in the 5'-upstream region. However, despite the high degree of nucleic acid identity (94%), only the 5'-upstream region of the functional gene was able to induce significant transcriptional activity in transfected cells of epithelial origin. Using chimeric upstream regions consisting of different fragments from the pseudogene and the functional gene, we made the surprising observation that cis elements in the proximal 5'-upstream region of the pseudogene promoter can cooperate with distal enhancer elements of the functional gene to induce strong chloramphenicol-O-acetyltransferase activity in transfected HeLa cells. A major site in the proximal upstream region was identified by deoxyribonuclease protection experiments to be necessary for this cooperative effect. The structure and properties of this element were further analysed by transfection of different chloramphenicol-O-acetyltransferase gene constructs, and by nucleic acid sequence comparison to corresponding regions of the related cytokeratins 14 and 16. It is concluded that the upstream regions identified in this study contribute to the strong expression of the human cytokeratin 17 gene in a coordinated fashion.

Programming gene expression in developing epidermis

As the major proteins of adult keratinocytes, keratins provide biochemical markers for exploring mouse epidermal embryogenesis. Here, we used a modified method of whole-mount in situ hybridization to track skin-specific expression of endogenous keratin mRNAs throughout embryogenesis. To monitor transcriptional regulation, we coupled this with beta-galactosidase expression of a human epidermal keratin promoter-driven transgene. These studies have radically changed our perception of how the program of gene expression becomes established during epidermal development. Specifically, we have discovered that (1) basal keratin (K5 and K14) genes are first detected at E9.5 in a highly regional fashion, and surprisingly as early as the single layered ectodermal stage; (2) the early patterns do not correlate with morphogenesis per se, but rather with regional variations in the embryonic origin of underlying mesenchyme, supporting morphogenetic criteria that early inductive cues are mesenchymal; (3) epidermal keratin genes are expressed in periderm, supporting the notion that this layer arises from ectodermal stratification, even though it is simple epithelial-like in morphology and is subsequently sloughed during development; (4) later embryonic patterns of K5 and K14 gene expression parallel proliferative capacity and not stratification; and (5) K1 and K10 mRNAs are first detected as early as E13.5, and their patterns correlate with differentiation and not stratification. These patterns of epidermal gene expression led us to explore whether potential transcriptional regulators of these genes are expressed similarly. We show that AP2 (but not Sp1) cRNAs hybridize in a pattern similar to, but preceding that of basal keratin cRNAs. Finally, using gene expression in cultured cells, we demonstrate that AP2 has a strong inductive effect on basal keratin expression in a cellular environment that does not normally possess AP2 activity.

Mutations in the non-helical linker segment L1-2 of keratin 5 in patients with Weber-Cockayne epidermolysis bullosa simplex

Keratins are the major structural proteins of the epidermis. Analyzing keratin gene sequences, appreciating the switch in keratin gene expression that takes place as epidermal cells commit to terminally differentiate, and elucidating how keratins assemble into 10 nm filaments, have provided the foundation that has led to the discoveries of the genetic bases of two major classes of human skin diseases, epidermolysis bullosa simplex (EBS) and epidermolytic hyperkeratosis (EH). These diseases involve point mutations in either the basal epidermal keratin pair, K5 and K14 (EBS), or the suprabasal pair, K1 and K10 (EH). In severe cases of EBS and EH, mutations are found in the highly conserved ends of the alpha-helical rod domain, regions that, by random mutagenesis, had already been found to be important for 10 nm filament assembly. In order to identify regions of the keratin polypeptides that might be more subtly involved in 10 nm filament assembly and to explore the diversity in mutations within milder cases of these diseases, we have focused on Weber-Cockayne EBS, where mild blistering occurs primarily on the hands and feet in response to mechanical stress. In this report, we show that affected members of two different W-C EBS families have point mutations within 1 residue of each other in the non-helical linker segment of the K5 polypeptide. Genetic linkage analyses, the absence of this mutation in &gt; 150 wild-type alleles and filament assembly studies suggest that these mutations are responsible for the W-C EBS phenotype. These findings provide the best evidence to date that the non-helical linker region in the middle of the keratin polypeptides plays a subtle but significant role in intermediate filament structure and/or intermediate filament cytoskeletal architecture.

Conservation of the structure of keratin intermediate filaments: molecular mechanism by which different keratin molecules integrate into preexisting keratin intermediate filaments during differentiation

During development and differentiation, the intermediate filament component of the cytoskeleton of many cells and tissues is rebuilt by a dynamic exchange process in which one set of protein chains is replaced by another, without recourse to creation of a new network. One major example is the replacement of keratin 5/keratin 14 (K5/K14) keratin intermediate filaments (KIFs) by K1/K10 KIFs during terminal differentiation in the epidermis. The present work was undertaken to explore how this may occur. We have induced lysine-lysine cross-links with disulfosuccinimidyl tartrate in K5/K14 KIFs in order to determine the axial dimensions and relative axial alignments of the K5/K14 molecules. Many of the cross-links induced in subfilamentous oligomers containing one, two, or three molecules were also found in the intact KIF, indicating that the body of data thus generated provides physiologically relevant information on the structural organization in the KIF. A least-squares analysis using as data the positions of lysine residues involved in 23 induced cross-links has allowed the axial alignments of the various coiled-coil segments in the rod domain to be determined. Three modes of antiparallel alignment of two neighboring molecules were found: A11 (staggered by -16.7 nm), A22 (staggered by 28.8 nm), and A12 (almost in register; staggered by only 0.3 nm). Since the axial repeat length is about 1 nm less than the molecular length, the data require a fourth mode of molecule alignment, termed ACN, in which similarly directed molecules are overlapped by the equivalent of about 5-10 residues.(ABSTRACT TRUNCATED AT 250 WORDS)

A 300 bp 5′-upstream sequence of a differentiation-dependent rabbit K3 keratin gene can serve as a keratinocyte-specific promoter

Keratinocytes of the suprabasal compartment of many stratified epithelia synthesize as a major differentiation product a keratin pair, consisting of an acidic and a basic keratin, which accounts for 10–20% of the newly synthesized proteins. While genes of several differentiation-related keratins have been cloned and studied, relatively little is known about the molecular basis underlying their tissue-specific and differentiation-dependent expression. We have chosen to study, as a prototype of these genes, the gene of K3 keratin, which has the unique property of being expressed in the majority of corneal epithelial basal cells but suprabasally in peripheral cornea, the site of corneal epithelial stem cells. Using a monoclonal antibody, AE5, specific for K3 keratin, and a fragment of human K3 gene as probes, we have isolated several cDNA and genomic clones of rabbit K3 keratin. One genomic clone has been sequenced and characterized, and the identity of its coding sequence with that of cDNAs indicates that it corresponds to the single, functional rabbit K3 gene. Transfection assays showed that its 3.6 kb 5′-upstream sequence can drive a chloramphenicol acetyl transferase (CAT) reporter gene to express in cultured corneal and esophageal epithelial cells, but not in mesothelial and kidney epithelial cells or fibroblasts, all of rabbit origin. Serial deletion experiments narrowed this keratinocyte-specific promoter to within -300 bp upstream of the transcription initiation site. Its activity is not regulated by the coding or 3′-noncoding sequences that have been tested so far. This 300 bp 5′-upstream sequence of K3 keratin gene, which can function in vitro as a keratinocyte-specific promoter, contains two clusters of partially overlapping motifs, one with an NFkB consensus sequence and another with a GC box. The combinatorial effects of these multiple motifs and their cognate binding proteins may play an important role in regulating the expression of this tissue-restricted and differentiation-dependent keratin gene.

Keratin 14 protein in cultured nonparenchymal rat hepatic epithelial cells: characterization of keratin 14 and keratin 19 as antigens for the commonly used mouse monoclonal antibody OV-6

We have recently reported that cell lines of nonparenchymal origin isolated from rat liver and pancreas, which have been suggested to be the progeny of a facultative stem cell compartment in vivo, express an unusual combination of keratins (K). These cell lines express K8 and K14 but not K18 and K5, their normal partners in filament formation (Bisgaard HC, Thorgeirsson SS, J Cell Physiol 147:333-343, 1991). However, upon spontaneous transformation and differentiation toward a hepatoblastlike progeny, K14 expression is abrogated and replaced by expression of K18 (Wirth et al., Electrophoresis 13:305-332, 1992). In the study presented here, we confirmed by protein sequence analysis that K14 was a major component of the intermediate filaments in a nonparenchymal cell line of hepatic origin. Immunocytochemical analysis of the cells in monolayer demonstrated that K8 as well as K14 were incorporated in the cellular cytoskeleton. Further analysis by immunoprecipitation showed that filament complexes were formed between K8 and K14 as atypical partners. Thus, we concluded that in some nonparenchymal cell lines isolated from rat liver, K8 and K14 form a major intermediate filament network. Finally, we showed that an antibody widely used in studies of the cell lineages of hepatic and pancreatic tissues and their neoplasms, the mouse monoclonal antibody OV-6, recognizes a common epitope in K14 and K19.

Complete sequence of a hair-like intermediate filament type II keratin gene

The Intermediate Filament (IF) superfamily comprises several multigene families, of which the two keratin families are the largest. The keratin IF genes are expressed in epithelial tissues in differentiation-specific patterns and recently we reported the sequence and expression of a hair IF type II keratin gene (KRT2.9). Two related genes were present in the cosmid containing KRT2.9 and we have now sequenced one of them and found that it encodes a hair-like IF type II protein (KRT2.13). However, KRT2.13 is not expressed in the hair follicle. Interestingly there is significant sequence homology between introns 1, 5 and 6 of KRT2.13 and KRT2.9 to suggest gene conversion of these regions or possibly conservation of functional sequences.

Nuclear proteins involved in transcription of the human K5 keratin gene

Keratin K5 is expressed in the basal layer of stratified epithelia in mammals and its synthesis is regulated by hormones and vitamins such as retinoic acid. The molecular mechanisms that regulate K5 expression are not known. To initiate analysis of the protein factors that interact with the human K5 keratin gene upstream region, we have used gel-retardation and DNA-mediated cell-transfection assays. We found five DNA sites that specifically bind nuclear proteins. DNA-protein interactions at two of the sites apparently increase transcription levels, at one decrease it. The importance of the remaining two sites is, at present, unclear. In addition, the location of the retinoic acid and thyroid hormone nuclear receptor action site has been determined, and we suggest that it involves a cluster of five sites similar to the consensus recognition elements. The complex constellation of protein binding sites upstream from the K5 gene probably reflects the complex regulatory circuits that govern the expression of the K5 keratin in mammalian tissues.

Regional assignment of the human keratin 5 (KRT5) gene to chromosome 12q near D12S14 by PCR analysis of somatic cell hybrids and multicolor in situ hybridization

Keratin 5 is the major type II keratin of the basal cells of epidermis and of other stratified epithelia. With its type I partner, keratin 14, it constitutes a major fraction of the cytoskeleton of the basal cells. Because the inheritance of epidermolysis bullosa simplex, a disease of epidermal basal cell fragility, was mapped in one family to chromosome 12q close to D12S14, we undertook to localize the gene for keratin 5. Polymerase chain reaction analysis of somatic cell hybrids mapped the keratin 5 gene to chromosome 12, and multicolor fluorescence in situ hybridization localized it to 12q very near D12S14. This sublocalization exemplifies the utility of in situ physical localization in assessing the candidacy of genes thought to underlie inherited disorders.

Intermediate filaments as differentiation markers of normal pancreas and pancreas cancer

Expression of intermediate filaments (IF) is regulated during development and differentiation. The authors have studied the expression of vimentin and cytokeratins (CK) 4, 7, 8, 13, 18, 19 in normal pancreas, chronic pancreatitis, and pancreas cancer using monoclonal antibodies. Immunohistochemical assays were performed on fresh frozen tissue sections and on cultured pancreas cancer cells using the streptavidin-peroxidase method. In normal pancreas, acinar cells expressed CK 8 and 18, whereas ductal cells expressed CK 7, 8, 18, and 19. CK 4 was expressed by 5-10% of pancreas duct cells in all specimens of normal pancreas. CK 13 was not detected in any epithelial cells of normal pancreas or pancreatitis. CK 7, 8, 18, and 19 were homogeneously expressed in all pancreas cancers, whereas CK 4 was expressed only in 5-50% of cells in 10/16 tumors. Foci of squamous metaplasia expressed CK 13 but showed partial loss of expression of CK 7, 8, 18, and 19. Thirteen pancreas cancer cell lines examined showed homogeneous expression of CK 7, 8, 18, and 19; 2/11 lines expressed CK 4 weakly, and 6/11 expressed vimentin. CK 13 was not detected in any of the lines. These results indicate that pancreas cancer cells consistently express cytokeratin polypeptides characteristic of ductal epithelial cells and that this phenotype is retained in pancreas cancer cell lines. In addition, squamous metaplasia is associated with a coordinate change in the expression of CK polypeptides.

A mutation in the conserved helix termination peptide of keratin 5 in hereditary skin blistering

In the hereditary blistering condition epidermolysis bullosa simplex, the skin blisters on trauma following rupture of epidermal basal cells. Clinical variations range from severely incapacitating, especially in early childhood, to mild forms that may not even present clinically. Dowling-Meara epidermolysis bullosa simplex is characterized by clusters of epidermal blisters and keratin clumping in the cytoplasm; recent reports describe potentially causal mutations in keratin 14 (refs 2, 3). Here we describe a 'complementary' mutation at the other end of the other keratin expressed by these cells (K5, coexpressed with K14), a change from a Glu to a Gly in the helix termination peptide, detected by altered antibody binding and confirmed by sequencing using the polymerase chain reaction. The two conserved helix boundary peptides are predicted to be essential for filament assembly, and the requirement for two complementary (type I and type II) keratins is absolute. Epidermolysis bullosa simplex diseases demonstrate the function of the keratin cytoskeleton in resisting compaction stresses which otherwise lead to cell lysis.