Cracks in the foundation: keratin filaments and genetic disease

Potential localization of putative stem/progenitor cells in human bulbar conjunctival epithelium

Although the conjunctival fornix appears to contain the greatest proportion of stem cells, it is likely that pockets of conjunctival epithelial stem cells may also exist throughout the conjunctival epithelium. This study was to investigate the potential localization of putative stem/progenitor cells in the human bulbar conjunctival epithelium by evaluating 6 keratins and 13 molecules that have been previously proposed stem cell associated or differentiation markers. We found that cornea specific cytokeratin (CK) 3 was not expressed by the bulbar conjunctival epithelial cells. In contrast, CK4 and CK7 were expressed by the superficial cells of bulbar conjunctival epithelium. CK14 and CK15 were confined to the basal cell layer. CK19 was strongly expressed by all layers of the bulbar conjunctival epithelium. The expression patterns of molecular markers in the basal cells of human bulbar conjunctival epithelium were found to be similar to the corneal epithelium. Basal conjunctival epithelial cells strongly expressed stem cell associated markers, including ABCG2, p63, nerve growth factor (NGF) with its receptors tyrosine kinase receptor A (TrkA) and neurotrophin low-affinity receptor p75NTR, glial cell-derived neurotrophic factor (GDNF) with its receptor GDNF family receptor alpha 1 (GFRalpha-1), integrin beta1, alpha-enolase, and epidermal growth factor receptor (EGFR). The differentiation associated markers nestin, E-cadherin and involucrin were not expressed by these cells. These findings indicate that the basal cells of bulbar conjunctival epithelium shares a similar expression pattern of stem cell associated markers to the corneal epithelium, but has a unique pattern of differentiation associated cytokeratin expression.

The use of transgenic and knock-out mice in the investigation of ocular surface cell biology

The transgenic and knock-out mice created by transgenesis and gene targeting techniques are very useful for elucidating the pathophysiology of human diseases caused by altered genetic functions. Many of the experimental mouse lines exhibit ocular surface disorders. However, embryonic lethality and congenital defects found in many of the transgenic and knock-out mice preclude their use for studying the consequences of altered genetic functions in adult animals. To circumvent these difficulties, we have established binary inducible mouse models, using the corneal keratocyte-specific keratocan promoter, and the tetracycline-inducible gene expression system (reverse tetracycline transcription activator--rtTA). In these models, the animals function normally until they are fed doxycycline, thus inducing the overexpression of inserted transgenes by keratocytes. We have also developed inserted rtTA and Cre reporter gene constructs to create genetically modified mouse lines that have tissue-specific gene alterations to study acquired conditions, e.g., wound healing and irregular hormone and cytokine signaling that offsets homeostasis in adults. Furthermore, the genes that are ubiquitously expressed in many tissues can be specifically ablated solely in ocular surface tissues to examine their function, since the loss of such a gene in ocular surface tissues will not be life-threatening. It is noteworthy that these altered mouse lines can also be used as models for the development of therapeutic treatment regimens of diseases using gene therapy and stem cell strategies.

Apoptosis and keratin intermediate filaments

Intermediate filament (IF) proteins utilize central alpha-helical domains to generate polymeric fibers intermediate in size between actin microfilaments and microtubules. The regions flanking the central structural domains have diverged greatly to permit IF proteins to adopt specialized functions. Keratins represent the largest two groups of IF proteins. Most keratins serve structural functions in hair or epidermis. Intracellular epidermal keratins also provide strength to epithelial sheets. The intracellular type I keratins and other IF proteins are cleaved by caspases during apoptosis to ensure the disposal of the relatively insoluble cellular components. However, recent studies have also revealed an unexpected protective role for keratin 8 during TNF and Fas mediated apoptosis. Evidence for possible functions of keratins both upstream and downstream of apoptotic signaling are considered.

Transduction of a preselected population of human epidermal stem cells: consequences for gene therapy

Continuously renewing tissues, such as the epidermis, are populated by a hierarchy of dividing transient amplifying cells, which are maintained by stem cells. Transient amplifying cells divide to maintain the tissue, but they are limited to a finite number of cell divisions before they differentiate and are sloughed. Only the stem cells remain for the life of the tissue. Thus, it is critical to target stem cells when designing gene therapy regimes for genetically inherited diseases, such as epidermolysis bullosa simplex (EBS). Unfortunately, isolating pure epithelial stem cells has been problematic. In this study, we used rapid adherence to collagen type IV to successfully enrich for epidermal stem cells from adult human skin. These preselected stem cells were slow to proliferate, but they ultimately formed large colonies. When recombined with the dermal substrate AlloDerm, the stem cells re-formed a stratified squamous epidermis within 1 week after raising the AlloDerm to the air-liquid interface. These organotypic cultures grew continuously and, even after 6 weeks in culture, they maintained a proliferative basal layer. When transduced with a retroviral LacZ vector, preselected stem cells formed beta-galactosidase-positive clones in submerged and organotypic cultures. Transduced cells showed persistent expression through 12 weeks in organotypic culture, demonstrating the feasibility of using preselected stem cells for gene therapy. Currently, we are developing two models of EBS to test a gene therapy approach, which is based on the premise that EBS stem cells with a mutant keratin (K)14 gene corrected to wild type will have a growth advantage over noncorrected EBS stem cells.

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

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

Chronic esophagitis dissecans: an unrecognized clinicopathologic entity?

BACKGROUND

We report the clinical and histologic features of a distinctive form of chronic esophagitis for which we propose the term chronic esophagitis dissecans.

METHODS

The study group included five patients diagnosed at Hôpital Beaujon, Clichy, from 1988 to 1994. Clinical and endoscopic examinations were performed. Samples of esophageal biopsy specimens were analyzed by histologic and ultrastructural examinations and by immunohistochemistry with antibodies directed against cell adhesion molecules.

RESULTS

All patients were elderly (mean age, 66 years). They presented the following combination of clinical and endoscopic features: (1) long-standing history of chronic dysphagia, without symptoms of reflux, (2) shedding of mucosal fragments, occurring spontaneously or after mechanical trauma, (3) existence of localized esophageal strictures, (4) lack of concurrent chronic cutaneomucous lesions. Two patients presented with thymoma. Histologic examination showed evidence of mucosal blistering, in the absence of significant inflammatory lesions. Altered cell-cell adhesion was suggested by the reduced number of desmosomes on ultrastructural examination and the decreased expression of immunoreactive intercellular adhesion molecule E-cadherin.

CONCLUSION

Chronic esophagitis dissecans likely represents a hitherto unrecognized clinicopathologic entity and must be added to the causes of chronic dysphagia.

Mutation of human keratin 18 in association with cryptogenic cirrhosis

Mutations in 11 of the more than 20 keratin intermediate filaments cause several epidermal and oral associated diseases. No disease-associated mutations have been described in keratin 8 or 18 (K8/18) which are the major keratin pair in simple-type epithelia, as found in the liver, pancreas, and intestine. However, transgenic mice that express mutant keratin 18 develop chronic hepatitis, and have an increased susceptibility to drug-induced hepatotoxicity. Also, ectopic expression of epidermal K14 in mouse liver results in chronic hepatitis, and disruption of mouse K8 leads to embryo lethality with extensive liver hemorrhage. We tested if patients with liver disease of unknown cause may harbor mutations in K18. We describe a his127-->leu (H127L) K18 mutation in a patient with cryptogenic cirrhosis that is germline transmitted. The K18 H127L isolated from the liver explant, or after expression in bacteria, showed an altered migration on two-dimensional gel analysis as compared with normal human liver or bacterially expressed K18. Electron microscopy of in vitro assembled K18 H127L and wild type K8 showed an assembly defect as compared with normal K8/18 assembly. Our results suggest that mutations in K18 may be predispose to, or result in cryptogenic cirrhosis in humans.

Implications of intermediate filament protein phosphorylation

Intermediate filament (IF) proteins, a large family of tissue specific proteins, undergo several posttranslational modifications, with phosphorylation being the most studied modification. IF protein phosphorylation is highly dynamic and involves the head and/or tail domains of these proteins, which are the domains that impart most of the structural heterogeneity and hence presumed tissue specific functions. Although the function of IF proteins remains poorly understood, several regulatory roles for IF protein phosphorylation have been identified or are emerging. Those roles include filament disassembly and reorganization, solubility, localization within specific cellular domains, association with other cytoplasmic or membrane associated proteins, protection against physiologic stress and mediation of tissue-specific functions. Understanding the mechanistic and functional aspects of IF protein phosphorylation is providing insights not only regarding the function of this modification, but also regarding the function of IF proteins.

Susceptibility to hepatotoxicity in transgenic mice that express a dominant-negative human keratin 18 mutant

Keratins 8 and 18 (K8/18) are intermediate filament phosphoglycoproteins that are expressed preferentially in simple-type epithelia. We recently described transgenic mice that express point-mutant human K18 (Ku, N.-O., S. Michie, R.G. Oshima, and M.B. Omary. 1995. J. Cell Biol. 131:1303-1314) and develop chronic hepatitis and hepatocyte fragility in association with hepatocyte keratin filament disruption. Here we show that mutant K18 expressing transgenic mice are highly susceptible to hepatotoxicity after acute administration of acetaminophen (400 mg/Kg) or chronic ingestion of griseofulvin (1.25% wt/wt of diet). The predisposition to hepatotoxicity results directly from the keratin mutation since nontransgenic or transgenic mice that express normal human K18 are more resistant. Hepatotoxicity was manifested by a significant difference in lethality, liver histopathology, and biochemical serum testing. Keratin glycosylation decreased in all griseofulvin-fed mice, whereas keratin phosphorylation increased dramatically preferentially in mice expressing normal K18. The phosphorylation increase in normal K18 after griseofulvin feeding appears to involve sites that are different to those that increase after partial hepatectomy. Our results indicate that hepatocyte intermediate filament disruption renders mice highly susceptible to hepatotoxicity, and raises the possibility that K18 mutations may predispose to drug hepatotoxicity. The dramatic phosphorylation increase in nonmutant keratins could provide survival advantage to hepatocytes.

14-3-3 proteins associate with phosphorylated simple epithelial keratins during cell cycle progression and act as a solubility cofactor

14-3-3 is a ubiquitous protein family that interacts with several signal transduction kinases. We show that 14-3-3 proteins associate with keratin intermediate filament polypeptides 8 and 18 (K8/18) that are expressed in simple-type epithelia. The association is stoichiometrically significant (> or = one 14-3-3 molecule/keratin tetramer), occurs preferentially with K18, and is phosphorylation- and cell cycle-dependent in that it occurs during S/G2/M phases of the cell cycle when keratins become hyperphosphorylated. Binding of phospho-K8/18 to 14-3-3 can be reconstituted in vitro using recombinant 14-3-3 or using total cellular cytosol. Phosphatase treatment results in dissociation of 14-3-3, and dephosphorylation of phospho-K8/18 prevents reconstitution of the binding. Three cellular keratin subpopulations were analyzed that showed parallel gradients of keratin phosphorylation and 14-3-3 binding. Incubation of 14-3-3 with keratins during or after in vitro filament assembly results in sequestering of additional soluble keratin, only in cases when the keratins were hyperphosphorylated. Our results demonstrate a stoichiometrically significant cell cycle- and phosphorylation-regulated binding of 14-3-3 proteins to K18 and in vitro evidence of a simple epithelial keratin sequestering role for 14-3-3 proteins.

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

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

Dynamics of human keratin 18 phosphorylation: polarized distribution of phosphorylated keratins in simple epithelial tissues

Phosphorylation of keratin polypeptides 8 and 18 (K8/18) and other intermediate filament proteins results in their reorganization in vitro and in vivo. In order to study functional aspects of human K18 phosphorylation, we generated and purified a polyclonal antibody (termed 3055) that specifically recognizes a major phosphorylation site (ser52) of human K18 but not dephosphorylated K18 or a ser52-->ala K18 mutant. Pulse-chase experiments followed by immunoprecipitation and peptide mapping of in vivo 32PO4-labeled K8/18 indicated that the overall phosphorylation turnover rate is faster for K18 versus K8, and that ser52 of K18 is a highly dynamic phosphorylation site. Isoelectric focusing of 32PO4 labeled K18 followed by immunoblotting with 3055 showed that the major phosphorylated K18 species contain ser52 phosphorylation but that some K18 molecules exist that are preferentially phosphorylated on K18 sites other than ser52. Immunoblotting of total cell lysates obtained from cells at different stages of the cell cycle showed that ser52 phosphorylation increases three to fourfold during the S and G2/M phases of the cell cycle. Immunofluorescence staining of cells at different stages of mitosis, using 3055 or other antibodies that recognize the total keratin pool, resulted in preferential binding of the 3055 antibody to the reorganized keratin fraction. Staining of human tissues or tissues from transgenic mice that express human K18 showed that the phospho-ser52 K18 species are located preferentially in the basolateral and apical domains in the liver and pancreas, respectively, but no preferential localization was noted in other simple epithelial organs examined. Our results support a model whereby phosphorylated intermediate filaments are localized in specific cellular domains depending on the tissue type and site(s) of phosphorylation. In addition, ser52 of human K18 is a highly dynamic phosphorylation site that undergoes modulation during the S and G2/M phases of the cell cycle in association with filament reorganization.

Chronic hepatitis, hepatocyte fragility, and increased soluble phosphoglycokeratins in transgenic mice expressing a keratin 18 conserved arginine mutant

The two major intermediate filament proteins in glandular epithelia are keratin polypeptides 8 and 18 (K8/18). To evaluate the function and potential disease association of K18, we examined the effects of mutating a highly conserved arginine (arg89) of K18. Expression of K18 arg89-->his/cys and its normal K8 partner in cultured cells resulted in punctate staining as compared with the typical filaments obtained after expression of wild-type K8/18. Generation of transgenic mice expressing human K18 arg89-->cys resulted in marked disruption of liver and pancreas keratin filament networks. The most prominent histologic abnormalities were liver inflammation and necrosis that appeared at a young age in association with hepatocyte fragility and serum transaminase elevation. These effects were caused by the mutation since transgenic mice expressing wild-type human K18 showed a normal phenotype. A relative increase in the phosphorylation and glycosylation of detergent solubilized K8/18 was also noted in vitro and in transgenic animals that express mutant K18. Our results indicate that the highly conserved arg plays an important role in glandular keratin organization and tissue fragility as already described for epidermal keratins. Phosphorylation and glycosylation alterations in the arg mutant keratins may account for some of the potential changes in the cellular function of these proteins. Mice expressing mutant K18 provide a novel animal model for human chronic hepatitis, and for studying the tissue specific function(s) of K8/18.