The human keratin genes and their differential expression

Identification of a nonapeptide motif in the vimentin head domain involved in intermediate filament assembly

The assembly of soluble vimentin subunits into intermediate filaments (IFs) is dependent on information located in the amino-terminal domain. Using site-directed mutagenesis of a Xenopus laevis vimentin cDNA and an Escherichia coli production system to obtain pure mutated protein, we have identified, in the head domain, a nine amino acid motif (SSYRRIFGG), evolutionarily conserved from amphibia to man, which plays an important role in the orderly formation of IFs. Exchanges in the central di-arginine and in the two aromatic residues interfere with IF assembly of vimentin in vitro: on assembly under standard assembly conditions (160 mM-NaCl) most of the protein is included in dense aggregates, with a variable and minor proportion of IFs, whereas at lower ionic concentrations short and incomplete IF-like structures are formed. The deletion of the whole motif results in a protein that under standard assembly conditions (e.g. 160 mM-NaCl) predominantly and rapidly precipitates into large aggregates of non-IF material, whereas at lower ionic strength (e.g. 50 mM-NaCl) both IFs and dense aggregates are formed simultaneously. Our results show that the mutated protein can assume different forms at the same time and under the same conditions. This motif alone is insufficient for the formation of normal IFs as demonstrated by a mutant in which the motif has been brought closer to the alpha-helical rod domain by deletion of 55 internal amino acid residues. Corresponding observations have been made, by immunofluorescence microscopy, upon transfection of cultured epithelial cells lacking vimentin IFs. The importance of the head domain motif for the assembly and higher-order arrangement of IFs is discussed.

Intermediate filaments formed de novo from tail-less cytokeratins in the cytoplasm and in the nucleus

The roles of the different molecular domains of intermediate filament (IF) proteins in the assembly and higher order organization of IF structures have recently been studied by various groups but with partially controversial results. To examine the requirement of the aminoterminal (head) and the carboxyterminal (tail) domain of cytokeratins (CKs) for de novo IF formation in the living cell, we have constructed cDNAs coding for intact as well as head- and/or tail-less human CKs 8 and 18 and the naturally tail-less human CK 19, all under the control of the human beta-actin promoter. After transient and stable transfections of mouse 3T3-L1 cells, which are devoid of any CKs, we have studied, with such constructs, the resulting gene products by gel electrophoresis and immunolocalization techniques. By light and electron microscopy we show that extended cytoplasmic IF meshworks are formed from pairs of the type II CK 8 with the type I CKs 18 or 19 as well as from pairs of tail-less CK 8 with tail-less CKs 18 or 19 in the transfected cells, proving that the absence of the tail domain in both types of CKs does not prevent the de novo formation of regular IFs. Most surprisingly, however, we have observed spectacular alterations in the nucleocytoplasmic distribution of the IFs formed from tail-less CKs. In many of the transfected cells, a large part, or all, of the detectable CKs was found to occur in extensive IF bundles in the nucleoplasm. Intranuclear accumulations of CK deposits, however mostly nonfibrillar, were also observed when the cells had been transfected with cDNAs encoding tail-less CKs also lacking their head domains, whereas CKs deleted only in the head domain were found exclusively in the cytoplasm. The specific domain requirements for the assembly of cytoplasmic IF bundles are discussed and possible mechanisms of intranuclear accumulation of IFs are proposed.

Hair-specific keratins: characterization and expression of a mouse type I keratin gene

A genomic clone for a member of the mouse type I hair keratin protein family has been isolated and analyzed in order to study the regulation of this keratin during the hair growth cycle. The coding sequence is divided into seven exons. The gene structure is typical of keratins in particular and intermediate filaments in general in that the intron-exon borders are not located at the domain borders of the protein. Comparison with a sheep wool keratin gene shows that the splice sites in the two hair keratin genes are found at identical locations relative to the amino acid sequence of the proteins. Similarly, comparison of the promoter areas of these genes shows several areas of nucleotide sequence conservation, including the area around the TATA box and an SV40 core enhancer sequence. In addition, a high degree of sequence identity exists in the fourth intron. In situ hybridization shows that transcripts of this gene are first found in the relatively undifferentiated proximal cortex area in the keratogenous zone of mouse vibrissae.

Sorting out IF networks: consequences of domain swapping on IF recognition and assembly

Vimentin and keratin are coexpressed in many cells, but they segregate into two distinct intermediate filament (IF) networks. To understand the molecular basis for the sorting out of these IF subunits, we genetically engineered cDNAs encoding hybrid IF proteins composed of part vimentin and part type I keratin. When these cDNAs were transiently expressed in cells containing vimentin, keratin, or both IFs, the hybrid IF proteins all recognized one or the other or both networks. The ability to distinguish networks was dependent upon which segments of IF proteins were present in each construct. Constructs containing sequences encoding either helix 1B or helix 2B seemed to be the most critical in conferring IF recognition. At least for type I keratins, recognition was exerted at the level of dimer formation with wild-type type II keratin, as demonstrated by anion exchange chromatography. Interestingly, despite the fact that swapping of helical domains was not as deleterious to IF structure/function as deletion of helical domains, keratin/vimentin hybrids still caused structural aberrations in one or more of the cytoplasmic IF network. Thus, sequence diversity among IF proteins seems to influence not only coiled-coil but also higher ordered associations leading to 10-nm filament formation and/or IF interactions with other cellular organelles/proteins.

Dynamics of differentiation in human epidermoid squamous carcinoma cells (A431) with continuous, long-term gamma-IFN treatment

We investigated the long-term effects of continuous gamma interferon (gamma-IFN) treatment on A431, a human squamous carcinoma cell line. Cells were grown in an in vitro culture system, which over time produces cohesive cell masses ("tumoroids") exhibiting three-dimensional, histotypically differentiated structures, e.g., keratin "pearls", intercellular bridges (desmosomes), elongated flattened cells (squames) and stratification. The effects of gamma-IFN on cell growth, morphology and stage of differentiation were assessed at different treatment times by light and electron microscopy and by immunohistochemical staining using antibodies to keratins 1 and 14 and to filaggrin, markers of specific stages of keratinocyte differentiation. Our results show that A431 cells have the capacity for spontaneous differentiation, that this capacity is significantly enhanced and accelerated by gamma-IFN treatment leading to terminal differentiation and extensive cell death by 2 wk. Despite continuous exposure to IFN, a small number of viable, undifferentiated cells remain. Their proliferation, evident by 3 wk, reconstitutes the tumoroid which once again contains the full range of differentiating cell types.

Cell origin and identity in limb regeneration and development

The occurrence of limb regeneration in adult urodele amphibians raises fundamental questions about the relationship between development and regeneration. The use of monoclonal antibodies as cell markers has provided clear evidence that blastemal cells, the progenitor cells of the regenerate, are not the same as limb bud cells in the embryo. For one of these antibodies the distinction has been traced to the relationship between limb regeneration and the nerve supply. Innervation of the limb bud during development appears to establish nerve-dependent growth control for regeneration. The cell markers have also contributed to the problem of how the blastemal cells arise after amputation, although several important questions remain to be answered.

Expression of low molecular mass cytokeratins in oocytes of Schistosoma mansoni

We studied the distribution of acidic 45 kDa keratin 18 and 40/42 kDa keratin 19 in Schistosoma mansoni, a trematode of medical importance in many tropical regions. The monoclonal antibodies which were produced against the cytoskeleton of mammary carcinoma cell line BT-20 recognized cytokeratins preferentially in parasite oocytes. As has been described in mammalian oocytes, the acidic cytokeratins were present in a nonfibrillar form. The two monoclonal antibodies also recognized testicular cells. No keratin immunoreactivity could be demonstrated by immunofluorescence microscopy at the larval stage, the miracidium. In immunoblotting, the molecular mass as determined by SDS-polyacrylamide gel electrophoresis of schistosome cytokeratins was about 15 kDa higher than that of the corresponding cytokeratins recognized by the monoclonal antibodies in BT-20 cells. The results suggest that acidic low molecular mass cytokeratins in trematodes have a phylogenetically conserved major function in oocytes which is unrelated to the documented cytoskeletal role in differentiated mammalian epithelial cells.

Cell type-specific and efficient synthesis of human cytokeratin 19 in transgenic mice

In studies designed to identify cis-regulatory elements involved in the cell-type-specific expression of human cytokeratin (CK) genes we have dissected from the major type I CK gene locus on chromosome 17 a region containing the gene that encodes CK 19, with flanking segments of different lengths, and have examined the expression of related gene constructs in transgenic mice. Adult transgenic mice have been characterized by immunohistochemistry, gel-electrophoretic analyses of cytoskeletal proteins and genomic DNA (Southern blots). We have found that a construct harbouring the transcriptional unit plus approximately 0.7 kb downstream from the polyA-addition site and an immediately adjacent 5' upstream segment of approximately 3.6 kb, when combined with a further 5' upstream element of -6.4 - -8.6 kb, is sufficient to guarantee the synthesis of human CK 19 in the same cells and to a similar extent as the murine genome expresses its endogenous CK 19 gene. The findings demonstrate that all cis-elements necessary for the specific and efficient expression of a single type I CK gene, in the context of epithelial differentiation, can be located in the vicinity of the gene itself and that more-distant elements are not required.

Topography-related expression of individual cytokeratins in normal and pathological (non-neoplastic and neoplastic) human oral mucosa

Recently, regional changes of cytokeratin patterns in human normal non-keratinized or keratinized oral mucosa have been demonstrated and the expression of individual cytokeratin polypeptides in lesions of oral mucosa has been compared with that of normal tissues. In particular, the presence of cytokeratin 19 in the suprabasal cell layers of oral epithelia has been shown to be strongly correlated with premalignancy. In the present study, we describe the results of an immunohistochemical investigation performed using a monoclonal antibody specific for cytokeratin 1 on normal oral mucosa and benign or malignant oral lesions. We show the different distribution of this polypeptide in non-neoplastic lesions from different sites of oral mucosa and describe the presence of cytokeratin 19. Our results are in agreement with the data obtained previously. In the malignant cases we demonstrate that the distribution of the two cytokeratins is characterized by complementary patterns.

Identification of protein IT of the intestinal cytoskeleton as a novel type I cytokeratin with unusual properties and expression patterns

A major cytoskeletal polypeptide (Mr approximately 46,000; protein IT) of human intestinal epithelium was characterized by biochemical and immunological methods. The polypeptide, which was identified as a specific and genuine mRNA product by translation in vitro, reacted, in immunoblotting after SDS-PAGE, only with one of numerous cytokeratin (CK) antisera tested but with none of many monoclonal CK antibodies. In vitro, it formed heterotypic complexes with the type II CK 8, as shown by blot binding assays and gel electrophoresis in 4 M urea, and these complexes assembled into intermediate filaments (IFs) under appropriate conditions. A chymotrypsin-resistant Mr approximately 38,000 core fragment of protein IT could be obtained from cytoskeletal IFs, indicating its inclusion in a coiled coil. Antibodies raised against protein IT decorated typical CK fibril arrays in normal and transformed intestinal cells. Four proteolytic peptide fragments obtained from purified polypeptide IT exhibited significant amino acid sequence homology with corresponding regions of coils I and II of the rod domain of several other type I CKs. Immunocytochemically, the protein was specifically detected as a prominent component of intestinal and gastric foveolar epithelium, urothelial umbrella cells, and Merkel cells of epidermis. Sparse positive epithelial cells were noted in the thymus, bronchus, gall bladder, and prostate gland. The expression of protein IT was generally maintained in primary and metastatic colorectal carcinomas as well as in cell cultures derived therefrom. A corresponding protein was also found in several other mammalian species. We conclude that polypeptide IT is an integral IF component which is related, though somewhat distantly, to type I CKs, and, therefore, we propose to add it to the human CK catalogue as CK 20.

Elucidating the early stages of keratin filament assembly

Because of extraordinarily tight coiled-coil associations of type I and type II keratins, the composition and structure of keratin subunits has been difficult to determine. We report here the use of novel genetic and biochemical methods to explore the early stages of keratin filament assembly. Using bacterially expressed humans K5 and K14, we show that remarkably, these keratins behave as 1:1 complexes even in 9 M urea and in the presence of a reducing agent. Gel filtration chromatography and chemical cross-linking were used to identify heterodimers and heterotetramers as the most stable building blocks of keratin filament assembly. EM suggested that the dimer consists of a coiled-coil of K5 and K14 aligned in register and in parallel fashion, and the tetramer consists of two dimers in antiparallel fashion, without polarity. In 4 M urea, both end-to-end and lateral packing of tetramers occurred, leading to a variety of larger heteromeric complexes. The coexistence of multiple, higher-ordered associations under strongly denaturing conditions suggests that there may not be a serial sequence of events leading to the assembly of keratin intermediate filaments, but rather a number of associations may take place in parallel.

In vitro posttranslational modification of lamin B cloned from a human T-cell line

Autoimmune diseases are characterized by spontaneously occurring autoantibodies which have proven to be useful reagents for the characterization of specific nuclear proteins. Using a monoclonal autoantibody (72B9) derived from a murine lupus strain, we have cloned a cDNA from the human T-cell line MOLT-4, which encodes nuclear lamin B. The identity of the encoded protein as lamin B was established by both biochemical and immunological criteria. Inspection of the deduced amino acid sequence of lamin B revealed the presence in coil 1B of the alpha-helical domain of a leucine heptad repeat region. Analysis of mRNA in HL60 and MOLT-4 cells, which express only lamin B, or HeLa cells, which express all three major lamins (A, B, and C), together with the comigration of in vitro-translated product with isolated HeLa cell lamin B by two-dimensional gel electrophoresis, suggests that a single lamin B is expressed in mammalian somatic cells. In vitro translation with the cDNA clone revealed an EDTA-sensitive posttranslational modification which resulted in an increase in the apparent molecular weight to that equivalent to the native in vivo-synthesized lamin B protein. This in vitro modification included incorporation of a product of mevalonolactone and required an intact carboxy terminus.

In vitro posttranslational modification of lamin B cloned from a human T-cell line

Autoimmune diseases are characterized by spontaneously occurring autoantibodies which have proven to be useful reagents for the characterization of specific nuclear proteins. Using a monoclonal autoantibody (72B9) derived from a murine lupus strain, we have cloned a cDNA from the human T-cell line MOLT-4, which encodes nuclear lamin B. The identity of the encoded protein as lamin B was established by both biochemical and immunological criteria. Inspection of the deduced amino acid sequence of lamin B revealed the presence in coil 1B of the alpha-helical domain of a leucine heptad repeat region. Analysis of mRNA in HL60 and MOLT-4 cells, which express only lamin B, or HeLa cells, which express all three major lamins (A, B, and C), together with the comigration of in vitro-translated product with isolated HeLa cell lamin B by two-dimensional gel electrophoresis, suggests that a single lamin B is expressed in mammalian somatic cells. In vitro translation with the cDNA clone revealed an EDTA-sensitive posttranslational modification which resulted in an increase in the apparent molecular weight to that equivalent to the native in vivo-synthesized lamin B protein. This in vitro modification included incorporation of a product of mevalonolactone and required an intact carboxy terminus.

Selective accumulation of IF proteins at a focal juxtanuclear site in COS-1 cells transfected with mouse keratin 18 cDNA

COS-1 cells contain two keratins analogous to human keratins 8 (type II) and 18 (type I), and vimentin. Transfection of a plasmid, pSVK18, containing a mouse keratin 18 cDNA regulated by the SV40 early region promoter, was used to force the synthesis of exogenous (but homologous) type I keratin and to assess the effect of the oversynthesis of a keratin on endogenous keratins and vimentin intermediate filaments (IFs). Double immunofluorescence microscopy with monoclonal antimouse keratin 18 and monoclonal anti-human keratins 8 and 18 antibodies which cross-react with monkey keratins, showed that mouse keratin 18 formed typical IFs with the endogenous keratins but also accumulated in a focal area near the nucleus. Vimentin and its associated protein, p50, also colocalized at the juxtanuclear focal region, but the vimentin IFs of the outer cytoplasm vanished. Similar analyses with anti-tubulin and anti-actin antibodies indicated that the accumulated mouse keratin 18 colocalized with the centrosome but did not disturb the organization of microtubules or microfilaments. Anti-lamin and anti-SV40 large T antibodies showed that the oversynthesis of mouse keratin 18 had no effect on the distribution of these proteins. The accumulation was therefore selective for the cytoplasmic IF proteins. Electron microscopy and immunogold labeling of whole-mount detergent-extracted cells demonstrated that the accumulated IFs in the centrosomal region extended as a dense IF plexus-like network anchored to part of the nuclear surface.

Organization and sequence of the human gene encoding cytokeratin 8

Among the various intermediate filament (IF) proteins, cytokeratin 8 (CK8) is especially remarkable as it is produced early in embryogenesis, is the only type-II CK occurring in many simple epithelial cells, and can also be synthesized in certain non-epithelial cells. Using a cDNA probe specific for human CK8 we have isolated an approx. 14-kb genomic clone (in phage lambda EMBL3) which contains the gene encoding human CK8. The gene comprising a total of 7766 nucleotides (nt) from the transcription start point, determined by primer extension analysis, to the polyadenylation site, determined from cDNA sequencing, and a 1030-nt 5' upstream region have been sequenced. The sequence of 485 amino acids (aa) deduced from the exon sequences (Mr 53,532) shows strong homology with the corresponding gene products of bovine, murine and amphibian (Xenopus laevis) origins. Surprisingly, the ck8 gene contains only seven introns instead of eight as found in all other genes of the same (type-II) CK subfamily; while all seven introns occur in identical positions as in the other type-II CK-encoding genes, intron V of these genes is missing in the ck8 gene. Intron I of ck8 is remarkably long (2534 nt) and contains a cluster of potential regulatory sequences, including three Sp1 sites, and an extended Alu-element. In Southern-blot analyses, we found only one intron-containing gene encoding CK8 in the human genome, and by heterologous transfection experiments we showed that this gene is correctly transcribed in non-human cells expressing the orthologous gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Extensive changes in cytokeratin expression patterns in pathologically affected human gingiva

The stratified squamous epithelium of the oral gingiva and the hard palate is characterized by a tissue architecture and a cytoskeletal composition similar to, although not identical with, that of the epidermis and fundamentally different from that of the adjacent non-masticatory oral mucosa. Using immunocytochemistry with antibodies specific for individual cytokeratins, in situ hybridization and Northern blots of RNA with riboprobes specific for individual cytokeratin mRNAs, and gel electrophoresis of cytoskeletal proteins of microdissected biopsy tissue samples, we show changes in the pattern of expression of cytokeratins and their corresponding mRNAs in pathologically altered oral gingiva. Besides a frequently, although not consistently, observed increase in the number of cells producing cytokeratins 4 and 13 (which are normally found as abundant components in the sulcular epithelium and the alveolar mucosa but not in the oral gingiva) and a reduction in the number of cells producing cytokeratins 1, 10 and 11, the most extensive change was noted for cytokeratin 19, a frequent cytokeratin in diverse one-layered and complex epithelia. While in normal oral gingiva cytokeratin 19 is restricted to certain, sparsely scattered cells of --or near--the basal cell layer, probably neuroendocrine (Merkel) cells, in altered tissue of inflamed samples it can appear in larger regions of the basal cell layer(s) and, in apparently more advanced stages, also in a variable number of suprabasal cells. Specifically, our in situ hybridization experiments show that this altered suprabasal cytokeratin 19 expression is more extended at the mRNA than at the protein level, indicating that cytokeratin 19 mRNA synthesis may be a relatively early event during the alteration. These changes in cytokeratin expression under an external pathological influence are discussed in relation to other factors known to contribute to the expression of certain cytokeratins and with respect to changes occurring during dysplasia and malignant transformation of oral epithelia.

Cytokeratins and cytokeratin filaments in subpopulations of cultured human and rodent cells of nonepithelial origin: modes and patterns of formation

Using immunofluorescence microscopy, we observed that in several established cell culture lines derived from different nonepithelial tissues and species, cells spontaneously emerge, usually at low frequencies, which contain cytoplasmic structures decorated by antibodies specific for cytokeratins 8 and 18. This phenomenon was further examined at both the protein (gel electrophoreses of cytoskeletal proteins, followed by immunoblotting) and the RNA (Northern blots, "nuclear run-on" analysis, in situ hybridization) level. Positive cell lines included simian virus (SV40)-transformed human fibroblasts (HF-SV80, WI-38 VA13), human astrocytic glioma cells (U333 CG/343MG), rat (RVF-SMC) and hamster (BHK-21/13) cells derived from vascular smooth muscle and murine sarcoma MS-180 cells. In two cell lines (HF-SV80 and BHK-21/13), the frequency of the cytokeratin-containing cells and of the cytokeratin fibril arrays per cell was drastically increased upon treatment with 5-azacytidine. The structural appearance of the cytokeratins was variable in the different cell lines but could also differ among cells of the same culture: While small granular or comma-shaped structures or bizarrely shaped filament arrays prevailed in WI-38, RVF and normally grown BHK-21 cells, most of the other lines revealed extended normal-looking, fibrillar arrays. In one line (MS-180), the appearance of cytokeratins was associated with a morphological change, as it was only found in a subpopulation of cells that had lost their typical elongated and spindle-shaped phenotype and assumed a rounded ("coccoid") shape. Our results show that the expression of the genes encoding cytokeratins 8 and 18 is not necessarily restricted to programs of epithelial differentiation and that factors stochastically effective appear in cultured cell lines that allow the synthesis of these cytoskeletal components. Mechanisms possibly involved in this spontaneous and selective advent of cytokeratins 8 and 18 and implications for tumor diagnosis are discussed.

Expression of keratin K14 in the epidermis and hair follicle: insights into complex programs of differentiation

Keratins K14 and K5 have long been considered to be biochemical markers of the stratified squamous epithelia, including epidermis (Moll, R., W. Franke, D. Schiller, B. Geiger, and R. Krepler. 1982. Cell. 31:11-24; Nelson, W., and T.-T. Sun. 1983. J. Cell Biol. 97:244-251). When cells of most stratified squamous epithelia differentiate, they downregulate expression of mRNAs encoding these two keratins and induce expression of new sets of keratins specific for individual programs of epithelial differentiation. Frequently, as in the case of epidermis, the expression of differentiation-specific keratins also leads to a reorganization of the keratin filament network, including denser bundling of the keratin fibers. We report here the use of monospecific antisera and cRNA probes to examine the differential expression of keratin K14 in the complex tissue of human skin. Using in situ hybridizations and immunoelectron microscopy, we find that the patterns of K14 expression and filament organization in the hair follicle are strikingly different from epidermis. Some of the mitotically active outer root sheath (ORS) cells, which give rise to ORS under normal circumstances and to epidermis during wound healing, produce only low levels of K14. These cells have fewer keratin filaments than basal epidermal cells, and the filaments are organized into looser, more delicate bundles than is typical for epidermis. As these cells differentiate, they elevate their expression of K14 and produce denser bundles of keratin filaments more typical of epidermis. In contrast to basal cells of epidermis and ORS, matrix cells, which are relatively undifferentiated and which can give rise to inner root sheath, cuticle and hair shaft, show no evidence of K14, K14 mRNA expression, or keratin filament formation. As matrix cells differentiate, they produce hair-specific keratins and dense bundles of keratin filaments but they do not induce K14 expression. Collectively, the patterns of K14 and K14 mRNA expression and filament organization in mitotically active epithelial cells of the skin correlate with their relative degree of pluripotency, and this suggests a possible basis for the deviation of hair follicle programs of differentiation from those of other stratified squamous epithelia.

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

The mitotically active basal layers of most stratified squamous epithelia express 10 to 30% of their total protein as keratin. The two keratins specifically expressed in these cells are the type II keratin K5 (58 kilodaltons) and its corresponding partner, type I keratin K14 (50 kilodaltons), both of which are essential for the formation of 8-nm filaments. Dissecting the molecular mechanisms underlying the coordinate regulation of the two keratins is an important first step in understanding epidermal differentiation and in designing promoters that will enable delivery and expression of foreign gene products in stratified squamous epithelia, e.g., skin. Previously, we reported the sequence of the gene encoding human K14 (D. Marchuk, S. McCrohon, and E. Fuchs, Cell 39:491-498, 1984; Marchuk et al., Proc. Natl. Acad. Sci. USA 82:1609-1613, 1985). We have now isolated and characterized the gene encoding human K5. The sequence of the coding portion of this gene matched perfectly with that of a partial K5 cDNA sequence obtained from a cultured human epidermal library (R. Lersch and E. Fuchs, Mol. Cell. Biol. 8:486-493, 1988), and gene transfection studies indicated that the gene is functional. Nuclear runoff experiments demonstrated that the K5 and K14 genes were both transcribed at dramatically higher levels in cultured human epidermal cells than in fibroblasts, indicating that at least part of the regulation of the expression of this keratin pair is at the transcriptional level. When the K5 gene was transfected transiently into NIH 3T3 fibroblasts, foreign expression of the gene caused the appearance of endogenous mouse K14 and the subsequent formation of a keratin filament array in the cells. In this case, transcriptional changes did not appear to be involved in the regulation, suggesting that there may be multiple control mechanisms underlying the pairwise expression of keratins.

Spontaneous losses of control of cytokeratin gene expression in transformed, non-epithelial human cells occurring at different levels of regulation

Intermediate filaments (IFs) of the cytokeratin (CK) type are cytoskeletal elements typical for epithelial differentiation. However, in diverse transformed culture lines of non-epithelial origin, rare cells emerge spontaneously, which synthesize, in addition to their vimentin IFs, CKs 8 and 18. We enriched such cells by cloning and studied the level(s) of regulation at which these changes occur. We found that in SV40-transformed fibroblasts the CK 18 gene is constitutively transcribed into translatable mRNA but that the protein is rapidly degraded in the absence of its complex partner, CK 8. In contrast, cells immunocytochemically positive for CK IFs contained both CKs 8 and 18, which apparently stabilized in heterotypic complexes. These findings and related observations of active genes for CKs 8 and/or 18 in several other transformed non-epithelial cell lines indicate that the genes for CKs 18 and, less frequently, 8 can be active in diverse different non-epithelial cell lines; synthesis of type I and type II CK pair partners can be uncoupled; control of CK IF formation can take place at different levels. We suggest that the intrinsic instability of the inactive state of these genes is responsible for the occurrence of CKs 8 and 18 in certain non-epithelial tissues and tumors, a caveat in tumor diagnosis.

A family of type I keratin genes and the homeobox-2 gene complex are closely linked to the rex locus on mouse chromosome 11

Type I and type II keratins are major constituents of intermediate filaments that play a fundamental role in the cytoskeletal network. By using both somatic cell hybrids and conventional and interspecific linkage crosses, several genes encoding type I keratins, including the epidermal keratin K10, were shown to be closely linked to the homeobox-2 complex and the rex locus on mouse chromosome 11. The absence of crossovers between type I keratin-encoding genes and rex (N = 239), a locus affecting hair development, raises the possibility that mutations at rex and neighboring loci affecting skin and hair development involve type I keratin genes.

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

The mitotically active basal layers of most stratified squamous epithelia express 10 to 30% of their total protein as keratin. The two keratins specifically expressed in these cells are the type II keratin K5 (58 kilodaltons) and its corresponding partner, type I keratin K14 (50 kilodaltons), both of which are essential for the formation of 8-nm filaments. Dissecting the molecular mechanisms underlying the coordinate regulation of the two keratins is an important first step in understanding epidermal differentiation and in designing promoters that will enable delivery and expression of foreign gene products in stratified squamous epithelia, e.g., skin. Previously, we reported the sequence of the gene encoding human K14 (D. Marchuk, S. McCrohon, and E. Fuchs, Cell 39:491-498, 1984; Marchuk et al., Proc. Natl. Acad. Sci. USA 82:1609-1613, 1985). We have now isolated and characterized the gene encoding human K5. The sequence of the coding portion of this gene matched perfectly with that of a partial K5 cDNA sequence obtained from a cultured human epidermal library (R. Lersch and E. Fuchs, Mol. Cell. Biol. 8:486-493, 1988), and gene transfection studies indicated that the gene is functional. Nuclear runoff experiments demonstrated that the K5 and K14 genes were both transcribed at dramatically higher levels in cultured human epidermal cells than in fibroblasts, indicating that at least part of the regulation of the expression of this keratin pair is at the transcriptional level. When the K5 gene was transfected transiently into NIH 3T3 fibroblasts, foreign expression of the gene caused the appearance of endogenous mouse K14 and the subsequent formation of a keratin filament array in the cells. In this case, transcriptional changes did not appear to be involved in the regulation, suggesting that there may be multiple control mechanisms underlying the pairwise expression of keratins.

Cytokeratin filaments and desmosomes in the epithelioid cells of the perineurial and arachnoidal sheaths of some vertebrate species

Using electron microscopy and immunohistochemistry with a large panel of antibodies to various cytoskeletal proteins we have noted that the single- or multi-layered sheaths of epithelioid cells ("neurothelia") surrounding peripheral nerves (perineurial cells) or structures of the central nervous system, including the optic nerve (arachnoid cells), show remarkable interspecies differences in their cytoskeletal complements. In two anuran amphibia examined (Xenopus laevis, Rana ridibunda), the cells of both forms of neurothelia, i.e., perineurial and arachnoid, are interconnected by true desmosomes and are rich intermediate-sized filaments (IFs) of the cytokeratin type. Among higher vertebrates, a similar situation is found in the bovine and chicken nervous systems, in which the arachnoid cells of the meninges contain desmosomes and IFs of both the cytokeratin (apparently with restricted epitope accessibilities in the chicken) and the vimentin type, whereas the perineurial cells of many nerves contain cytokeratin IFs, often together with vimentin, but no desmosomes. In contrast, in rat arachnoidal and perineurial cells significant reactions have been observed neither for cytokeratins nor for desmosomes. In the human nervous system, cytokeratins and desmosomes have also not been seen in the various perineuria studied whereas desmosomes are frequent in arachnoidal cell layers which are dominated by vimentin IFs and only in certain small regions of the brain contain some additional cytokeratins. The occurrence of cytokeratins in the tissues found positive by immunohistochemistry has been confirmed by gel electrophoresis of cytoskeletal proteins, followed by immunoblotting. Our results emphasize both similarities and differences between the neurothelia on the one hand and epithelia or endothelia on the other, justifying classification as a separate kind of tissue, i.e., neurothelium. The observations of interspecies differences lead to the challenging conclusion that neither desmosomes nor cytokeratins are essential for the basic functions of neurothelial sheaths nor does the specific type of IF protein expressed in these cells appear to matter in this respect. The results are also discussed in relation to the cytoskeletal characteristics of other epithelioid tissues and of human neurothelium-derived tumors.

Expression of mutant keratin cDNAs in epithelial cells reveals possible mechanisms for initiation and assembly of intermediate filaments

We have deleted cDNA sequences encoding portions of the amino- and carboxy-terminal end of a human type I epidermal keratin K14, and examined the molecular consequences of forcing the expression of these mutants in simple epithelial and squamous cell carcinoma lines. To follow the expression of our mutant products in transfected cells, we have tagged the 3' end of the K14 coding sequence with a sequence encoding an antigenic domain of the neuropeptide substance P. Using DNA transfection and immunohistochemistry (with an antibody against substance P), we have defined the limits of K14 sequence necessary to incorporate into a keratin filament network in vivo without disrupting its architecture. We have also uncovered major differences in the behavior of carboxy- and amino-terminal alpha-helical mutants which do perturb the cytoskeletal network of IFs: whereas carboxy terminal mutants give rise to aggregates of keratin in the cytoplasm, amino-terminal mutants tend to produce aggregates of keratins which seem to localize at the nuclear surface. An examination of the phenotypes generated by the carboxy and amino-terminal mutants and the behavior of cells at late times after transfection suggests a model whereby initiation of filament assembly occurs at discrete sites on the nuclear envelope and filaments grow from the nucleus toward the cytoplasm.

Tissue-specific and differentiation-specific expression of a human K14 keratin gene in transgenic mice

A construct containing approximately 2500 base pairs (bp) of 5' upstream and approximately 700 bp of 3' downstream sequence was used to drive the expression of an intronless human K14 gene in vitro and in vivo. To track the expression of the gene, a small sequence encoding the antigenic portion of neuropeptide substance P was inserted in frame 5' to the TGA translation stop codon of the gene. Surprisingly, this gene was expressed promiscuously in a wide variety of cultured cells transiently transfected with the construct. In contrast, when introduced into the germ line of transgenic mice, the construct was expressed in a fashion analogous to the endogenous K14 gene--namely, in the basal layer of stratified squamous epithelia. Our results suggest that some regulatory mechanism is overridden as a consequence of transient transfection but that sequences that can control proper K14 expression are present in the construct. The appropriate tissue-specific and differentiation-specific expression of K14.P in transgenic mice is an important first step in characterizing a promoter that could be employed to drive the foreign expression of drug-related genes in the epidermis of skin grafts.

A new look into an old problem: keratins as tools to investigate determination, morphogenesis, and differentiation in skin

We have investigated keratin and keratin mRNA expression during (1) differentiation of stem cells into epidermis and hair follicles and (2) morphogenesis of follicles. Our results indicate that a type I keratin K14 is expressed early in embryonal basal cells. Subsequently, its expression is elevated in the basal layer of developing epidermis but suppressed in developing matrix cells. This difference represents an early and major biochemical distinction between the two diverging cell types. Moreover, because expression of this keratin is not readily influenced by extracellular regulators or cell culture, it suggests a well-defined and narrow window of development during which an irreversible divergence in basal and matrix cells may take place. In contrast to K14, which is expressed very early in development and coincident with basal epidermal differentiation, a hair-specific type I keratin and its mRNA is expressed late in hair matrix development and well after follicle morphogenesis. Besides providing an additional developmental difference between epidermal and hair matrix cells, the hair-specific keratins provide the first demonstration that keratin expression may be a consequence rather than a cause of cell organization and differentiation.

Localization of cytokeratins in tissues of the rainbow trout: fundamental differences in expression pattern between fish and higher vertebrates

Using a panel of antibodies against different cytokeratins in immunofluorescence microscopy on frozen tissue sections and two-dimensional gel electrophoresis of cytoskeletal proteins from these tissues, we have studied the tissue distribution of cytokeratins in a fish, the rainbow trout Salmo gairdneri. We have distinguished at least 14 different cytokeratin polypeptides in only a limited number of tissues, thus demonstrating the great complexity of the cytokeratin pattern in a fish species. The simplest cytokeratin pattern was that present in hepatocytes, comprising one type-II (L1) and two type-I (L2, L3) polypeptides that appear to be related to mammalian cytokeratins 8 and 18, respectively. Two or all three cytokeratins of this group were also identified in several other epithelial tissues, such as kidney. Epithelia associated with the digestive tract contained, in addition, other major tissue-specific cytokeratins, such as components D1-D3 (stomach, intestine and swim bladder) and B1 and B2 (biliary tract). With the exception of D1, all these polypeptides were also found in a cultured cell line (RTG-2). Epidermal keratinocytes contained D1 and six other major cytokeratins, termed E1-E6. The most complex cytokeratin pattern was that found in the gill epithelium. Surprisingly, antibodies specific for cytokeratins of the L1-L3 group also reacted with certain cell-sheet-forming tissues that are not considered typical epithelia and in higher vertebrates express primarily, if not exclusively, vimentin. Such tissues were (a) endothelia, including the pillar cells of the "gill filaments", (b) scale-associated cells, and (c) the ocular lens epithelium, and also several nonepithelial cell types, such as (d) fibroblasts and other mesenchymal cells, (e) chondrocytes, (f) certain vascular smooth muscle cells, and (g) astroglial cells of the optic nerve. The differences between the patterns of cytokeratin expression in this fish species and those of higher vertebrates are discussed. It is concluded that the diversity of cytokeratins has already been established in lower vertebrates such as fish, but that the tissue-expression pattern of certain cytokeratins has been restricted during vertebrate evolution. We discuss the value of antibodies specific for individual cytokeratin polypeptides as marker molecules indicating cell and tissue differentiation in fish histology, embryology, and pathology.

Isolation, sequence, and differential expression of a human K7 gene in simple epithelial cells

Simple epithelial cells synthesize a different set of keratins than epidermal cells. In experiments reported in this manuscript, we show that the base level of keratin expression in simple epithelial cells is variable for different cell types, and that, in some simple epithelia, this level can be upregulated by increasing the exposure of cells to retinoids, but not glucocorticoids or estradiol. To elucidate the molecular mechanisms underlying simple epithelial keratin gene regulation, we have isolated and characterized a human gene encoding the simple epithelial keratin K7. By examining the possible regulatory elements of this gene and by investigating the behavior of this gene introduced transiently into simple epithelial cells, we have uncovered a possible basis for the differential expression of epidermal and simple epithelial keratin genes.

Use of monospecific antisera and cRNA probes to localize the major changes in keratin expression during normal and abnormal epidermal differentiation

We report here the isolation and characterization of three antisera, each of which is specific for a single keratin from one of the three different pairs (K1/K10, K14/K5, K16/K6) that are differentially expressed in normal human epidermis and in epidermal diseases of hyperproliferation. We have used these antisera in conjunction with monospecific cRNA probes for epidermal keratin mRNAs to investigate pathways of differentiation in human epidermis and epidermal diseases in vivo and in epidermal cells cultured from normal skin and from squamous cell carcinomas in vitro. Specifically, our results suggest that: (a) the basal-specific keratin mRNAs are down-regulated upon commitment to terminal differentiation, but their encoded proteins are stable, and can be detected throughout the spinous layers; (b) the hyperproliferation-associated keratin mRNAs are expressed at a low level throughout normal epidermis when their encoded proteins are not expressed, but are synthesized at high levels in the suprabasal layers of hyperproliferating epidermis, coincident with the induced expression of the hyperproliferation-associated keratins in these cells; and (c) concomitantly with the induction of the hyperproliferation-associated keratins in the suprabasal layers of the epidermis is the down-regulation of the expression of the terminal differentiation-specific keratins. These data have important implications for our understanding of normal epidermal differentiation and the deviations from this process in the course of epidermal diseases of hyperproliferation.

A group of type I keratin genes on human chromosome 17: characterization and expression

The human type I keratins K16 and K14 are coexpressed in a number of epithelial tissues, including esophagus, tongue, and hair follicles. We determined that two genes encoding K16 and three genes encoding K14 were clustered in two distinct segments of chromosome 17. The genes within each cluster were tightly linked, and large parts of the genome containing these genes have been recently duplicated. The sequences of the two K16 genes showed striking homology not only within the coding sequences, but also within the intron positions and sequences and extending at least 400 base pairs 5' upstream and 850 base pairs 3' downstream from these genes. Despite the strong homologies between these two genes, only one of the genes encoded a protein which assembled into keratin filaments when introduced into simple epithelial cells. While there were no obvious abnormalities in the sequence of the other gene, its promoter seemed to be significantly weaker, and even a hybrid gene with the other gene's promoter gave rise to a much reduced mRNA level after gene transfection. To demonstrate that the functional K16 gene that we identified was in fact responsible for the K16 expressed in human tissues, we made a polyclonal antiserum which recognized our functional K16 gene product in both denatured and filamentous form and which was specific for bona fide human K16.

A group of type I keratin genes on human chromosome 17: characterization and expression

The human type I keratins K16 and K14 are coexpressed in a number of epithelial tissues, including esophagus, tongue, and hair follicles. We determined that two genes encoding K16 and three genes encoding K14 were clustered in two distinct segments of chromosome 17. The genes within each cluster were tightly linked, and large parts of the genome containing these genes have been recently duplicated. The sequences of the two K16 genes showed striking homology not only within the coding sequences, but also within the intron positions and sequences and extending at least 400 base pairs 5' upstream and 850 base pairs 3' downstream from these genes. Despite the strong homologies between these two genes, only one of the genes encoded a protein which assembled into keratin filaments when introduced into simple epithelial cells. While there were no obvious abnormalities in the sequence of the other gene, its promoter seemed to be significantly weaker, and even a hybrid gene with the other gene's promoter gave rise to a much reduced mRNA level after gene transfection. To demonstrate that the functional K16 gene that we identified was in fact responsible for the K16 expressed in human tissues, we made a polyclonal antiserum which recognized our functional K16 gene product in both denatured and filamentous form and which was specific for bona fide human K16.

Maturation of hagfish gland thread cells: composition and characterization of intermediate filament polypeptides

Previous studies with the hagfish, a primitive vertebrate, have shown that the gland thread cells (GTCs) each contain a single thread (approximately 60 cm long in average-sized cells) in the form of a concisely coiled cytoskeletal entity destined for export by holocrine secretion. The thread in relatively immature GTCs consists almost entirely of intermediate filaments (IFs) bundled in parallel alignment with far fewer microtubules (MTs). The three thread polypeptides described earlier (alpha, basic; beta, acidic; gamma, most acidic; each with a Mr of 63-64 kD) are now further evaluated with respect to in vitro assembly, cross-reactivity with IF polypeptides from higher vertebrates, and peptide sequence homology with known IF polypeptides. The overall results mainly suggest that the hagfish polypeptides are keratinlike substances but lamins or a new type of IF is not ruled out. However, cross-reactivity is weak with mammalian keratins; the 8-11-nm filaments formed from mixtures of alpha and gamma in vitro are generally linear rather than the curvilinear structures usually formed by keratin and nonkeratin IFs; and mixtures of alpha and beta tend to yield 9-12-nm granules or granular strings. Polypeptide analyses on GTCs segregated on the basis of maturational stage show a progressive increase in beta/gamma values which correlates with cell maturation, but the alpha/(beta + gamma) ratios remain near 1. Inasmuch as beta and gamma have many similar properties, the documented increase in the amount of the beta component in aging GTCs might in part be the result of a failure in a posttranslational modification system and may contribute to the ultrastructural changes that accompany thread maturation in preparation for holocrine secretion and subsequent modulation of the viscoelastic properties of mucus.