Lessons from keratin 18 knockout mice: formation of novel keratin filaments, secondary loss of keratin 7 and accumulation of liver-specific keratin 8-positive aggregates

Skeletal muscle adaptations following eccentric contractions are not mediated by keratin 18

The molecular mechanisms that drive muscle adaptations after eccentric exercise training are multifaceted and likely impacted by age. Previous studies have reported that many genes and proteins respond differently in young and older muscles following training. Keratin 18 (Krt18), a cytoskeletal protein involved in force transduction and organization, was found to be upregulated after muscles performed repeated bouts of eccentric contractions, with higher levels observed in young muscle compared with older muscle. Therefore, the purpose of this study was to determine if Krt18 mediates skeletal muscle adaptations following eccentric exercise training. The anterior crural muscles of Krt18 knockout (KO) and wild-type (WT) mice were subjected to either a single bout or repeated bouts of eccentric contractions, with isometric torque assessed across the initial and final bouts. Functionally, Krt18 KO and WT mice did not differ prior to performing any eccentric contractions (P ≥ 0.100). Muscle strength (tetanic isometric torques) and the ability to adapt to eccentric exercise training were also consistent across strains at all time points (P ≥ 0.169). Stated differently, immediate strength deficits and the recovery of strength following a single bout or multiple bouts of eccentric contractions were similar between Krt18 KO and WT mice. In summary, the absence of Krt18 does not impede the muscle's ability to adapt to repeated eccentric contractions, suggesting it is not essential for exercise-induced remodeling.NEW & NOTEWORTHY The molecular processes that underlie the changes in skeletal muscle following eccentric exercise training are complex and involve multiple factors. Our findings indicate that Krt18 may not play a significant role in muscle adaptations following eccentric exercise training, likely due to its low expression in skeletal muscle. These results underscore the complexity of the molecular mechanisms that contribute to muscle plasticity and highlight the need for further research in this area.

Hepatic stearoyl-CoA desaturase-1 deficiency induces fibrosis and hepatocellular carcinoma-related gene activation under a high carbohydrate low fat diet

Stearoyl-CoA desaturase-1 (SCD1) is a pivotal enzyme in lipogenesis, which catalyzes the synthesis of monounsaturated fatty acids (MUFA) from saturated fatty acids, whose ablation downregulates lipid synthesis, preventing steatosis and obesity. Yet deletion of SCD1 promotes hepatic inflammation and endoplasmic reticulum stress, raising the question of whether hepatic SCD1 deficiency promotes further liver damage, including fibrosis. To delineate whether SCD1 deficiency predisposes the liver to fibrosis, cirrhosis, and hepatocellular carcinoma (HCC), we employed in vivo SCD1 deficient global and liver-specific mouse models fed a high carbohydrate low-fat diet and in vitro established AML12 mouse cells. The absence of liver SCD1 remarkably increased the saturation of liver lipid species, as indicated by lipidomic analysis, and led to hepatic fibrosis. Consistently, SCD1 deficiency promoted hepatic gene expression related to fibrosis, cirrhosis, and HCC. Deletion of SCD1 increased the circulating levels of Osteopontin, known to be increased in fibrosis, and alpha-fetoprotein, often used as an early marker and a prognostic marker for patients with HCC. De novo lipogenesis or dietary supplementation of oleate, an SCD1-generated MUFA, restored the gene expression related to fibrosis, cirrhosis, and HCC. Although SCD1 deficient mice are protected against obesity and fatty liver, our results show that MUFA deprivation results in liver injury, including fibrosis, thus providing novel insights between MUFA insufficiency and pathways leading to fibrosis, cirrhosis, and HCC under lean non-steatotic conditions.

The expression of intermediate filaments in the abomasum of ruminants: A comparative study

Intermediate filaments (IFs) are key molecular factors of the cell and have been reported to play an important role in maintaining the structural integrity and functionality of the abomasum. This study was designed to determine the regional distribution, cellular localization and expression of several IFs, including CK8, CK18, CK19, vimentin, desmin, peripherin and nestin, as well as the connective tissue component laminin, in the bovine, ovine and caprine abomasa. Immunohistochemical analyses demonstrated varying levels of expression of CK8, CK18, CK19, vimentin, desmin, nestin, peripherin and laminin in the bovine, ovine and caprine abomasa. CK8 immunoreactions were particularly evident in the luminal and glandular epithelia of the glands found in the abomasal cardia, fundus and pylorus in all three species. In the bovine abomasum, CK18 immunoreactions were stronger in the parietal cells, compared to the chief cells. In the abomasum of all three species, the smooth muscle as well as the smooth muscle cells of the vascular media in the cardiac, fundic and pyloric regions showed strong immunoreactivity. In all three species, the cardiac, fundic and pyloric regions of the abomasum showed strong peripherin and nestin immunoreactions in the luminal and glandular epithelial cells, stromal and smooth muscle cells, nervous plexuses and blood vessels. The expression patterns of IFs and laminin in the ruminant abomasum suggest that these proteins play a structural role in the cytoskeleton and are effective in maintaining abomasal tissue integrity and stability.

The keratin-desmosome scaffold of internal epithelia in health and disease - The plot is thickening

Keratin (K) intermediate filaments are attached to desmosomes and constitute the orchestrators of epithelial cell and tissue architecture. While their relevance in the epidermis is well recognized, our review focuses on their emerging importance in internal epithelia. The significance of keratin-desmosome scaffolds (KDSs) in the intestine is highlighted by transgenic mouse models and individuals with inflammatory bowel disease who display profound KDS alterations. In lung, high K8 expression defines a transitional cell subset during regeneration, and K8 variants are associated with idiopathic pulmonary fibrosis. Inherited variants in desmosomal proteins are overrepresented in idiopathic lung fibrosis, and familiar eosinophilic esophagitis. K18 serum fragments are established hepatocellular injury markers that correlate with the extent of histological inflammation. K17 expression is modified in multiple tumors, and K17 levels might be of prognostic relevance. These data should spur further studies on biological roles of these versatile tissue protectors and efforts on their therapeutic targeting.

Anatomy of the female reproductive tract organs of the brown anole (Anolis sagrei)

Female reproduction in squamate reptiles (lizards and snakes) is highly diverse and mode of reproduction, clutch size, and reproductive tract morphology all vary widely across this group of ~11,000 species. Recently, CRISPR genome editing techniques that require manipulation of the female reproductive anatomy have been developed in this group, making a more complete understanding of this anatomy essential. We describe the adult female reproductive anatomy of the model reptile the brown anole (Anolis sagrei). We show that the brown anole female reproductive tract has three distinct anterior-to-posterior regions, the infundibulum, the glandular uterus, and the nonglandular uterus. The infundibulum has a highly ciliated epithelial lip, a region where the epithelium is inverted so that cilia are present on the inside and outside of the tube. The glandular uterus has epithelial ducts that are patent with a lumen as well as acinar structures with a lumen. The nonglandular uterus has a heterogeneous morphology from anterior to posterior, with a highly folded, ciliated epithelium transitioning to a stratified squamous epithelium. This transition is accompanied by a loss of keratin-8 expression and together, these changes are similar to the morphological and gene expression changes that occur in the mammalian cervix. We recommend that description of the nonglandular uterus include the regional sub-specification of a "cervix" and "vagina" as this terminology change more accurately describes the morphology. Our data extend histological studies of reproductive organ morphology in reptiles and expand our understanding of the variation in reproductive system anatomy across squamates and vertebrates.

Deacetylation via SIRT2 prevents keratin-mutation-associated injury and keratin aggregation

Keratin (K) and other intermediate filament (IF) protein mutations at conserved arginines disrupt keratin filaments into aggregates and cause human epidermolysis bullosa simplex (EBS; K14-R125C) or predispose to mouse liver injury (K18-R90C). The challenge for more than 70 IF-associated diseases is the lack of clinically utilized IF-targeted therapies. We used high-throughput drug screening to identify compounds that normalized mutation-triggered keratin filament disruption. Parthenolide, a plant sesquiterpene lactone, dramatically reversed keratin filament disruption and protected cells and mice expressing K18-R90C from apoptosis. K18-R90C became hyperacetylated compared with K18-WT and treatment with parthenolide normalized K18 acetylation. Parthenolide upregulated the NAD-dependent SIRT2, and increased SIRT2-keratin association. SIRT2 knockdown or pharmacologic inhibition blocked the parthenolide effect, while site-specific Lys-to-Arg mutation of keratin acetylation sites normalized K18-R90C filaments. Treatment of K18-R90C-expressing cells and mice with nicotinamide mononucleotide had a parthenolide-like protective effect. In 2 human K18 variants that associate with human fatal drug-induced liver injury, parthenolide protected K18-D89H- but not K8-K393R-induced filament disruption and cell death. Importantly, parthenolide normalized K14-R125C-mediated filament disruption in keratinocytes and inhibited dispase-triggered keratinocyte sheet fragmentation and Fas-mediated apoptosis. Therefore, keratin acetylation may provide a novel therapeutic target for some keratin-associated diseases.

A Kaleidoscope of Keratin Gene Expression and the Mosaic of Its Regulatory Mechanisms

Keratins are a family of intermediate filament-forming proteins highly specific to epithelial cells. A combination of expressed keratin genes is a defining property of the epithelium belonging to a certain type, organ/tissue, cell differentiation potential, and at normal or pathological conditions. In a variety of processes such as differentiation and maturation, as well as during acute or chronic injury and malignant transformation, keratin expression undergoes switching: an initial keratin profile changes accordingly to changed cell functions and location within a tissue as well as other parameters of cellular phenotype and physiology. Tight control of keratin expression implies the presence of complex regulatory landscapes within the keratin gene loci. Here, we highlight patterns of keratin expression in different biological conditions and summarize disparate data on mechanisms controlling keratin expression at the level of genomic regulatory elements, transcription factors (TFs), and chromatin spatial structure.

Revisiting the significance of keratin expression in complex epithelia

A large group of keratin genes (n=54 in the human genome) code for intermediate filament (IF)-forming proteins and show differential regulation in epithelial cells and tissues. Keratin expression can be highly informative about the type of epithelial tissue, differentiation status of constituent cells and biological context (e.g. normal versus diseased settings). The foundational principles underlying the use of keratin expression to gain insight about epithelial cells and tissues primarily originated in pioneering studies conducted in the 1980s. The recent emergence of single cell transcriptomics provides an opportunity to revisit these principles and gain new insight into epithelial biology. Re-analysis of single-cell RNAseq data collected from human and mouse skin has confirmed long-held views regarding the quantitative importance and pairwise regulation of specific keratin genes in keratinocytes of surface epithelia. Furthermore, such analyses confirm and extend the notion that changes in keratin gene expression occur gradually as progenitor keratinocytes commit to and undergo differentiation, and challenge the prevailing assumption that specific keratin combinations reflect a mitotic versus a post-mitotic differentiating state. Our findings provide a blueprint for similar analyses in other tissues, and warrant a more nuanced approach in the use of keratin genes as biomarkers in epithelia.

Priming, Triggering, Adaptation and Senescence (PTAS): A Hypothesis for a Common Damage Mechanism of Steatohepatitis

Understanding the pathomechanism of steatohepatitis (SH) is hampered by the difficulty of distinguishing between causes and consequences, by the broad spectrum of aetiologies that can produce the phenotype, and by the long time-span during which SH develops, often without clinical symptoms. We propose that SH develops in four phases with transitions: (i) priming lowers stress defence; (ii) triggering leads to acute damage; (iii) adaptation, possibly associated with cellular senescence, mitigates tissue damage, leads to the phenotype, and preserves liver function at a lower level; (iv) finally, senescence prevents neoplastic transformation but favours fibrosis (cirrhosis) and inflammation and further reduction in liver function. Escape from senescence eventually leads to hepatocellular carcinoma. This hypothesis for a pathomechanism of SH is supported by clinical and experimental observations. It allows organizing the various findings to uncover remaining gaps in our knowledge and, finally, to provide possible diagnostic and intervention strategies for each stage of SH development.

Keratin 8/18 Regulate the Akt Signaling Pathway

Keratin 8 and keratin 18 (K8/K18) are intermediate filament proteins that form the obligate heteropolymers in hepatocytes and protect the liver against toxins. The mechanisms of protection include the regulation of signaling pathway associated with cell survival. Previous studies show K8/K18 binding with Akt, which is a well-known protein kinase involved in the cell survival signaling pathway. However, the role of K8/K18 in the Akt signaling pathway is unclear. In this study, we found that K8/K18-Akt binding is downregulated by K8/K18 phosphorylation, specifically phosphorylation of K18 ser7/34/53 residues, whereas the binding is upregulated by K8 gly-62-cys mutation. K8/K18 expression in cultured cell system tends to enhance the stability of the Akt protein. A comparison of the Akt signaling pathway in a mouse system with liver damage shows that the pathway is downregulated in K18-null mice compared with nontransgenic mice. K18-null mice with Fas-induced liver damage show enhanced apoptosis combined with the downregulation of the Akt signaling pathway, i.e., lower phosphorylation levels of GSK3β and NFκB, which are the downstream signaling factors in the Akt signaling pathway, in K18-null mice compared with the control mice. Our study indicates that K8/K18 expression protects mice from liver damage by participating in enhancing the Akt signaling pathway.

Keratin 7 Is a Constituent of the Keratin Network in Mouse Pancreatic Islets and Is Upregulated in Experimental Diabetes

Keratin (K) 7 is an intermediate filament protein expressed in ducts and glands of simple epithelial organs and in urothelial tissues. In the pancreas, K7 is expressed in exocrine ducts, and apico-laterally in acinar cells. Here, we report K7 expression with K8 and K18 in the endocrine islets of Langerhans in mice. K7 filament formation in islet and MIN6 β-cells is dependent on the presence and levels of K18. K18-knockout (K18^‒/‒) mice have undetectable islet K7 and K8 proteins, while K7 and K18 are downregulated in K8^‒/‒ islets. K7, akin to F-actin, is concentrated at the apical vertex of β-cells in wild-type mice and along the lateral membrane, in addition to forming a fine cytoplasmic network. In K8^‒/‒ β-cells, apical K7 remains, but lateral keratin bundles are displaced and cytoplasmic filaments are scarce. Islet K7, rather than K8, is increased in K18 over-expressing mice and the K18-R90C mutation disrupts K7 filaments in mouse β-cells and in MIN6 cells. Notably, islet K7 filament networks significantly increase and expand in the perinuclear regions when examined in the streptozotocin diabetes model. Hence, K7 represents a significant component of the murine islet keratin network and becomes markedly upregulated during experimental diabetes.

Revealing the Roles of Keratin 8/18-Associated Signaling Proteins Involved in the Development of Hepatocellular Carcinoma

Although hepatocellular carcinoma (HCC) is developed with various etiologies, protection of hepatocytes seems basically essential to prevent the incidence of HCC. Keratin 8 and keratin 18 (K8/K18) are cytoskeletal intermediate filament proteins that are expressed in hepatocytes. They maintain the cell shape and protect cells under stress conditions. Their protective roles in liver damage have been described in studies of mouse models, and K8/K18 mutation frequency in liver patients. Interestingly, K8/K18 bind to signaling proteins such as transcription factors and protein kinases involved in HCC development. Since K8/K18 are abundant cytoskeletal proteins, K8/K18 binding with the signaling factors can alter the availability of the factors. Herein, we discuss the potential roles of K8/K18 in HCC development.

Keratin intermediate filaments in the colon: guardians of epithelial homeostasis

Keratin intermediate filament proteins are major cytoskeletal components of the mammalian simple layered columnar epithelium in the gastrointestinal tract. Human colon crypt epithelial cells express keratins 18, 19 and 20 as the major type I keratins, and keratin 8 as the type II keratin. Keratin expression patterns vary between species, and mouse colonocytes express keratin 7 as a second type II keratin. Colonic keratin patterns change during cell differentiation, such that K20 increases in the more differentiated crypt cells closer to the central lumen. Keratins provide a structural and mechanical scaffold to support cellular stability, integrity and stress protection in this rapidly regenerating tissue. They participate in central colonocyte processes including barrier function, ion transport, differentiation, proliferation and inflammatory signaling. The cell-specific keratin compositions in different epithelial tissues has allowed for the utilization of keratin-based diagnostic methods. Since the keratin expression pattern in tumors often resembles that in the primary tissue, it can be used to recognize metastases of colonic origin. This review focuses on recent findings on the biological functions of mammalian colon epithelial keratins obtained from pivotal in vivo models. We also discuss the diagnostic value of keratins in chronic colonic disease and known keratin alterations in colon pathologies. This review describes the biochemical properties of keratins and their molecular actions in colonic epithelial cells and highlights diagnostic data in colorectal cancer and inflammatory bowel disease patients, which may facilitate the recognition of disease subtypes and the establishment of personal therapies in the future.

Keratins are asymmetrically inherited fate determinants in the mammalian embryo

To implant in the uterus, the mammalian embryo first specifies two cell lineages: the pluripotent inner cell mass that forms the fetus, and the outer trophectoderm layer that forms the placenta1. In many organisms, asymmetrically inherited fate determinants drive lineage specification2, but this is not thought to be the case during early mammalian development. Here we show that intermediate filaments assembled by keratins function as asymmetrically inherited fate determinants in the mammalian embryo. Unlike F-actin or microtubules, keratins are the first major components of the cytoskeleton that display prominent cell-to-cell variability, triggered by heterogeneities in the BAF chromatin-remodelling complex. Live-embryo imaging shows that keratins become asymmetrically inherited by outer daughter cells during cell division, where they stabilize the cortex to promote apical polarization and YAP-dependent expression of CDX2, thereby specifying the first trophectoderm cells of the embryo. Together, our data reveal a mechanism by which cell-to-cell heterogeneities that appear before the segregation of the trophectoderm and the inner cell mass influence lineage fate, via differential keratin regulation, and identify an early function for intermediate filaments in development.

Keratin expression by corneal and limbal stem cells during development

Keratins are the forming units of intermediate filaments (IF) that provide mechanical support, and formation of desmosomes between cells and hemi desmosomes with basement membranes for epithelium integrity. Keratin IF are polymers of obligate heterodimer consisting one type I keratin and one type II keratin molecules. There are 54 functional keratin genes in human genome, which are classified into three major groups, i.e., epithelial keratins, hair follicle cell-specific epithelial keratins and hair keratins. Their expression is cell type-specific and developmentally regulated. Corneal epithelium expresses a subgroup of keratins similar to those of epidermal epithelium. Limbal basal stem cells express K5/K14, and K8/K18 and K8/K19 IF suggesting that there probably are two populations of limbal stem cells (LSCs). In human, LSCs at limbal basal layer can directly stratify and differentiate to limbal suprabasal cells that express K3/K12 IF, or centripetally migrate then differentiate to corneal basal transient amplifying cells (TAC) that co-express both K3/K12 and K5/K14 prior to moving upward and assuming suprabasal cells phenotype of only K3/K12 expression that signifies corneal type epithelium differentiation. In rodent, the differentiated cornea epithelial cells express K5/K12 in lieu of K3/K12, because K3 allele exists as a pseudogene and does not encode a functional K3 protein. The basal corneal cells of new-born mice originate from surface ectoderm during embryonic development slowly commit to differentiation of becoming TAC co-expressing K5/K12 and K5/K14 IF. However, the centripetal migration may still occur at a slower rate in young mice, which is accelerated during wound healing. In this review, we will discuss and compare the cornea-specific keratins expression patterns between corneal and epidermal epithelial cells during mouse development, and between human and mouse during development and homeostasis in adult, and pathology caused by a mutation of keratins.

Intestinal intermediate filament polypeptides in C. elegans: Common and isotype-specific contributions to intestinal ultrastructure and function

The abundance and diversity of intermediate filaments (IFs) in the C. elegans intestine indicate important contributions to intestinal function and organismal wellbeing. Fluorescent IF reporters localize below the actin-rich brush border and are highly enriched in the lumen-enveloping endotube, which is attached to the C. elegans apical junction. Mapping intestinal viscoelasticity by contact-free Brillouin microscopy reveals that the IF-rich endotube is positioned at the interface between the stiff brush border and soft cytoplasm suggesting a mechanical buffering function to deal with the frequent luminal distortions occurring during food intake and movement. In accordance, depletion of IFB-2, IFC-2 and IFD-2 leads to intestinal lumen dilation although depletion of IFC-1, IFD-1 and IFP-1 do not. Ultrastructural analyses of loss of function mutants further show that IFC-2 mutants have a rarefied endotube and IFB-2 mutants lack an endotube altogether. Remarkably, almost all IFB-2- and IFC-2-deficient animals develop to fertile adults. But developmental retardation, reduced brood size, altered survival and increased sensitivity to microbial toxin, osmotic and oxidative stress are seen in both mutants albeit to different degrees. Taken together, we propose that individual intestinal IF polypeptides contribute in different ways to endotube morphogenesis and cooperate to cope with changing environments.

14-3-3 targets keratin intermediate filaments to mechanically sensitive cell-cell contacts

Intermediate filament (IF) cytoskeletal networks simultaneously support mechanical integrity and influence signal transduction pathways. Marked remodeling of the keratin IF network accompanies collective cellular morphogenetic movements that occur during early embryonic development in the frog Xenopus laevis. While this reorganization of keratin is initiated by force transduction on cell–cell contacts mediated by C-cadherin, the mechanism by which keratin filament reorganization occurs remains poorly understood. In this work, we demonstrate that 14-3-3 proteins regulate keratin reorganization dynamics in embryonic mesendoderm cells from Xenopus gastrula. 14-3-3 colocalizes with keratin filaments near cell–cell junctions in migrating mesendoderm. Coimmunoprecipitation, mass spectrometry, and bioinformatic analyses indicate 14-3-3 is associated with Keratin 19 (K19) in the whole embryo and, more specifically, mesendoderm tissue. Inhibition of 14-3-3 results in both the decreased exchange of keratin subunits into filaments and blocks keratin filament recruitment toward cell–cell contacts. Synthetically coupling 14-3-3 to K19 through a unique fusion construct conversely induces the localization of this keratin population to the region of cell–cell contacts. Taken together, these findings indicate that 14-3-3 acts on keratin IFs and is involved in their reorganization to sites of cell adhesion.

Filaments and phenotypes: cellular roles and orphan effects associated with mutations in cytoplasmic intermediate filament proteins

Cytoplasmic intermediate filaments (IFs) surround the nucleus and are often anchored at membrane sites to form effectively transcellular networks. Mutations in IF proteins (IFps) have revealed mechanical roles in epidermis, muscle, liver, and neurons. At the same time, there have been phenotypic surprises, illustrated by the ability to generate viable and fertile mice null for a number of IFp-encoding genes, including vimentin. Yet in humans, the vimentin ( VIM) gene displays a high probability of intolerance to loss-of-function mutations, indicating an essential role. A number of subtle and not so subtle IF-associated phenotypes have been identified, often linked to mechanical or metabolic stresses, some of which have been found to be ameliorated by the over-expression of molecular chaperones, suggesting that such phenotypes arise from what might be termed "orphan" effects as opposed to the absence of the IF network per se, an idea originally suggested by Toivola et al. and Pekny and Lane.

Corneal keratin aggresome (CKAGG) formation and clearance by proteasome activation

Purpose

To understand the mechanism of corneal keratin expression and clearance in corneal epithelium with Limbal Stem Cell Deficiency (LSCD). The hypothesis is that LSCD-induced proteasome dysfunction is a contributing factor to keratin aggregation, causing corneal keratin aggresome (CKAGG) formation.

Method

LSCD was surgically induced in rabbit corneas. LSCD corneal epithelial cells (D-CEC) were collected to investigate keratin K4 and K13 expression and CKAGG formation. Oral mucosal epithelial cells (OMECS) were isolated and cultured to study K4 and K13 expression. Cultured cells were treated with proteasome inhibitor to induce CKAGG formation.

Results

K4 and K13 were strongly expressed in D-CEC, with additional higher molecular weight bands of K4 and K13, suggesting CKAGG formation. Double staining of K4/K13 and ubiquitin showed co-localization of these keratins with ubiquitin in D-CEC. Proteasome inhibition also showed K4/K13 modification and accumulation in cultured OMECS, similar to D-CEC. Proteasome activation was then performed in cultured OMEC. There was no accumulation of keratins, and levels of unmodified keratins were found significantly reduced.

Conclusion

Results showed an abnormal expression of K4 and K13 after LSCD-induced proteasome dysfunction, which coalesce to form CKAGG in Corneal Epithelial Cells (CEC). We propose that CKAGG formation may be one of the causative factors of morphological alterations in the injured corneal epithelium, and that CKAGG could potentially be cleared by enhancing proteasome activity.

The role of keratins in the digestive system: lessons from transgenic mouse models

Keratins are the largest subfamily of intermediate filament proteins. They are either type I acidic or type II basic keratins. Keratins form obligate heteropolymer in epithelial cells and their expression patterns are tissue-specific. Studies have shown that keratin mutations are the cause of many diseases in humans or predispose humans to acquiring them. Using mouse models to study keratin-associated human diseases is critical, because they allow researchers to get a better understanding of these diseases and their progressions, and so many such studies have been conducted. Acknowledging the importance, researches with genetically modified mice expressing human disease-associated keratin mutants have been widely done. Numerous studies using keratin knockout mice, keratin-overexpressed mice, or transgenic mice expressing keratin mutants have been conducted. This review summarizes the mouse models that have been used to study type I and type II keratin expression in the digestive organs, namely, the liver, pancreas, and colon.

Intermediate Filaments at the Junction of Mechanotransduction, Migration, and Development

Mechanically induced signal transduction has an essential role in development. Cells actively transduce and respond to mechanical signals and their internal architecture must manage the associated forces while also being dynamically responsive. With unique assembly-disassembly dynamics and physical properties, cytoplasmic intermediate filaments play an important role in regulating cell shape and mechanical integrity. While this function has been recognized and appreciated for more than 30 years, continually emerging data also demonstrate important roles of intermediate filaments in cell signal transduction. In this review, with a particular focus on keratins and vimentin, the relationship between the physical state of intermediate filaments and their role in mechanotransduction signaling is illustrated through a survey of current literature. Association with adhesion receptors such as cadherins and integrins provides a critical interface through which intermediate filaments are exposed to forces from a cell's environment. As a consequence, these cytoskeletal networks are posttranslationally modified, remodeled and reorganized with direct impacts on local signal transduction events and cell migratory behaviors important to development. We propose that intermediate filaments provide an opportune platform for cells to both cope with mechanical forces and modulate signal transduction.

A mutation in keratin 18 that causes caspase-digestion resistance protects homozygous transgenic mice from hepatic apoptosis and injury

Cytoskeletal keratin 18 (K18) undergoes caspase-mediated digestion during apoptosis, which leads to dramatic disassembly of keratin filaments. We studied the significance of K18 caspase digestion in a mouse model and generated transgenic mice expressing the human K18 caspase digestion-resistant double-mutant K18-D238/397E in a mouse (m) K18-null background, and compared their response to injury mediated by administration of antibody against tumor necrosis factor receptor superfamily member 6 (Fas), anti-FasAb. Notably, K18-D238/397E;mK18-null mice were significantly more resistant to anti-FasAb-induced injury as compared with K18-WT;mK18-null mice (23% vs 57% lethality, respectively; P<0.001). The same applied when the toxin microcystin-LR (MLR) was used to induce liver injury, i.e. lethality of K18-D238/397E;mK18-null mice in response to MLR treatment was reduced compared with the control mouse strain. The lesser rate of apoptosis in K18-D238/397E;mK18-null livers is associated with delayed degradation and, thus, sustained activation of cell-survival-related protein kinases, including stress-activated protein kinases and the NF-κB transcription factor, up to 6-8 h after administration of anti-FasAb. However, activation of the kinases and NF-κB in K18-WT-reconstituted livers decreases dramatically 8 h after anti-FasAb administration. In addition, the D238/397E double-mutation results in prolonged stability of K18 protein in transfected cells and transgenic livers. Therefore, our results show that the caspase digestion-resistant K18 helps to maintain keratin filament organization and delays apoptosis, thereby resulting in protection from liver injury.

Keratin 8 limits TLR-triggered inflammatory responses through inhibiting TRAF6 polyubiquitination

Toll-like receptors (TLRs) have critical roles in innate immunity and inflammation and the detailed mechanisms by which TLR signaling is fine tuned remain unclear. Keratin 8 (CK8) belongs to the type II keratin family and is the major compontent of the intermediate filaments of simple or single-layered epithelia. Here we report that down-regulation of CK8 in mice enhanced TLR-mediated responses, rendering mice more susceptible to lipopolysaccharide (LPS)-induced endotoxin shock and Escherichia coli–caused septic peritonitis with reduced survival, elevated levels of inflammation cytokines and more severe tissue damage. We found that CK8 suppressed TLR-induced nuclear factor (NF)-κB activation and interacted with the adaptor tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) to prevent its polyubiquitination. Our findings demonstrate a novel role of CK8 in negative regulation of TLR/NF-κB signaling and highlight a previously unidentified nonclassical function for CK8 in limiting inflammatory responses.

Intermediate Filaments and Polarization in the Intestinal Epithelium

The cytoplasmic intermediate filament cytoskeleton provides a tissue-specific three-dimensional scaffolding with unique context-dependent organizational features. This is particularly apparent in the intestinal epithelium, in which the intermediate filament network is localized below the apical terminal web region and is anchored to the apical junction complex. This arrangement is conserved from the nematode Caenorhabditis elegans to humans. The review summarizes compositional, morphological and functional features of the polarized intermediate filament cytoskeleton in intestinal cells of nematodes and mammals. We emphasize the cross talk of intermediate filaments with the actin- and tubulin-based cytoskeleton. Possible links of the intermediate filament system to the distribution of apical membrane proteins and the cell polarity complex are highlighted. Finally, we discuss how these properties relate to the establishment and maintenance of polarity in the intestine.

Multiple roles for keratin intermediate filaments in the regulation of epithelial barrier function and apico-basal polarity

As multicellular organisms evolved a family of cytoskeletal proteins, the keratins (types I and II) expressed in epithelial cells diversified in more than 20 genes in vertebrates. There is no question that keratin filaments confer mechanical stiffness to cells. However, such a number of genes can hardly be explained by evolutionary advantages in mechanical features. The use of transgenic mouse models has revealed unexpected functional relationships between keratin intermediate filaments and intracellular signaling. Accordingly, loss of keratins or mutations in keratins that cause or predispose to human diseases, result in increased sensitivity to apoptosis, regulation of innate immunity, permeabilization of tight junctions, and mistargeting of apical proteins in different epithelia. Precise mechanistic explanations for these phenomena are still lacking. However, immobilization of membrane or cytoplasmic proteins, including chaperones, on intermediate filaments (“scaffolding”) appear as common molecular mechanisms and may explain the need for so many different keratin genes in vertebrates.

Alleviation mechanisms against hepatocyte oxidative stress in patients with chronic hepatic disorders

AIM

Autophagy induction and Mallory-Denk body (MDB) formation have been considered to have cytoprotective effects from cellular stress in liver diseases. We investigated the relations among oxidative stress, autophagy and MDB formation in patients with chronic hepatitis B (CHB), chronic hepatitis C (CHC) and non-alcoholic fatty liver disease (NAFLD) to clarify the alleviation mechanisms against oxidative stress of hepatocytes.

METHODS

First, we treated cultured cells with proteasome inhibitor (PI) or free fatty acid (FFA) and evaluated endoplasmic reticulum (ER) stress, oxidative stress, ubiquitinated proteins and p62 by western blotting. Then, we used human liver biopsy samples to evaluate oxidative stress, autophagy and MDB formation by immunohistochemical analysis.

RESULTS

Treatment with PI or FFA increased ER stress, oxidative stress, ubiquitinated proteins and p62 in cultured cells. Human liver biopsy samples of CHC and NAFLD showed that MDB formed in areas with strong oxidative stress and that the MDB-containing cells circumvented oxidative stress. Keratin 8 (K8) expression was strong in MDB-containing cells in CHC and NAFLD. However, in CHB samples, the expression of K8 was not increased in response to oxidative stress and MDB aggregates did not appear. Aminotransferase values were significantly lower in patients with CHC and NAFLD in whom light chain 3 antibody expression was increased in response to oxidative stress.

CONCLUSION

Strong expression of K8 was considered to be important for MDB formation. MDB protect liver cells from oxidative stress at a cellular level and autophagy reduced hepatic damage when it was induced in the hepatocytes exposed to strong oxidative stress.

Absence of keratin 8 or 18 promotes antimitochondrial autoantibody formation in aging male mice

Human mutations in keratin 8 (K8) and keratin 18 (K18), the intermediate filament proteins of hepatocytes, predispose to several liver diseases. K8-null mice develop chronic liver injury and fragile hepatocytes, dysfunctional mitochondria, and Th2-type colitis. We tested the hypothesis that autoantibody formation accompanies the liver damage that associates with K8/K18 absence. Sera from wild-type control, K8-null, and K18-null mice were analyzed by immunoblotting and immunofluorescence staining of cell and mouse tissue homogenates. Autoantibodies to several antigens were identified in 81% of K8-null male mice 8 mo or older. Similar autoantibodies were detected in aging K18-null male mice that had a related liver phenotype but normal colon compared with K8-null mice, suggesting that the autoantibodies are linked to liver rather than colonic disease. However, these autoantibodies were not observed in nontransgenic mice subjected to 4 chronic injury models. The autoantigens are ubiquitous and partition with mitochondria. Mass spectrometry and purified protein analysis identified, mitochondrial HMG-CoA synthase, aldehyde dehydrogenase, and catalase as the primary autoantigens, and glutamate dehydrogenase and epoxide hydrolase-2 as additional autoantigens. Therefore, absence of the hepatocyte keratins results in production of anti-mitochondrial autoantibodies (AMA) that recognize proteins involved in energy metabolism and oxidative stress, raising the possibility that AMA may be found in patients with keratin mutations that associate with liver and other diseases.

Human keratin 8 variants promote mouse acetaminophen hepatotoxicity coupled with c-jun amino-terminal kinase activation and protein adduct formation

Keratins 8 and 18 (K8/K18) are the intermediate filaments proteins of simple-type digestive epithelia and provide important cytoprotective function. K8/K18 variants predispose humans to chronic liver disease progression and poor outcomes in acute acetaminophen (APAP)-related liver failure. Given that K8 G62C and R341H/R341C are common K8 variants in European and North American populations, we studied their biological significance using transgenic mice. Mice that overexpress the human K8 variants, R341H or R341C, were generated and used together with previously described mice that overexpress wild-type K8 or K8 G62C. Mice were injected with 600 mg/kg of APAP or underwent bile duct ligation (BDL). Livers were evaluated by microarray analysis, quantitative real-time polymerase chain reaction, immunoblotting, histological and immunological staining, and biochemical assays. Under basal conditions, the K8 G62C/R341H/R341C variant-expressing mice did not show an obvious liver phenotype or altered keratin filament distribution, whereas K8 G62C/R341C animals had aberrant disulphide cross-linked keratins. Animals carrying the K8 variants displayed limited gene expression changes, but had lower nicotinamide N-methyl transferase (NNMT) levels and were predisposed to APAP-induced hepatotoxicity. NNMT represents a novel K8/K18-associated protein that becomes up-regulated after K8/K18 transfection. The more pronounced liver damage was accompanied by increased and prolonged JNK activation; elevated APAP protein adducts; K8 hyperphosphorylation at S74/S432 with enhanced keratin solubility; and prominent pericentral keratin network disruption. No differences in APAP serum levels, glutathione, or adenosine triphosphate levels were noted. BDL resulted in similar liver injury and biliary fibrosis in all mouse genotypes.

CONCLUSION

Expression of human K8 variants G62C, R341H, or R341C in mice predisposes to acute APAP hepatotoxicity, thereby providing direct evidence for the importance of these variants in human acute liver failure.

Tumor necrosis factor-α confers cardioprotection through ectopic expression of keratins K8 and K18

Tumor necrosis factor-α (TNF-α), one of the major stress-induced proinflammatory cytokines, is upregulated in the heart after tissue injury, and its sustained expression can contribute to the development of heart failure. Whether TNF-α also exerts cytoprotective effects in heart failure is not known. Here we provide evidence for a cardioprotective function of TNF-α in a genetic heart failure model, desmin-deficient mice. The cardioprotective effects of TNF-α are a consequence of nuclear factor-κB (NF-κB)-mediated ectopic expression in cardiomyocytes of keratin 8 (K8) and keratin 18 (K18), two epithelial-specific intermediate filament proteins. In cardiomyocytes, K8 and K18 (K8/K18) formed an alternative cytoskeletal network that localized mainly at intercalated discs (IDs) and conferred cardioprotection by maintaining normal ID structure and mitochondrial integrity and function. Ectopic induction of K8/K18 expression in cardiomyocytes also occurred in other genetic and experimental models of heart failure. Loss of the K8/K18 network resulted in a maladaptive cardiac phenotype following transverse aortic constriction. In human failing myocardium, where TNF-α expression is upregulated, K8/K18 were also ectopically expressed and localized primarily at IDs, which did not contain detectable amounts of desmin. Thus, TNF-α- and NF-κB-mediated formation of an alternative, stress-induced intermediate filament cytoskeleton has cardioprotective function in mice and potentially in humans.

HSP70-binding protein HSPBP1 regulates chaperone expression at a posttranslational level and is essential for spermatogenesis

Molecular chaperones play key roles during growth, development, and stress survival. The ability to induce chaperone expression enables cells to cope with the accumulation of nonnative proteins under stress and complete developmental processes with an increased requirement for chaperone assistance. Here we generate and analyze transgenic mice that lack the cochaperone HSPBP1, a nucleotide-exchange factor of HSP70 proteins and inhibitor of chaperone-assisted protein degradation. Male HSPBP1(-/-) mice are sterile because of impaired meiosis and massive apoptosis of spermatocytes. HSPBP1 deficiency in testes strongly reduces the expression of the inducible, antiapoptotic HSP70 family members HSPA1L and HSPA2, the latter of which is essential for synaptonemal complex disassembly during meiosis. We demonstrate that HSPBP1 affects chaperone expression at a posttranslational level by inhibiting the ubiquitylation and proteasomal degradation of inducible HSP70 proteins. We further provide evidence that the cochaperone BAG2 contributes to HSP70 stabilization in tissues other than testes. Our findings reveal that chaperone expression is determined not only by regulated transcription, but also by controlled degradation, with degradation-inhibiting cochaperones exerting essential prosurvival functions.

Beyond expectations: novel insights into epidermal keratin function and regulation

The epidermis is a stratified epithelium that relies on its cytoskeleton and cell junctions to protect the body against mechanical injury, dehydration, and infections. Keratin intermediate filament proteins are involved in many of these functions by forming cell-specific cytoskeletal scaffolds crucial for the maintenance of cell and tissue integrity. In response to various stresses, the expression and organization of keratins are altered at transcriptional and posttranslational levels to restore tissue homeostasis. Failure to restore tissue homeostasis in the presence of keratin gene mutations results in acute and chronic skin disorders for which currently no rational therapies are available. Here, we review the recent progress on the role of keratins in cytoarchitecture, adhesion, signaling, and inflammation. By focusing on epidermal keratins, we illustrate the contribution of keratin isotypes to differentiated epithelial functions.

Cytokeratin 18 is not required for morphogenesis of developing prostates but contributes to adult prostate regeneration

Cytokeratin 18 (CK18) is a key component of keratin-containing intermediate filaments and has long been used as a classic luminal cell marker in prostatic tissue. However, the in vivo function of CK18 in prostate is not known so far. We reported in this study, unexpectedly, that deletion of CK18 in a mouse model did not affect the morphological or the histological structures of adult prostate, as the CK18 knockout prostate displayed a normal glandular ductal structure, branching pattern, and composition of both luminal and basal cells. However, CK18 loss compromised the regenerative tubular branching in dorsolateral prostate after castration and androgen replacement. Therefore, in contrast to its importance as luminal cell marker, CK18 is dispensable for the prostate morphogenesis but contributes to adult prostate regeneration.

Keratin 9 is required for the structural integrity and terminal differentiation of the palmoplantar epidermis

Keratin 9 (K9) is a type I intermediate filament protein whose expression is confined to the suprabasal layers of the palmoplantar epidermis. Although mutations in the K9 gene are known to cause epidermolytic palmoplantar keratoderma, a rare dominant-negative skin disorder, its functional significance is poorly understood. To gain insight into the physical requirement and importance of K9, we generated K9-deficient (Krt9^−/−) mice. Here, we report that adult Krt9^−/−mice develop calluses marked by hyperpigmentation that are exclusively localized to the stress-bearing footpads. Histological, immunohistochemical, and immunoblot analyses of these regions revealed hyperproliferation, impaired terminal differentiation, and abnormal expression of keratins K5, K14, and K2. Furthermore, the absence of K9 induces the stress-activated keratins K6 and K16. Importantly, mice heterozygous for the K9-null allele (Krt9^+/−) show neither an overt nor histological phenotype, demonstrating that one Krt9 allele is sufficient for the developing normal palmoplantar epidermis. Together, our data demonstrate that complete ablation of K9 is not tolerable in vivo and that K9 is required for terminal differentiation and maintaining the mechanical integrity of palmoplantar epidermis.

Generation and characterisation of keratin 7 (K7) knockout mice

Keratin 7 (K7) is a Type II member of the keratin superfamily and despite its widespread expression in different types of simple and transitional epithelia, its functional role in vivo remains elusive, in part due to the lack of any appropriate mouse models or any human diseases that are associated with KRT7 gene mutations. Using conventional gene targeting in mouse embryonic stem cells, we report here the generation and characterisation of the first K7 knockout mouse. Loss of K7 led to increased proliferation of the bladder urothelium although this was not associated with hyperplasia. K18, a presumptive type I assembly partner for K7, showed reduced expression in the bladder whereas K20, a marker of the terminally differentiated superficial urothelial cells was transcriptionally up-regulated. No other epithelia were seen to be adversely affected by the loss of K7 and western blot and immunofluorescence microscopy analysis revealed that the expression of K8, K18, K19 and K20 were not altered in the absence of K7, with the exception of the kidney where there was reduced K18 expression.

Keratins control intercellular adhesion involving PKC-α-mediated desmoplakin phosphorylation

Keratins limit PKC-α phosphorylation activity and desmosome turnover to ensure the stability of epithelial intracellular adhesion.

Maintenance of epithelial cell adhesion is crucial for epidermal morphogenesis and homeostasis and relies predominantly on the interaction of keratins with desmosomes. Although the importance of desmosomes to epidermal coherence and keratin organization is well established, the significance of keratins in desmosome organization has not been fully resolved. Here, we report that keratinocytes lacking all keratins show elevated, PKC-α–mediated desmoplakin phosphorylation and subsequent destabilization of desmosomes. We find that PKC-α activity is regulated by Rack1–keratin interaction. Without keratins, desmosomes assemble but are endocytosed at accelerated rates, rendering epithelial sheets highly susceptible to mechanical stress. Re-expression of the keratin pair K5/14, inhibition of PKC-α activity, or blocking of endocytosis reconstituted both desmosome localization at the plasma membrane and epithelial adhesion. Our findings identify a hitherto unknown mechanism by which keratins control intercellular adhesion, with potential implications for tumor invasion and keratinopathies, settings in which diminished cell adhesion facilitates tissue fragility and neoplastic growth.

Proteomics identifies molecular networks affected by tetradecylthioacetic acid and fish oil supplemented diets

Fish oil (FO) and tetradecylthioacetic acid (TTA) - a synthetic modified fatty acid have beneficial effects in regulating lipid metabolism. In order to dissect the mechanisms underlying the molecular action of those two fatty acids we have investigated the changes in mitochondrial protein expression in a long-term study (50weeks) in male Wistar rats fed 5 different diets. The diets were as follows: low fat diet; high fat diet; and three diets that combined high fat diet with fish oil, TTA or combination of those two as food supplements. We used two different proteomics techniques: a protein centric based on 2D gel electrophoresis and mass spectrometry, and LC-MS(E) based peptide centric approach. As a result we provide evidence that fish oil and TTA modulate mitochondrial metabolism in a synergistic manner yet the effects of TTA are much more dramatic. We demonstrate that fatty acid metabolism; lipid oxidation, amino acid metabolism and oxidative phosphorylation pathways are involved in fish oil and TTA action. Evidence for the involvement of PPAR mediated signalling is provided. Additionally we postulate that down regulation of components of complexes I and II contributes to the strong antioxidant properties of TTA.

BIOLOGICAL SIGNIFICANCE

This study for the first time explores the effect of fish oil and TTA - tetradecyl-thioacetic acid and the combination of those two as diet supplements on mitochondria metabolism in a comprehensive and systematic manner. We show that fish oil and TTA modulate mitochondrial metabolism in a synergistic manner yet the effects of TTA are much more dramatic. We demonstrate in a large scale that fatty acid metabolism and lipid oxidation are affected by fish oil and TTA, a phenomenon already known from more directed molecular biology studies. Our approach, however, shows additionally that amino acid metabolism and oxidative phosphorylation pathways are also strongly affected by TTA and also to some extent by fish oil administration. Strong evidence for the involvement of PPAR mediated signalling is provided linking the different metabolic effects. The global and systematic viewpoint of this study compiles many of the known phenomena related to the effects of fish oil and fatty acids giving a solid foundation for further exploratory and more directed studies of the mechanisms behind the beneficial and detrimental effects of fish oil and TTA diet supplementation. This work is already a second article in a series of studies conducted using this model of dietary intervention. In the previous study (Vigerust et al., [21]) the effects of fish oil and TTA on the plasma lipids and cholesterol levels as well as key metabolic enzymes in the liver have been studied. In an ongoing study more work is being done to explore in detail for example the link between the down regulation of the components of the respiratory chain (observed in this study) and the strong antioxidant effects of TTA. The reference diet in this study has been designed to mimic an unhealthy - high fat diet that is thought to contribute to the development of metabolic syndrome - a condition that is strongly associated with diabetes, obesity and heart failure. Fish oil and TTA are known to have beneficial effects for the fatty acid metabolism and have been shown to alleviate some of the symptoms of the metabolic syndrome. To date very little is known about the molecular mechanisms behind these beneficial effects and the potential pitfalls of the consumption of those two compounds. Only studies of each compound separately and using only small scale molecular biology approaches have been carried out. The results of this work provide an excellent starting point for further studies that will help to understand the metabolic effects of fish oil and TTA and will hopefully help to design dietary programs directed towards reduction of the prevalence of metabolic syndrome and associated diseases.

Understanding the role of keratins 8 and 18 in neoplastic potential of breast cancer derived cell lines

Background

Breast cancer is a complex disease which cannot be defined merely by clinical parameters like lymph node involvement and histological grade, or by routinely used biomarkers like estrogen receptor (ER), progesterone receptor (PGR) and epidermal growth factor receptor 2 (HER2) in diagnosis and prognosis. Breast cancer originates from the epithelial cells. Keratins (K) are cytoplasmic intermediate filament proteins of epithelial cells and changes in the expression pattern of keratins have been seen during malignant transformation in the breast. Expression of the K8/18 pair is seen in the luminal cells of the breast epithelium, and its role in prognostication of breast cancer is not well understood.

Methodology/Principal Findings

In this study, we have modulated K8 expression to understand the role of the K8/18 pair in three different breast epithelium derived cell lines: non-transformed MCF10A, transformed but poorly invasive MDA MB 468 and highly invasive MDA MB 435. The up-regulation of K8 in the invasive MDA MB 435 cell line resulted in a significant decrease in proliferation, motility, in-vitro invasion, tumor volume and lung metastasis. The down-regulation of K8 in MDA MB 468 resulted in a significant increase in transformation potential, motility and invasion in-vitro, while MCF10A did not show any changes in cell transformation assays.

Conclusions/Significance

These results indicate the role of K8/18 in modulating invasion in breast cancer -its presence correlating with less invasive phenotype and absence correlating with highly invasive, dedifferentiated phenotype. These data may have important implications for prognostication of breast cancer.

Understanding the role of keratins 8 and 18 in neoplastic potential of breast cancer derived cell lines

BACKGROUND

Breast cancer is a complex disease which cannot be defined merely by clinical parameters like lymph node involvement and histological grade, or by routinely used biomarkers like estrogen receptor (ER), progesterone receptor (PGR) and epidermal growth factor receptor 2 (HER2) in diagnosis and prognosis. Breast cancer originates from the epithelial cells. Keratins (K) are cytoplasmic intermediate filament proteins of epithelial cells and changes in the expression pattern of keratins have been seen during malignant transformation in the breast. Expression of the K8/18 pair is seen in the luminal cells of the breast epithelium, and its role in prognostication of breast cancer is not well understood.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we have modulated K8 expression to understand the role of the K8/18 pair in three different breast epithelium derived cell lines: non-transformed MCF10A, transformed but poorly invasive MDA MB 468 and highly invasive MDA MB 435. The up-regulation of K8 in the invasive MDA MB 435 cell line resulted in a significant decrease in proliferation, motility, in-vitro invasion, tumor volume and lung metastasis. The down-regulation of K8 in MDA MB 468 resulted in a significant increase in transformation potential, motility and invasion in-vitro, while MCF10A did not show any changes in cell transformation assays.

CONCLUSIONS/SIGNIFICANCE

These results indicate the role of K8/18 in modulating invasion in breast cancer -its presence correlating with less invasive phenotype and absence correlating with highly invasive, dedifferentiated phenotype. These data may have important implications for prognostication of breast cancer.

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.

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.

Oxidative stress, Nrf2 and keratin up-regulation associate with Mallory-Denk body formation in mouse erythropoietic protoporphyria

Mallory-Denk bodies (MDBs) are hepatocyte inclusions commonly seen in steatohepatitis. They are induced in mice by feeding 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) for 12 weeks, which also causes porphyrin accumulation. Erythropoietic protoporphyria (EPP) is caused by mutations in ferrochelatase (fch), and a fraction of EPP patients develop liver disease that is phenocopied in Fech(m1Pas) mutant (fch/fch) mice, which have an inactivating fch mutation. fch/fch mice develop spontaneous MDBs, but the molecular factors involved in their formation and whether they relate to DDC-induced MDBs are unknown. We tested the hypothesis that fch mutation creates a molecular milieu that mimics experimental drug-induced MDBs. In 13- and 20-week-old fch/fch mice, serum alkaline phosphatase, alanine aminotransferase, and bile acids were increased. The 13-week-old fch/fch mice did not develop histologically evident MDBs but manifested biochemical alterations required for MDB formation, including increased transglutaminase-2 and keratin overexpression, with a greater keratin 8 (K8)-to-keratin 18 (K18) ratio, which are critical for drug-induced MDB formation. In 20-week-old fch/fch mice, spontaneous MDBs were readily detected histologically and biochemically. Short-term (3-week) DDC feeding markedly induced MDB formation in 20-week-old fch/fch mice. Under basal conditions, old fch/fch mice had significant alterations in mitochondrial oxidative-stress markers, including increased protein oxidation, decreased proteasomal activity, reduced adenosine triphosphate content, and Nrf2 (redox sensitive transcription factor) up-regulation. Nrf2 knockdown in HepG2 cells down-regulated K8, but not K18.

CONCLUSION

Fch/fch mice develop age-associated spontaneous MDBs, with a marked propensity for rapid MDB formation upon exposure to DDC, and therefore provide a genetic model for MDB formation. Inclusion formation in the fch/fch mice involves oxidative stress which, together with Nrf2-mediated increase in K8, promotes MDB formation.

Genetic background effects of keratin 8 and 18 in a DDC-induced hepatotoxicity and Mallory-Denk body formation mouse model

Keratin 8 (K8) and keratin 18 (K18) form the major hepatocyte cytoskeleton. We investigated the impact of genetic loss of either K8 or K18 on liver homeostasis under toxic stress with the hypothesis that K8 and K18 exert different functions. krt8⁻/⁻ and krt18⁻/⁻ mice crossed into the same 129-ola genetic background were treated by acute and chronic administration of 3,5-diethoxy-carbonyl-1,4-dihydrocollidine (DDC). In acutely DDC-intoxicated mice, macrovesicular steatosis was more pronounced in krt8⁻/⁻ and krt18⁻/⁻ compared with wild-type (wt) animals. Mallory-Denk bodies (MDBs) appeared in krt18⁻/⁻ mice already at an early stage of intoxication in contrast to krt8⁻/⁻ mice that did not display MDB formation when fed with DDC. Keratin-deficient mice displayed significantly lower numbers of apoptotic hepatocytes than wt animals. krt8⁻/⁻, krt18⁻/⁻ and control mice displayed comparable cell proliferation rates. Chronically DDC-intoxicated krt18⁻/⁻ and wt mice showed a similarly increased degree of steatohepatitis with hepatocyte ballooning and MDB formation. In krt8⁻/⁻ mice, steatosis was less, ballooning, and MDBs were absent. krt18⁻/⁻ mice developed MDBs whereas krt8⁻/⁻ mice on the same genetic background did not, highlighting the significance of different structural properties of keratins. They are independent of the genetic background as an intrinsic factor. By contrast, toxicity effects may depend on the genetic background. krt8⁻/⁻ and krt18⁻/⁻ mice on the same genetic background show similar sensitivity to DDC intoxication and almost resemble wt animals regarding survival, degree of porphyria, liver-to-body weight ratio, serum bilirubin and liver enzyme levels. This stands in contrast to previous work where krt8⁻/⁻ and krt18⁻/⁻ mice on different genetic backgrounds were investigated.

Generation of a cholangiocyte-specific cDNA expression library for the identification of B and T cell autoantigens in murine biliary disease

AIM

  Several mouse models of inflammatory cholangiopathies exist, including biliary atresia, primary biliary cirrhosis, autoimmune hepatitis, and primary sclerosing cholangitis. In an ongoing effort to identify the target antigens of both infiltrating autoreactive T cells and serum autoantibodies, we aimed to generate a cholangiocyte-derived cDNA library capable of expressing a wide variety of proteins.

METHODS

  mRNA was isolated from a normal mouse cholangiocyte cell line and reverse transcribed into cDNA. After initial cloning of the cDNA into a transfer vector (pDONR222), the entire library was shuttled into an Escherichia coli expression vector (pDEST160).

RESULTS

  The library contains 2.3 × 10(6) independent clones and expresses proteins up to 100 kD in molecular weight. Using a variety of techniques, including western blot analysis, mass spectrometry of individual clones, and direct DNA sequencing of plasmids, a number of both ubiquitously expressed and cholangiocyte-specific proteins (e.g. cytokeratin 19) have been identified within.

CONCLUSION

  A comprehensive mouse cholangiocyte cDNA expression library has been generated and is available for use as a source of multiple cholangiocyte-specific antigens for immunological studies. The library can be used to screen for specificity of T cell lines or hybridomas. Furthermore, this library has potential uses in SEREX analysis of autoantibody reactivity. The cholangiocyte-specific cDNA library is a powerful tool for the identification of target antigens in murine inflammatory cholangiopathies and is available as a shared resource.

Aging modulates susceptibility to mouse liver Mallory-Denk body formation

Mallory-Denk bodies (MDBs) are hepatocyte cytoplasmic inclusions found in several liver diseases and consist primarily of the cytoskeletal proteins, keratins 8 and 18 (K8/K18). Recent evidence indicates that the extent of stress-induced protein misfolding, a K8>K18 overexpression state, and transglutaminase-2 activation promote MDB formation. In addition, the genetic background and gender play an important role in mouse MDB formation, but the effect of aging on this process is unknown. Given that oxidative stress increases with aging, the authors hypothesized that aging predisposes to MDB formation. They used an established mouse MDB model-namely, feeding non-transgenic male FVB/N mice (1, 3, and 8 months old) with 3,5 diethoxycarbonyl-1,4-dihydrocollidine for 2 months. MDB formation was assessed using immunofluorescence staining and biochemically by demonstrating keratin and ubiquitin-containing crosslinks generated by transglutaminase-2. Immunofluorescence staining showed that old mice had a significant increase in MDB formation compared with young mice. MDB formation paralleled the generation of high molecular weight ubiquitinated keratin-containing complexes and induction of p62. Old mouse livers had increased oxidative stress. In addition, 20S proteasome activity and autophagy were decreased, and endoplasmic reticulum stress was increased in older livers. Therefore, aging predisposes to experimental MDB formation, possibly by decreased activity of protein degradation machinery.