Keratin 1 maintains skin integrity and participates in an inflammatory network in skin through interleukin-18

Keratin 1 (KRT1) and its heterodimer partner keratin 10 (KRT10) are major constituents of the intermediate filament cytoskeleton in suprabasal epidermis. KRT1 mutations cause epidermolytic ichthyosis in humans, characterized by loss of barrier integrity and recurrent erythema. In search of the largely unknown pathomechanisms and the role of keratins in barrier formation and inflammation control, we show here that Krt1 is crucial for maintenance of skin integrity and participates in an inflammatory network in murine keratinocytes. Absence of Krt1 caused a prenatal increase in interleukin-18 (IL-18) and the S100A8 and S100A9 proteins, accompanied by a barrier defect and perinatal lethality. Depletion of IL-18 partially rescued Krt1(-/-) mice. IL-18 release was keratinocyte-autonomous, KRT1 and caspase-1 dependent, supporting an upstream role of KRT1 in the pathology. Finally, transcriptome profiling revealed a Krt1-mediated gene expression signature similar to atopic eczema and psoriasis, but different from Krt5 deficiency and epidermolysis bullosa simplex. Our data suggest a functional link between KRT1 and human inflammatory skin diseases.

Keratin K18 increases cystic fibrosis transmembrane conductance regulator (CFTR) surface expression by binding to its C-terminal hydrophobic patch

BACKGROUND

CFTR function is tightly regulated by many interacting proteins.

RESULTS

Intermediate filament protein keratin 18 increases the cell surface expression of CFTR by interacting with the C-terminal hydrophobic patch of CFTR.

CONCLUSION

K18 controls the function of CFTR.

SIGNIFICANCE

These findings offer novel insights into the regulation of CFTR and suggest that K18 and its dimerization partner, K8, may be modifier genes in cystic fibrosis. Malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) leads to cystic fibrosis, but the regulation of CFTR is not fully understood. Here, we identified the intermediate filament protein keratin K18 (K18) as a CFTR-binding protein by various approaches. We mapped a highly conserved "hydrophobic patch" ((1413)FLVI(1416)) in the CFTR C-terminus, known to determine plasmalemmal CFTR stability, as the K18-binding site. On the other hand, the C-terminal tail of K18 was found to be a critical determinant for binding CFTR. Overexpression of K18 in cells robustly increased the surface expression of wild-type CFTR, whereas depletion of K18 through RNA interference specifically diminished it. K18 binding increased the surface expression of CFTR by accelerating its apical recycling rate without altering CFTR biosynthesis, maturation, or internalization. Importantly, CFTR surface expression was markedly reduced in duodenal and gallbladder epithelia of K18(-/-) mice. Taken together, our results suggest that K18 increases the cell surface expression of CFTR by interacting with the CFTR C-terminal hydrophobic patch. These findings offer novel insights into the regulation of CFTR and suggest that K18 and its dimerization partner, K8, may be modifier genes in cystic fibrosis.

Verrucous carcinoma in epidermolysis bullosa simplex is possibly associated with a novel mutation in the keratin 5 gene

Epidermolysis bullosa simplex (EBS) is mainly caused by mutations in the KRT5 and KRT14 genes. Squamous cell carcinoma (SCC) represents the second most frequent skin neoplasia with complex aetiology. The molecular events disrupting the orchestrated interplay between the cytoskeleton, cell adhesion molecules and signalling proteins are ill understood in SCC. We describe the molecular background and the unusual course of the disease in a patient with EBS Dowling-Meara, severe keratoderma and a massive verrucous carcinoma. Skin and tumour samples from the patient were analysed using light microscopy, immunohistochemistry and immunofluorescence mapping. Mutation analysis of the KRT5 and KRT14 genes identified the novel KRT5 mutation p.E477D. Invasive tumour areas were characterized by downregulation of keratins 5 and 14, reduced and irregular desmocollin-2 expression and increased expression of keratins 6, 16 and 17. Levels of Ki-67 were increased and levels of E-cadherin strongly reduced in the tumour tissue. In this case a novel KRT5 mutation led to increased fragility of keratinocytes. Desmosome and adherens junctions were destabilized, which may trigger keratinocyte-mediated inflammation, possibly via p120-catenin-dependent signalling, suggesting a link between a keratin mutation and SCC, which adds weight to the hypothesis that disturbance of the cytoskeleton represents a major cause in the appearance of the malignant phenotype. Some individuals with EBS may be at risk of developing secondary SCC.

Keratins mediate localization of hemidesmosomes and repress cell motility

The keratin (K)-hemidesmosome (HD) interaction is crucial for cell-matrix adhesion and migration in several epithelia, including the epidermis. Mutations in constituent proteins cause severe blistering skin disorders by disrupting the adhesion complex. Despite extensive studies, the role of keratins in HD assembly and maintenance is only partially understood. Here we address this issue in keratinocytes in which all keratins are depleted by genome engineering. Unexpectedly, such keratinocytes maintain many characteristics of their normal counterparts. However, the absence of the entire keratin cytoskeleton leads to loss of plectin from the hemidesmosomal plaque and scattering of the HD transmembrane core along the basement membrane zone. To investigate the functional consequences, we performed migration and adhesion assays. These revealed that, in the absence of keratins, keratinocytes adhere much faster to extracellular matrix substrates and migrate approximately two times faster compared with wild-type cells. Reexpression of the single keratin pair K5 and K14 fully reversed the above phenotype. Our data uncover a role of keratins, which to our knowledge is previously unreported, in the maintenance of HDs upstream of plectin, with implications for epidermal homeostasis and pathogenesis. They support the view that the downregulation of keratins observed during epithelial-mesenchymal transition supports the migratory and invasive behavior of tumor cells.

Involvement of the ubiquitin-proteasome system in the stabilization of cell-cell contacts in human keratinocytes

Desmosomes are highly organized intercellular junctions composed of a number of interacting proteins that provide mechanical integrity to epithelial tissues. Mutations in genes encoding desmosomal proteins, including desmoplakin (DP), are associated with human hereditary diseases affecting skin integrity. The detailed mechanism of desmosome assembly remains, despite many efforts, incompletely understood. Recently, the ubiquitin-proteasome system (UPS) has been suggested to be an important regulatory system for the proper intracellular trafficking of proteins. Here, we provide evidence for a calcium-independent, but UPS-dependent, stabilization of cell-cell contacts in human keratinocytes, which might be mediated by the maintenance of DP at desmosomes.

Targeting the palm: a leap forward toward treatment of keratin disorders

Any rational therapy benefits from an understanding of basic biology and the simplicity of its strategy. Among keratinopathies, epidermolytic palmoplantar keratoderma stands out by virtue of hotspot mutations in the KRT9 gene, exclusively expressed in the palmoplantar epidermis. In this issue, Leslie Pedrioli et al. report on the successful application of KRT9-specific siRNAs in cultured cells and in a mouse model. The study beautifully illustrates the potency of a thorough experimental approach and the challenges that remain, especially in its delivery.

Keratin function and regulation in tissue homeostasis and pathogenesis

Epithelial tissues act as hubs in metabolism and communication and protect the organism against dehydration, infections, pharmacological and physical stress. Keratin intermediate filament proteins are well established as major cytoskeletal players in maintaining epithelial integrity. More recently, an involvement of keratins in growth control and organelle functions has emerged. Disruption of the keratin cytoskeleton by mutations or its reorganization following posttranslational modifications can render epithelia susceptible to tissue damage and various stresses, while loss of keratin expression is a hallmark of epithelial-mesenchymal transition (EMT). To understand the molecular mechanisms by which keratins perform their functions remains a formidable challenge. Based on selected examples, we will discuss how cell-specific expression of keratin isotypes affects cytoarchitecture and cell behavior. Further, we ask how posttranslational modifications alter keratin organization and interactions during signaling. Next, we discuss pathomechanisms of epidermal keratin disorders in the light of novel data. Finally, we raise open questions and point out future directives.

Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia

Epithelia are exposed to multiple forms of stress. Keratin intermediate filaments are abundant in epithelia and form cytoskeletal networks that contribute to cell type–specific functions, such as adhesion, migration, and metabolism. A perpetual keratin filament turnover cycle supports these functions. This multistep process keeps the cytoskeleton in motion, facilitating rapid and protein biosynthesis–independent network remodeling while maintaining an intact network. The current challenge is to unravel the molecular mechanisms underlying the regulation of the keratin cycle in relation to actin and microtubule networks and in the context of epithelial tissue function.

Placental vasculogenesis is regulated by keratin-mediated hyperoxia in murine decidual tissues

The mammalian placenta represents the interface between maternal and embryonic tissues and provides nutrients and gas exchange during embryo growth. Recently, keratin intermediate filament proteins were found to regulate embryo growth upstream of the mammalian target of rapamycin pathway through glucose transporter relocalization and to contribute to yolk sac vasculogenesis through altered bone morphogenetic protein 4 signaling. Whether keratins have vital functions in extraembryonic tissues is not well understood. Here, we report that keratins are essential for placental function. In the absence of keratins, we find hyperoxia in the decidual tissue directly adjacent to the placenta, because of an increased maternal vasculature. Hyperoxia causes impaired vasculogenesis through defective hypoxia-inducible factor 1α and vascular endothelial growth factor signaling, resulting in invagination defects of fetal blood vessels into the chorion. In turn, the reduced labyrinth, together with impaired gas exchange between maternal and embryonic blood, led to increased hypoxia in keratin-deficient embryos. We provide evidence that keratin-positive trophoblast secretion of prolactin-like protein a (Prlpa) and placental growth factor (PlGF) during decidualization are altered in the absence of keratins, leading to increased infiltration of uterine natural killer cells into placental vicinity and increased vascularization of the maternal decidua. Our findings suggest that keratin mutations might mediate conditions leading to early pregnancy loss due to hyperoxia in the decidua.

Keratins regulate yolk sac hematopoiesis and vasculogenesis through reduced BMP-4 signaling

Keratin intermediate filament proteins form the major cytoskeleton in all embryonic and adult epithelia. Increasing evidence suggests that keratins, besides their primary cytoskeletal function, can act as scaffolds which locally regulate cell growth and survival in epithelial cells. Many of these functions, however, are not understood in full, owing to keratin redundancy. We have recently created mice which lack all keratins and found that keratins act upstream of mTOR signaling to regulate protein biosynthesis via GLUT localization. Here, we report that keratins are necessary to maintain adhesion between endodermal and mesodermal cell layers of the yolk sac. As a consequence, keratin(-/-) embryos suffer from reduced yolk sac hematopoiesis and vasculogenesis. Pathway analysis revealed a reduction of the hedgehog target Foxf1 in yolk sac mesoderm of keratin(-/-) embryos, and subsequent reduction of BMP-4 and P-p38 MAPK. These defects may be caused by the overall reduction in protein biosynthesis and diminished adhesion. Our data show for the first time that keratins are necessary for the differentiation of a non-epithelial cell lineage through a combination of mechanical and signaling mechanisms.

Keratins regulate protein biosynthesis through localization of GLUT1 and -3 upstream of AMP kinase and Raptor

Removal of the entire keratin family of intermediate filament proteins from embryonic epithelia has surprising implications for mTOR signaling.

Keratin intermediate filament proteins form cytoskeletal scaffolds in epithelia, the disruption of which affects cytoarchitecture, cell growth, survival, and organelle transport. However, owing to redundancy, the global function of keratins has not been defined in full. Using a targeted gene deletion strategy, we generated transgenic mice lacking the entire keratin multiprotein family. In this study, we report that without keratins, embryonic epithelia suffer no cytolysis and maintain apical polarity but display mislocalized desmosomes. All keratin-null embryos die from severe growth retardation at embryonic day 9.5. We find that GLUT1 and -3 are mislocalized from the apical plasma membrane in embryonic epithelia, which subsequently activates the energy sensor adenosine monophosphate kinase (AMPK). Analysis of the mammalian target of rapamycin (mTOR) pathway reveals that AMPK induction activates Raptor, repressing protein biosynthesis through mTORC1's downstream targets S6 kinase and 4E-binding protein 1. Our findings demonstrate a novel keratin function upstream of mTOR signaling via GLUT localization and have implications for pathomechanisms and therapy approaches for keratin disorders and the analysis of other gene families.

Mechanisms of fibroblast cell therapy for dystrophic epidermolysis bullosa: high stability of collagen VII favors long-term skin integrity

Here, we report on the first systematic long-term study of fibroblast therapy in a mouse model for recessive dystrophic epidermolysis bullosa (RDEB), a severe skin-blistering disorder caused by loss-of-function of collagen VII. Intradermal injection of wild-type (WT) fibroblasts in >50 mice increased the collagen VII content at the dermal-epidermal junction 3.5- to 4.7-fold. Although the active biosynthesis lasted <28 days, collagen VII remained stable and dramatically improved skin integrity and resistance to mechanical forces for at least 100 days, as measured with a digital 3D-skin sensor for shear forces. Experiments using species-specific antibodies, collagen VII-deficient fibroblasts, gene expression analyses, and cytokine arrays demonstrated that the injected fibroblasts are the major source of newly deposited collagen VII. Apart from transitory mild inflammation, no adverse effects were observed. The cells remained within an area Collapse

Key Words Collapse

MESH Headings

• Animals
• Animals, Newborn
• Blotting, Western
• Cell- and Tissue-Based Therapy/methods
• Cells, Cultured
• Collagen Type VII/metabolism
• Epidermolysis Bullosa Dystrophica/metabolism
• Epidermolysis Bullosa Dystrophica/pathology
• Epidermolysis Bullosa Dystrophica/therapy
• Fibroblasts/metabolism
• Fibroblasts/physiology
• Fibroblasts/ultrastructure
• Flow Cytometry
• Humans
• In Situ Nick-End Labeling
• Mice
• Mice, Transgenic
• Microscopy, Electron, Transmission
• Reverse Transcriptase Polymerase Chain Reaction
• Skin/metabolism
• Skin/pathology

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Grants Collapse

Cytokines as genetic modifiers in K5-/- mice and in human epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is a skin disorder caused by fully-penetrant mutations in the keratin genes KRT5 and KRT14, leading to extensive cytolysis and cell fragility of basal keratinocytes. EBS is subject to environmental conditions and displays high intra- and interfamilial variability, suggesting modifying loci. Here, we demonstrate that upregulation of certain cytokines accompanies mutations in keratin 5 (K5) but not in keratin 14 (K14). We find for the first time that cytokines macrophage chemotactic protein (MCP)-1/[chemokine (C-C motif) ligand 2] (CCL2), macrophage inflammatory protein (MIP)-3beta/CCL19 and MIP-3alpha/CCL20, all regulated by nuclear factor kappa B (NFkappaB) and involved in the recruitment, maturation, and migration of Langerhans cells (LCs) in the epidermis, are upregulated in the skin of K5(-/-), but not of K14(-/-) mice. In neonatal K5(-/-) epidermis, the number of LCs was increased two-fold. At the same time, tumor necrosis factor alpha (TNFalpha) remained unaltered, demonstrating the specificity of that process. Most remarkably, enhanced LC recruitment within the epidermis was found in five EBS patients carrying mutations in the KRT5 gene but not in EBS patients with KRT14 gene mutations. In agreement with the NFkappaB-dependent regulation of these cytokines, we found a decrease in p120-catenin in the basal epidermis of K5(-/-) mice. These data provide the first explanation for distinct, keratin-type-specific genotype-phenotype correlations in EBS and represent a rationale to investigate gene loci affecting skin pathology in EBS.

Rescue of atypical protein kinase C in epithelia by the cytoskeleton and Hsp70 family chaperones

Atypical PKC (PKC iota) is a key organizer of cellular asymmetry. Sequential extractions of intestinal cells showed a pool of enzymatically active PKC iota and the chaperone Hsp70.1 attached to the apical cytoskeleton. Pull-down experiments using purified and recombinant proteins showed a complex of Hsp70 and atypical PKC on filamentous keratins. Transgenic animals overexpressing keratin 8 displayed delocalization of Hsp70 and atypical PKC. Two different keratin-null mouse models, as well as keratin-8 knockdown cells in tissue culture, also showed redistribution of Hsp70 and a sharp decrease in the active form of atypical PKC, which was also reduced by Hsp70 knockdown. An in-vitro turn motif rephosphorylation assay indicated that PKC iota is dephosphorylated by prolonged activity. The Triton-soluble fraction could rephosphorylate PKC iota only when supplemented with the cytoskeletal pellet or filamentous highly purified keratins, a function abolished by immunodepletion of Hsp70 but rescued by recombinant Hsp70. We conclude that both filamentous keratins and Hsp70 are required for the rescue rephosphorylation of mature atypical PKC, regulating the subcellular distribution and steady-state levels of active PKC iota.

Keratin 18 provides resistance to Fas-mediated liver failure in mice

BACKGROUND

Keratins are intermediate filament proteins of epithelial cells with pivotal functions for cell integrity. They comprise keratins 18 [K18] and 8 [K8] in hepatocytes. Keratins are of major importance for an intact cellular microarchitecture and have protective functions in human liver diseases. In mice, K8 has been demonstrated to protect against Fas-antibody-induced liver failure by direct interaction with apoptotic regulators, while the role of K18 remains unresolved.

MATERIALS AND METHODS

We analysed effects of K18 deficiency on Fas-induced liver failure in mice. We determined survival and analysed induction of apoptosis after injection of the agonistic Fas antibody Jo2 into K18(-/-) and wild-type control mice by TUNEL assay and fluorometrically analysed caspase-3, -8 and -9 activities 1, 2 and 3 h after Jo2 injection.

RESULTS

In K18(-/-) mice, survival of Fas-antibody treated mice was significantly shorter than that of wild-type controls (P = 0.02). However, shortened survival of K18(-/-) mice was caused by increased hepatic damage but was not correlated to enhanced induction of apoptotic pathways, as neither numbers of TUNEL positive apoptotic cells nor activities of caspases-3, -8 and -9 differed between K18(-/-) and K18(+/+) mice at any point of time.

CONCLUSION

K18(-/-) mice are significantly more susceptible to Fas-antibody-induced liver failure. The cytoprotective effect of K18 is not explained by a differential activation of caspases-3, -8 and -9, suggesting that K18 does not directly interfere with apoptotic regulators. Importantly, however, K18 exerts significant protective functions by other mechanisms.

Dominant cataract formation in association with a vimentin assembly disrupting mutation

Cataracts are characterized by an opacification of the eye lens, often caused by protein misfolding and aggregation. The intermediate filament protein vimentin, which is highly expressed in lens fiber cells and in mesenchymal tissues, is a main structural determinant in these cells forming a membrane-connected cytoskeleton. Additional functions of vimentin remain to be identified. Here, we demonstrate that a mutation in VIM causes a dominant, pulverulent cataract. We sequenced the complete human VIM gene in 90 individuals suffering from congenital cataract and found a G596A change in exon 1 in a single individual, causing the missense mutation E151K in coil 1B of vimentin. The mutant vimentin formed an aberrant vimentin cytoskeleton and increased the proteasome activity in transfected cells. Furthermore, this mutation causes a severe kinetic defect in vimentin assembly both in vitro and in vivo. Hence, in conjunction with available mouse and cell culture models, our results reveal for the first time an important functional role for vimentin in the maintenance of lens integrity. Finally, this invites novel therapy approaches for cataracts.

A dominant vimentin mutant upregulates Hsp70 and the activity of the ubiquitin-proteasome system, and causes posterior cataracts in transgenic mice

Vimentin is the main intermediate filament (IF) protein of mesenchymal cells and tissues. Unlike other IF-/- mice, vimentin-/- mice provided no evidence of an involvement of vimentin in the development of a specific disease. Therefore, we generated two transgenic mouse lines, one with a (R113C) point mutation in the IF-consensus motif in coil1A and one with the complete deletion of coil 2B of the rod domain. In epidermal keratins and desmin, point mutations in these parts of the alpha-helical rod domain cause keratinopathies and desminopathies, respectively. Here, we demonstrate that substoichiometric amounts of vimentin carrying the R113C point mutation disrupted the endogenous vimentin network in all tissues examined but caused a disease phenotype only in the eye lens, leading to a posterior cataract that was paralleled by the formation of extensive protein aggregates in lens fibre cells. Unexpectedly, central, postmitotic fibres became depleted of aggregates, indicating that they were actively removed. In line with an increase in misfolded proteins, the amounts of Hsp70 and ubiquitylated vimentin were increased, and proteasome activity was raised. We demonstrate here for the first time that the expression of mutated vimentin induces a protein-stress response that contributes to disease pathology in mice, and hypothesise that vimentin mutations cause cataracts in humans.

Induction of inflammatory cytokines by a keratin mutation and their repression by a small molecule in a mouse model for EBS

Epidermolysis bullosa simplex (EBS) is a skin disorder caused by mutations in keratin (K) 5 or K14 genes. It is widely regarded as a mechanobullous disease, resulting from a weakened cytoskeleton, causing extensive cytolysis. It was postulated by others that certain K14 mutations induce tumor necrosis factor-alpha (TNF-alpha) and increase apoptosis. Here, we report that in K5-/- mice and in a cell culture model of EBS, the mRNA and protein levels of TNF-alpha remain unaltered. Transcriptome analysis of K5-/- mice revealed, however, that the proinflammatory cytokines IL-6 and IL-1beta were significantly upregulated at the mRNA level in K5-/- mouse skin. These results were confirmed by TaqMan real-time PCR and ELISA assays. We hypothesize that keratin mutations contribute to EBS in a mouse model by inducing local inflammation that mediates a stress response. Following clinical reports, we applied the small molecule doxycycline to K5-/- mice. We demonstrate that doxycycline extended the survival of neonatal K5-/- mice from less than 1 to up to 8 hours. Microarray and TaqMan real-time PCR showed a downregulation of matrix metalloproteinase 13 and IL-1beta, indicating an effect of doxycycline on transcription. Our data offer a novel small molecule-based therapy approach for EBS.

A mutation of keratin 18 within the coil 1A consensus motif causes widespread keratin aggregation but cell type-restricted lethality in mice

Mutations in genes encoding epidermal keratins cause skin disorders, while those in internal epithelial keratins, such as K8 and K18, are risk factors for liver diseases. The effect of dominant mutations in K8 or K18 during embryonic development and tissue homeostasis has not been examined so far. Here we demonstrate that the dominant mutation hK18 R89C, that is highly similar to hK14 R125C, causing EBS in humans, leads to cell type-specific lethality in mice, depending on the ratio of mutant to endogenous keratins. Mice expressing hK18 R89C in the absence of endogenous K19 and K18 died at mid-gestation from defects in trophoblast giant cells, accompanied by haematomas. A single, endogenous K18 allele rescued embryonic lethality but caused aggregation of keratins in all adult internal epithelia, surprisingly without spontaneous cell fragility. Closer analysis revealed that both filaments and aggregates coexisted in the same cell, depending on the ratio of mutant to endogenous keratins. Our results demonstrate that balanced overexpression of a wild-type keratin rescued the lethal consequences of a dominant-negative mutation. This has important implications for therapy approaches of keratinopathies, suggesting that suppressing the mutant allele is not necessary in vivo.

Identification of a keratin-associated protein with a putative role in vesicle transport

Protection of skin against UV light requires a coordinated interaction between melanocytes and keratinocytes. Melanosomes are lysosome-related organelles that originate in melanocytes and are transferred into keratinocytes where they form a supranuclear cap. The mechanism responsible for melanosome transfer into keratinocytes and their intracellular distribution is poorly understood. Recently, we reported for the first time that loss-of-function mutations in the keratin K5 gene affect melanosome distribution in keratinocytes and results in a reticulate hyperpigmentation disorder, called Dowling-Degos disease. Here, we characterise the distribution and behaviour of individual K5 and K14 domains following transient and stable transfection into cells. We report that the K5 head domain is considerably more stable than the K14 head. Moreover, the distribution of the K5 head domain is altered following depolymerisation of microtubules. Following co-immunoprecipitation, we verified a specific interaction between the head domain of K5 with Hsc70, a chaperone also involved in vesicle uncoating. We hypothesise that this interaction is involved in melanosome formation or transport in keratinocytes. Alternatively, it may have a general function in the regulation of keratin assembly.

Structural and regulatory functions of keratins

The diversity of epithelial functions is reflected by the expression of distinct keratin pairs that are responsible to protect epithelial cells against mechanical stress and to act as signaling platforms. The keratin cytoskeleton integrates these functions by forming a supracellular scaffold that connects at desmosomal cell-cell adhesions. Multiple human diseases and murine knockouts in which the integrity of this system is destroyed testify to its importance as a mechanical stabilizer in certain epithelia. Yet, surprisingly little is known about the precise mechanisms responsible for assembly and disease pathology. In addition to these structural aspects of keratin function, experimental evidence accumulating in recent years has led to a much more complex view of the keratin cytoskeleton. Distinct keratins emerge as highly dynamic scaffolds in different settings and contribute to cell size determination, translation control, proliferation, cell type-specific organelle transport, malignant transformation and various stress responses. All of these properties are controlled by highly complex patterns of phosphorylation and molecular associations.

Keratin transgenic and knockout mice: functional analysis and validation of disease-causing mutations

The intermediate filament (IF) cytoskeleton of mammalian epithelia is generated from pairs of type I and type II keratins that are encoded by two large gene families, made up of 54 genes in humans and the mouse. These genes are expressed in a spatiotemporal and tissue-specific manner from the blastocyst stage onward. Since the discovery of keratin mutations leading to epidermolysis bullosa simplex, mutations in at least 18 keratin genes have been identified that result in keratinopathies of the epidermis and its appendages. Recently, noncanonical mutations in simple epithelial keratins were associated with pancreatic, liver, and intestinal disorders, demonstrating that keratins protect epithelia against mechanical and other forms of stress. In recent years, animal models provided novel insight and significantly improved understanding of IF function in tissue homeostasis and its role in disease. Pathological phenotypes detected in mutant mice generated so far range from embryonic lethality to tissue fragility to subtlety, which often depends on their genetic background. This range implies at least a partial influence of yet unidentified modifier genes on the phenotype after the ablation of the respective keratin. To date, nearly all available keratin mouse models were generated by taking advantage of conventional gene-targeting strategies. To reveal their cell type-specific functions and the mechanisms by which mutations lead to disease, it will be necessary to use conditional gene-targeting strategies and the introduction of point-mutated gene copies. Furthermore, conditional strategies offer the possibility to overcome embryonic or neonatal lethality in some of the keratin-deficient mice.

Keratin 8 modulation of desmoplakin deposition at desmosomes in hepatocytes

Keratins, the intermediate filament proteins of epithelial cells, connect to desmosomes, the cell-cell adhesion structures at the surface membrane. The building elements of desmosomes include desmoglein and desmocollin, which provide the actual cell adhesive properties, and desmoplakins, which anchor the keratin intermediate filaments to desmosomes. In the work reported here, we address the role of keratin 8 in modulating desmoplakin deposition at surface membrane in mouse hepatocytes. The experimental approach is based on the use of keratin 8- and keratin 18-null mouse hepatocytes as cell models. In wild-type mouse hepatocytes, desmoplakin is aligned with desmoglein and keratin 8 at the surface membrane. In keratin 8-null hepatocytes, the intermediate filament loss leads to alterations in desmoplakin distribution at the surface membrane, but not of desmoglein. Intriguingly, a significant proportion of keratin 18-null hepatocytes express keratin 8 at the surface membrane, associated with a proper desmoplakin alignment with desmoglein at desmosomes. A Triton treatment of the monolayer reveals that most of the desmoplakin present in either wild-type, keratin 8- or keratin 18-null hepatocytes is insoluble. Deletion analysis of keratin 8 further suggests that the recovery of desmoplakin alignment requires the keratin 8 rod domain. In addition, similarly to other works revealing a key role of desmoplakin phosphorylation on its interaction with intermediate filaments, we find that the phosphorylation status of the keratin 8 head domain affects desmoplakin distribution at desmosomes. Together, the data indicate that a proper alignment/deposition of desmoplakin with keratins and desmoglein in hepatocytes requires keratin 8, through a reciprocal phosphoserine-dependent process.

New consensus nomenclature for mammalian keratins

Keratins are intermediate filament–forming proteins that provide mechanical support and fulfill a variety of additional functions in epithelial cells. In 1982, a nomenclature was devised to name the keratin proteins that were known at that point. The systematic sequencing of the human genome in recent years uncovered the existence of several novel keratin genes and their encoded proteins. Their naming could not be adequately handled in the context of the original system. We propose a new consensus nomenclature for keratin genes and proteins that relies upon and extends the 1982 system and adheres to the guidelines issued by the Human and Mouse Genome Nomenclature Committees. This revised nomenclature accommodates functional genes and pseudogenes, and although designed specifically for the full complement of human keratins, it offers the flexibility needed to incorporate additional keratins from other mammalian species.

Denaturing temperature selection may underestimate keratin mutation detection by DHPLC

Keratins 8 and 18 (KRT8 and KRT18 genes; K8 and K18 proteins) variants are risk factors for developing end-stage liver disease and may be associated with inflammatory bowel disease and chronic pancreatitis. The frequency of K8/K18 variants in American, British, German, and Italian populations differs. For example, one study showed no amino acid-altering K8/K18 mutations in 256 German patients with liver disorders, while another found 58 out of 467 American liver disease patients with K8/K18 mutations. Both studies used the WaveSystem, which utilizes DHPLC. We hypothesized that experimental conditions contribute to the discrepancy, and we tested this hypothesis using previously described K8/K18 variants and a novel KRT18 c.1057C>G variant (K18 p.R353G) to optimize the DHPLC conditions in 10 examined exons under a range of denaturing temperatures. Six of 16 tested variants in three of the 10 exons, including the frequent KRT8 c.184G>T (K8 p.G62C), KRT8 c.187A>G (K8 p.I63V), and KRT8 c.1022G>A (K8 p.R341H), could not be reliably detected when using temperatures suggested by the prediction software, but all these variants were readily detectable at 2 degrees C higher denaturing temperatures. Using optimized temperatures, we then tested available genomic DNA from 151 out of the 256 German liver disease patients for the presence of K8 variants in exons 1 and 6, where most of the American cohort K8 variants occur. We identified 12 exonic and two intronic K8 variants: one KRT8 c.184G>T (K8 p.G62C), two KRT8 c.187A>G (K8 p.I63V), seven KRT8 c.1022G>A (K8 p.R341H), one KRT8 c.1128G>A (K8 p.E376E), two intronic KRT8 c.1202+46 A>T, and one hitherto undescribed KRT8 c.1138G>A (K8 p.V380I). Therefore, although DHPLC offers a robust and high throughput means for mutation analysis, assessment of denaturing temperature ranges, and possible inclusion of control mutants should be considered.

Keratin 5 knockout mice reveal plasticity of keratin expression in the corneal epithelium

We have recently demonstrated that the keratin K3 gene, which is active in the suprabasal human corneal epithelium, is missing in the genome of the mouse. We show that a normal K3 gene exists in a wide variety of mammals while in rodents the gene is converted to a pseudogene with a very strong sequence drift. The availability of K5-/- mice provides a unique opportunity to investigate type-specific keratin function during corneal differentiation in the absence of both K5 and K3. Here, we report that the deletion of K5, which in wild-type mice forms a cytoskeleton with K12, does neither cause keratin aggregation nor cytolysis in the cornea. This is due to the induction of K4 in corneal epithelial cells, normally restricted to corneal stem stem cells residing in the limbus. Using a combination of antibodies and RT-PCR, we identified additional keratins expressed in the mouse cornea including K23 which was previously thought to be specific for pancreatic carcinomas. This reflects an unexpected complexity of keratin expression in the cornea. Our data suggest that in the absence of mechanical stress, corneal differentiation does not depend on distinct keratin pairs, supporting a concept of functional redundancy, at least for certain keratins.

Loss-of-function mutations in the keratin 5 gene lead to Dowling-Degos disease

Dowling-Degos disease (DDD) is an autosomal dominant genodermatosis characterized by progressive and disfiguring reticulate hyperpigmentation of the flexures. We performed a genomewide linkage analysis of two German families and mapped DDD to chromosome 12q, with a total LOD score of 4.42 ( theta =0.0) for marker D12S368. This region includes the keratin gene cluster, which we screened for mutations. We identified loss-of-function mutations in the keratin 5 gene (KRT5) in all affected family members and in six unrelated patients with DDD. These represent the first identified mutations that lead to haploinsufficiency in a keratin gene. The identification of loss-of-function mutations, along with the results from additional functional studies, suggest a crucial role for keratins in the organization of cell adhesion, melanosome uptake, organelle transport, and nuclear anchorage.

Mutations in vimentin disrupt the cytoskeleton in fibroblasts and delay execution of apoptosis

To get new insights into the function of the intermediate filament (IF) protein vimentin in cell physiology, we generated two mutant cDNAs, one with a point mutation in the consensus motif in coil1A (R113C) and one with the complete deletion of coil 2B of the rod domain. In keratins and glia filament protein (GFAP), analogous mutations cause keratinopathies and Alexander disease, respectively. Both mutants prevented filament assembly in vitro and inhibited assembly of wild-type vimentin when present in equal amounts. In stably transfected preadipocytes, these mutants caused the complete disruption of the endogenous vimentin network, demonstrating their dominant-negative behaviour. Cytoplasmic vimentin aggregates colocalised with the chaperones alphaB-crystallin and HSP40. Moreover, vimR113C mutant cells were more resistant against staurosporine-induced apoptosis compared to controls. We hypothesise that mutations in the vimentin gene, like in most classes of IF genes, may contribute to distinct human diseases.

Type II keratins precede type I keratins during early embryonic development

We and others have recently demonstrated that the keratin (K) gene family in mammals is even more complex than previously thought [Eur. J. Cell Biol. 83, 19-26]. To address the function of keratins during early development, precise information on their spatio-temporal expression is required. Here, we examined the expression of selected mouse keratins from pre-implantation to mid-gestational embryonic stages using RT-PCR and immunofluorescence. At E0.5, transcripts encoding K5, K6, K7, K8, K14, K15, K18, and K19 are apparently absent. We report on a post-transcriptional regulation of type I keratins, preventing filament formation in 8- to 16-cell stage embryos. In these embryos, mRNAs coding for K7, K8, K18, and K19 are present, but only K7 and K8 are translated into protein which is deposited in aggregates. Following the accumulation of K18 protein at E3.5, keratin filaments are formed. Delayed onset of type I keratin protein expression was additionally observed in later embryonic stages for K5 and K14. K5 protein expression starts in the forelimb surface ectoderm as early as E9.25, while the expression of its partner, K14, begins at E9.75. From E9.25 to E9.75, K5 forms atypical filaments with K18. Remarkably, in embryonic K5-/- mice, K14 formed normal filaments until E12.5 despite the absence of its partner K5, due to the presence of K8. Following periderm formation, K14-containing filaments disappeared and K14 became localized in aggregates in basal keratinocytes. Despite the absence of a keratin cytoskeleton, there was no cytolysis. We suggest that the formation of the first embryonic cytoskeleton from soluble keratins is regulated by unknown mechanisms. Whether the premature expression of type II keratins relates to their proposed role in TNF- and Fas-mediated signalling is presently unknown.

A fast and robust quantitative time-lapse assay for cell migration

We describe a simple and widely applicable method to measure cell migration in time-lapse sequences of fluorescently labeled cells in culture. Briefly, binarized cell images obtained after thresholding were cumulatively projected, and the covered areas were measured. This procedure determines the time course of the track area successively covered by the cell population. Under conditions where cell growth is negligible, a robust index of cell motility is derived from normalized plots for the displacement of cells over time. We applied this method to quantitatively examine the migration of B35 neuroblastoma cells transiently expressing GFP and to C6 glioma cells after staining with Hoechst 33258. This sensitive assay detected the influence of agents which inhibit actin polymerization (cytochalasin B) or interfere with the maintenance of cell polarity (methyl-beta-cyclodextrin) on cell migration. Thus, this assay is a versatile tool to measure quickly the migration of different cell types using different labeling strategies.

Loss of keratin 10 is accompanied by increased sebocyte proliferation and differentiation

Here, we present strong evidence that the targeted deletion of keratin 10 (K10) alters sebocyte differentiation in mice, mediated by an increased proliferation and differentiation of cells located in the periphery of the glands. This was not accompanied by the induction of the proliferation-associated keratins K6, K16 and K17. Sebaceous gland cells of K10-/- mice showed an accelerated turnover and secreted more sebum including wax esters, triglycerides, and cholesterol esters. The levels of the major epidermal lipids ceramides and cholesterol were also increased, whereas glycosylceramides and sphingomyelin were decreased which was not based on altered sphingolipid biosynthesis. The amount of Cer(OS), covalently bound to the cornified envelope, remained unchanged, as well as the amount of loricrin and involucrin. In agreement with the unaltered expression of beta-catenin and its targets cyclin D1 and c-Myc, we conclude that the altered composition of the suprabasal intermediate filament cytoskeleton in K10-/- mice increased the differentiation of epidermal stem cells towards the sebocyte lineage.

Rescue of keratin 18/19 doubly deficient mice using aggregation with tetraploid embryos

We have previously shown that the targeted deletions of both type I keratins (K) 18 and 19 cause lethality by embryonic day (e) 9.5 due to fragility and cytolysis of trophoblast giant cells. The development of the embryo proper appeared to be unaffected and its death was caused by nutrient deficiency. In order to address the function of keratins within the embryo proper, lethality due to extraembryonic tissue failure must be overcome. One approach to rescue doubly deficient embryos is by aggregating knockout embryos with tetraploid wild-type embryos. As a general tool, tetraploid aggregation can be used to rescue embryonic lethality caused by defects in extraembryonic tissues like the placenta, trophoblast or yolk sac. We rescued K18-/- K19-/- embryos until e11.5, using this approach, proving that the loss of the keratin cytoskeleton causes defects in the trophoblast giant cell layer, but has no effect on early development of the embryo proper.

Cx31 andCx43 double-deficient mice reveal independent functions in murine placental and skin development

The overlapping expression of gap junctional connexins in tissues has indicated that the channels may compensate for each other. During development, Cx31 and Cx43 are coexpressed in preimplantation embryos, in the spongiotrophoblast of the placenta and in the epidermis. This study shows that Cx31/Cx43 double-deficient mice exhibit the known phenotypes of the single-knockout strains but no combined effects. Thus, Cx43, coexpressed with Cx31 at midgestation in the spongiotrophoblast of the placenta, cannot be responsible for a partial rescue of the lethal Cx31 knockout phenotype, as assumed before (Plum et al. [2001] Dev Biol 231:334-337). It follows that both connexins have unique functions in placental development. Despite an altered expression of other epidermal connexin mRNAs, epidermal differentiation and physiology was unaltered by the absence of Cx31 and Cx43. Therefore, in epidermal and preimplantation development, gap junctional communication can probably be compensated by other isoforms coexpressed with Cx31 and Cx43.

Emerging functions: diseases and animal models reshape our view of the cytoskeleton

Intermediate filaments (IFs), desmosomes, and their associates are built from multidomain proteins that form cytoskeletal scaffolds in the cytoplasm and the nucleus of vertebrate tissues. Mutations in more than 80 genes cause monogenic disorders that include severe skin fragility, myopathies, neurodegeneration, and premature ageing, and contribute to polygenic disorders including liver and inflammatory bowel disease. First interpreted as "mechanical weakness" disorders resulting from a weakened cytoskeleton, emerging data support the concept that changes in cytoskeletal architecture profoundly alter signal transduction and cellular transcription patterns. This is in line with cell type-specific interactions between cytoskeletal and their associated proteins, and may involve both soluble and insoluble forms of intermediate filament proteins. Understanding how mutation-induced disruption of the cytoskeleton and its upstream regulators causes disease at the molecular level presents one of the major challenges in future research.

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

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

Comprehensive analysis of keratin gene clusters in humans and rodents

Here, we present the comparative analysis of the two keratin (K) gene clusters in the genomes of man, mouse and rat. Overall, there is a remarkable but not perfect synteny among the clusters of the three mammalian species. The human type I keratin gene cluster consists of 27 genes and 4 pseudogenes, all in the same orientation. It is interrupted by a domain of multiple genes encoding keratin-associated proteins (KAPs). Cytokeratin, hair and inner root sheath keratin genes are grouped together in small subclusters, indicating that evolution occurred by duplication events. At the end of the rodent type I gene cluster, a novel gene related to K14 and K17 was identified, which is converted to a pseudogene in humans. The human type II cluster consists of 27 genes and 5 pseudogenes, most of which are arranged in the same orientation. Of the 26 type II murine keratin genes now known, the expression of two new genes was identified by RT-PCR. Kb20, the first gene in the cluster, was detected in lung tissue. Kb39, a new ortholog of K1, is expressed in certain stratified epithelia. It represents a candidate gene for those hyperkeratotic skin syndromes in which no K1 mutations were identified so far. Most remarkably, the human K3 gene which causes Meesmann's corneal dystrophy when mutated, lacks a counterpart in the mouse genome. While the human genome has 138 pseudogenes related to K8 and K18, the mouse and rat genomes contain only 4 and 6 such pseudogenes. Our results also provide the basis for a unified keratin nomenclature and for future functional studies.

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

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

Keratins: a structural scaffold with emerging functions

Intermediate filament proteins form an essential part of the cytoskeleton and provide topological order to cells and tissues. These features result from their intrinsic property of self-organization and their response to extrinsic cues. Keratins represent the largest subgroup among all intermediate filament proteins and are differentially expressed as pairs of type I and type II intermediate filament proteins in epithelia. Their primary function is to impart mechanical strength to cells. This function is illustrated by patients with keratin mutations and by gene-deficient mice. Additional functions include their participation in the response to stress, cell signalling and apoptosis, and thus the keratin cytoskeleton appears far more dynamic than previously anticipated. This may result from hyperphosphorylation and possibly from interaction with associated proteins. How signalling networks affect keratin organization, turnover and function and vice versa will be a major challenge for future investigations.

Defolliculated (dfl): a dominant mouse mutation leading to poor sebaceous gland differentiation and total elimination of pelage follicles

Defolliculated is a novel spontaneous mouse mutation that maps to chromosome 11 close to the type I keratin locus. Histology shows abnormal differentiation of the sebaceous gland, with the sebocytes producing little or no sebum and undergoing abnormal cornification. The hair follicles fail to regress during catagen leading to abnormally long follicles. In contrast the hair shafts are shorter than normal, suggesting altered differentiation or proliferation of matrix cells during anagen. The shafts emerge from the follicle with cornified material still attached. The dermis contains increased numbers of immune cells, including T cells (CD4-positive), macrophages, and mast cells, at all time points examined. Complete elimination of all pelage and tail follicles occurs after two to three hair cycles, apparently by necrosis. Defolliculated may be a useful model for determining further functions of the sebaceous gland, and for understanding the regulation of catagen and hair follicle immunology.

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

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

Ectopic expression of desmin in the epidermis of transgenic mice permits development of a normal epidermis

Cell architecture is largely based on the interaction of cytoskeletal proteins, which include intermediate filaments (IF), microfilaments, microtubules, as well as their type-specific membrane-attachment structures and associated proteins. In order to further our understanding of IF proteins and to address the fundamental issue whether different IF perform unique functions in different tissues, we expressed a desmin transgene in the basal epidermis of mice. Ectopic expression of desmin led to the formation of an additional, keratin-independent IF cytoskeleton and did not interfere with the keratin-desmosome interaction. We show that ectopic expression of a type III IF protein in basal keratinocytes did not interfere with the normal epidermal architecture and the program of terminal differentiation. This demonstrated that keratinocytes suffered no obvious detrimental effects from extra desmin filaments in their cytoplasm. In addition, we asked whether stable expression of desmin could rescue K5 null mice, which served as a model for severe EBS. Transgenic mice ectopically expressing desmin in the basal layer were mated with K5 heterozygous deficient animals to generate desmin rescue mice and analysed. In summary, our study support the notion that the different IF like desmin or keratins composing a IF network in vivo are central to cytoskeletal architecture and design in cells.

Hormone-sensitive lipase deficiency in mice causes diglyceride accumulation in adipose tissue, muscle, and testis

Hormone-sensitive lipase (HSL) is expressed predominantly in white and brown adipose tissue where it is believed to play a crucial role in the lipolysis of stored triglycerides (TG), thereby providing the body with energy substrate in the form of free fatty acids (FFA). From in vitro assays, HSL is known to hydrolyze TG, diglycerides (DG), cholesteryl esters, and retinyl esters. In the current study we have generated HSL knock-out mice and demonstrate three lines of evidence that HSL is instrumental in the catabolism of DG in vivo. First, HSL deficiency in mice causes the accumulation of DG in white adipose tissue, brown adipose tissue, skeletal muscle, cardiac muscle, and testis. Second, when tissue extracts were used in an in vitro lipase assay, a reduced FFA release and the accumulation of DG was observed in HSL knock-out mice which did not occur when tissue extracts from control mice were used. Third, in vitro lipolysis experiments with HSL-deficient fat pads demonstrated that the isoproterenol-stimulated release of FFA was decreased and DG accumulated intracellularly resulting in the essential absence of the isoproterenol-stimulated glycerol formation typically observed in control fat pads. Additionally, the absence of HSL in white adipose tissue caused a shift of the fatty acid composition of the TG moiety toward increased long chain fatty acids implying a substrate specificity of the enzyme in vivo. From these in vivo results we conclude that HSL is the rate-limiting enzyme for the cellular catabolism of DG in adipose tissue and muscle.

Disturbances in hepatic cell-cycle regulation in mice with assembly-deficient keratins 8/18

Simple epithelial tissues such as liver and pancreas express keratins 8 (K8) and 18 (K18) as their major intermediate filament proteins. K8 and K18 null mice and transgenic mice that express mutant K18 (K18C) manifest several hepatocyte abnormalities and demonstrate that K8/18 are important in maintaining liver tissue and cell integrity, although other potential functions remain uncharacterized. Here, we report an additional abnormal liver phenotype, which is similar in K8 null, K18 null, and K18C mouse models. Liver histologic examination showed large polynuclear areas that lacked cell membranes, desmosomal structures, and filamentous actin. Similar, but less prominent, areas were observed in the pancreas. The parenchyma outside the polynuclear areas displayed irregular sinusoidal structures and markedly enlarged nuclei. Most K8 null hepatocytes were positive for the proliferating cell nuclear antigen (PCNA) with a doubled DNA content in comparison with the predominantly PCNA-negative wild-type hepatocytes. The distribution of the 14-3-3zeta protein was also altered in K8 null mice. Taken together, our results indicate that absence of keratin filaments causes disturbances in cell-cycle regulation, driving cells into the S-G2 phase and causing aberrant cytokinesis. These effects could stem from disturbed functions of K8/18-dependent cell-cycle regulators, such as the signaling integrator, 14-3-3.

Genes for intermediate filament proteins and the draft sequence of the human genome

We screened the draft sequence of the human genome for genes that encode intermediate filament (IF) proteins in general, and keratins in particular. The draft covers nearly all previously established IF genes including the recent cDNA and gene additions, such as pancreatic keratin 23, synemin and the novel muscle protein syncoilin. In the draft, seven novel type II keratins were identified, presumably expressed in the hair follicle/epidermal appendages. In summary, 65 IF genes were detected, placing IF among the 100 largest gene families in humans. All functional keratin genes map to the two known keratin clusters on chromosomes 12 (type II plus keratin 18) and 17 (type I), whereas other IF genes are not clustered. Of the 208 keratin-related DNA sequences, only 49 reflect true keratin genes, whereas the majority describe inactive gene fragments and processed pseudogenes. Surprisingly, nearly 90% of these inactive genes relate specifically to the genes of keratins 8 and 18. Other keratin genes, as well as those that encode non-keratin IF proteins, lack either gene fragments/pseudogenes or have only a few derivatives. As parasitic derivatives of mature mRNAs, the processed pseudogenes of keratins 8 and 18 have invaded most chromosomes, often at several positions. We describe the limits of our analysis and discuss the striking unevenness of pseudogene derivation in the IF multigene family. Finally, we propose to extend the nomenclature of Moll and colleagues to any novel keratin.