Detection of Intestinal Inflammation by Vascular Adhesion Protein-1-Targeted [68Ga]Ga-DOTA-Siglec-9 Positron Emission Tomography in Murine Models of Inflammatory Bowel Disease

PURPOSE

Inflammatory bowel disease (IBD) can be imaged with positron emission tomography (PET), but existing PET radiopharmaceuticals have limited diagnostic accuracy. Vascular adhesion protein-1 (VAP-1) is an endothelial cell surface molecule that controls leukocyte extravasation into sites of inflammation. However, the role of inflammation-induced VAP-1 expression in IBD is still unclear. Therefore, this study investigated the utility of VAP-1-targeted [68Ga]Ga-DOTA-Siglec-9 positron emission tomography/computed tomography (PET/CT) for assessing inflammation in two mouse models of IBD.

PROCEDURES

Studies were performed using K8-/- mice that develop a chronic colitis-phenotype and C57Bl/6NCrl mice with acute intestinal inflammation chemically-induced using 2.5% dextran sodium sulfate (DSS) in drinking water. In both diseased and control mice, uptake of the VAP-1-targeting peptide [68Ga]Ga-DOTA-Siglec-9 was assessed in intestinal regions of interest using in vivo PET/CT, after which ex vivo gamma counting, digital autoradiography, and histopathological analyses were performed. Immunofluorescence staining was performed to determine VAP-1-expression in the intestine, including in samples from patients with ulcerative colitis.

RESULTS

Intestinal inflammation could be visualized by [68Ga]Ga-DOTA-Siglec-9 PET/CT in two murine models of IBD. In both models, the in vivo PET/CT and ex vivo studies of [68Ga]Ga-DOTA-Siglec-9 uptake were significantly higher than in control mice. The in vivo uptake was increased on average 1.4-fold in the DSS model and 2.0-fold in the K8-/- model. Immunofluorescence staining revealed strong expression of VAP-1 in the inflamed intestines of both mice and patients.

CONCLUSIONS

This study suggests that the VAP-1-targeting [68Ga]Ga-DOTA-Siglec-9 PET tracer is a promising tool for non-invasive imaging of intestinal inflammation. Future studies in patients with IBD and evaluation of the potential value of [68Ga]Ga-DOTA-Siglec-9 in diagnosis and monitoring of the disease are warranted.

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.

Defining a timeline of colon pathologies after keratin 8 loss: rapid crypt elongation and diarrhea are followed by epithelial erosion and cell exfoliation

Keratins are epithelial intermediate filament proteins that play a crucial role in cellular stress protection, with K8 being the most abundant in the colon. The intestinal epithelial-specific K8-deficient mouse model (K8flox/flox;Villin-Cre) exhibits characteristics of inflammatory bowel disease, including diarrhea, crypt erosion, hyperproliferation, and decreased barrier function. Nevertheless, the order in which these events occur and whether they are a direct cause of K8 loss or a consequence of one event inducing another remains unexplored. Increased knowledge about early events in the disruption of colon epithelial integrity would help to understand the early pathology of inflammatory and functional colon disorders and develop preclinical models and diagnostics of colonic diseases. Here, we aimed to characterize the order of physiological events after Krt8 loss by utilizing K8flox/flox;Villin-CreERt2 mice with tamoxifen-inducible Krt8 deletion in intestinal epithelial cells, and assess stool analysis as a noninvasive method to monitor real-time gene expression changes following Krt8 loss. K8 protein was significantly decreased within a day after induction, followed by its binding partners, K18 and K19 from day 4 onward. The sequential colonic K8 downregulation in adult mice leads to immediate diarrhea and crypt elongation with activation of proliferation signaling, followed by crypt loss and increased neutrophil activity within 6-8 days, highlighting impaired water balance and crypt elongation as the earliest colonic changes upon Krt8 loss. Furthermore, epithelial gene expression patterns were comparable between colon tissue and stool samples, demonstrating the feasibility of noninvasive monitoring of gut epithelia in preclinical research utilizing Cre-LoxP-based intestinal disease models.NEW & NOTEWORTHY Understanding the order in which physiological and molecular events occur helps to recognize the onset of diseases and improve their preclinical models. We utilized Cre-Lox-based inducible keratin 8 deletion in mouse intestinal epithelium to characterize the earliest events after keratin 8 loss leading to colitis. These include diarrhea and crypt elongation, followed by erosion and neutrophil activity. Our results also support noninvasive methodology for monitoring colon diseases in preclinical models.

Regulation of epithelial transitional states in murine and human pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease arising from impaired regeneration of the alveolar epithelium after injury. During regeneration, type 2 alveolar epithelial cells (AEC2s) assume a transitional state that upregulates multiple keratins and ultimately differentiate into AEC1s. In IPF, transitional AECs accumulate with ineffectual AEC1 differentiation. However, whether and how transitional cells cause fibrosis, whether keratins regulate transitional cell accumulation and fibrosis, and why transitional AECs and fibrosis resolve in mouse models but accumulate in IPF are unclear. Here, we show that human keratin 8 (KRT8) genetic variants were associated with IPF. Krt8-/- mice were protected from fibrosis and accumulation of the transitional state. Keratin 8 (K8) regulated the expression of macrophage chemokines and macrophage recruitment. Profibrotic macrophages and myofibroblasts promoted the accumulation of transitional AECs, establishing a K8-dependent positive feedback loop driving fibrogenesis. Finally, rare murine transitional AECs were highly senescent and basaloid and may not differentiate into AEC1s, recapitulating the aberrant basaloid state in human IPF. We conclude that transitional AECs induced and were maintained by fibrosis in a K8-dependent manner; in mice, most transitional cells and fibrosis resolved, whereas in human IPF, transitional AECs evolved into an aberrant basaloid state that persisted with progressive fibrosis.

The Effect of a Fish Oil and/or Probiotic Intervention from Early Pregnancy Onwards on Colostrum Immune Mediators: A Randomized, Placebo-Controlled, Double-Blinded Clinical Trial in Overweight/Obese Mothers

SCOPE

Modifying the composition of colostrum by external factors may provide opportunities to improve the infant's health. Here, we evaluated how fish oil and/or probiotics supplementation modify concentrations of colostrum immune mediators and their associations with perinatal clinical factors on mothers with overweight/obesity.

METHODS AND RESULTS

Pregnant women were randomized in a double-blind manner into four intervention groups, and the supplements were consumed daily from early pregnancy onwards. Colostrum samples were collected from 187 mothers, and 16 immune mediators were measured using bead-based immunoassays. Interventions modified colostrum composition; the fish oil+probiotics group had higher concentrations of IL-12p70 than probiotics+placebo and higher FMS-like tyrosine kinase 3 ligand (FLT-3L) than fish oil+placebo and probiotics+placebo (one-way analysis of variance, post-hoc Tukey's test). Although the fish oil+probiotics group had higher levels of IFNα2 compared to the fish oil+placebo group, these differences were not statistically significant after correction for multiple testing. Multivariate linear model revealed significant associations between several immune mediators and the perinatal use of medication.

CONCLUSION

Fish oil/probiotics intervention exerted a minor effect on concentrations of colostrum immune mediators. However, medication during the perinatal period modulated the immune mediators. These changes in colostrum's composition may contribute to immune system development in the infant.

Keratin 7 expression in different anatomical parts of colonic epithelium in inflammatory bowel diseases and its prognostic value: a 3-year follow-up study

The diagnosis of inflammatory bowel diseases (IBD) may be challenging and their clinical course, characterized by relapses and spontaneous or drug-induced remissions, is difficult to predict. Novel prognostic biomarkers are needed. Keratin 7 (K7) is a cytoskeletal intermediate filament protein which is not normally expressed in the colonic epithelium. It was recently shown that K7 expression in the colonic epithelium is associated with ulcerative colitis and Crohn's disease, the two main subtypes of IBD. Here we investigated IBD associated K7 neo-expression in different regions of colon and terminal ileum. The correlation of the K7 expression with the inflammatory activity of the epithelium was analyzed in each region. The prognostic value of K7 was estimated by comparing the clinical disease activity after 3 years with the K7 expression at the time of enrollment. Our data shows that the level of K7 expression in inflamed epithelium varies depending on the anatomical region and it is the most pronounced in ascending and descending colon, but it did not predict the severity of IBD for the following 3 years. These results warrant future studies focusing on the biological role of K7 in colon and its utilization as potential IBD biomarker.

Cytoplasmic keratins couple with and maintain nuclear envelope integrity in colonic epithelial cells

Keratin intermediate filaments convey mechanical stability and protection against stress to epithelial cells. Keratins are essential for colon health, as seen in keratin 8 knockout (K8^-/-) mice exhibiting a colitis phenotype. We hypothesized that keratins support the nuclear envelope and lamina in colonocytes. K8^-/- colonocytes in vivo exhibit significantly decreased levels of lamins A/C, B1, and B2 in a colon-specific and cell-intrinsic manner. CRISPR/Cas9- or siRNA-mediated K8 knockdown in Caco-2 cells similarly decreased lamin levels, which recovered after reexpression of K8 following siRNA treatment. Nuclear area was not decreased, and roundness was only marginally increased in cells without K8. Down-regulation of K8 in adult K8^flox/flox;Villin-CreER^t2 mice following tamoxifen administration significantly decreased lamin levels at day 4 when K8 levels had reduced to 40%. K8 loss also led to reduced levels of plectin, LINC complex, and lamin-associated proteins. While keratins were not seen in the nucleoplasm without or with leptomycin B treatment, keratins were found intimately located at the nuclear envelope and complexed with SUN2 and lamin A. Furthermore, K8 loss in Caco-2 cells compromised nuclear membrane integrity basally and after shear stress. In conclusion, colonocyte K8 helps maintain nuclear envelope and lamina composition and contributes to nuclear integrity.

Vimentin Suppresses Inflammation and Tumorigenesis in the Mouse Intestine

Vimentin has been implicated in wound healing, inflammation, and cancer, but its functional contribution to intestinal diseases is poorly understood. To study how vimentin is involved during tissue injury and repair of simple epithelium, we induced colonic epithelial cell damage in the vimentin null (Vim^−/−) mouse model. Vim^−/− mice challenged with dextran sodium sulfate (DSS) had worse colitis manifestations than wild-type (WT) mice. Vim^−/− colons also produced more reactive oxygen and nitrogen species, possibly contributing to the pathogenesis of gut inflammation and tumorigenesis than in WT mice. We subsequently describe that CD11b⁺ macrophages served as the mainly cellular source of reactive oxygen species (ROS) production via vimentin-ROS-pSTAT3–interleukin-6 inflammatory pathways. Further, we demonstrated that Vim^−/− mice did not develop colitis-associated cancer model upon DSS treatment spontaneously but increased tumor numbers and size in the distal colon in the azoxymethane/DSS model comparing with WT mice. Thus, vimentin has a crucial role in protection from colitis induction and tumorigenesis of the colon.

Targeted deletion of keratin 8 in intestinal epithelial cells disrupts tissue integrity and predisposes to tumorigenesis in the colon

Keratin 8 (K8) is the main intestinal epithelial intermediate filament protein with proposed roles for colonic epithelial cell integrity. Here, we used mice lacking K8 in intestinal epithelial cells (floxed K8 and Villin-Cre1000 and Villin-CreERt2) to investigate the cell-specific roles of intestinal epithelial K8 for colonocyte function and pathologies. Intestinal epithelial K8 deletion decreased K8 partner proteins, K18-K20, 75-95%, and the remaining keratin filaments were located at the colonocyte apical regions with type II K7, which decreased 30%. 2-Deoxy-2-[18F]-fluoroglucose positron emission tomography in vivo imaging identified a metabolic phenotype in the lower gut of the conditional K8 knockouts. These mice developed intestinal barrier leakiness, mild diarrhea, and epithelial damage, especially in the proximal colon. Mice exhibited shifted differentiation from enterocytes to goblet cells, displayed longer crypts and an increased number of Ki67 + transit-amplifying cells in the colon. Significant proproliferative and regenerative signaling occurred in the IL-22, STAT3, and pRb pathways, with minor effects on inflammatory parameters, which, however, increased in aging mice. Importantly, colonocyte K8 deletion induced a dramatically increased sensitivity to azoxymethane-induced tumorigenesis. In conclusion, intestinal epithelial K8 plays a significant role in colonocyte epithelial integrity maintenance, proliferation regulation and tumor suppression.

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.

Novel Selective Estrogen Receptor Modulator Ameliorates Murine Colitis

Estrogen-receptor-mediated signaling has been suggested to decrease the inflammatory response in monocyte macrophages. Previously, we showed that a novel selective estrogen receptor modulator (SERM2) promotes anti-inflammatory phenotype of monocytes in vitro. In this study, we demonstrate the potential of SERM2 in amelioration of colitis. We utilized a dextran sodium sulfate (DSS)-induced colitis model in FVB/n mice to demonstrate the effects of orally administered SERM2 on the clinical status of the mice and the histopathological changes in the colon, as well as proportion of Mrc-1 positive macrophages. SERM2 nuclear receptor affinities were measured by radioligand binding assays. Orally administered, this compound significantly alleviated DSS-induced colitis in male mice and induced local estrogen receptor activation in the inflamed colon, as well as promoting anti-inflammatory cytokine expression and infiltration of anti-inflammatory monocytes. We show that this novel drug candidate has an affinity to estrogen receptors α and β and progesterone receptors, but not to glucocorticoid receptor, thus expressing unique binding properties compared to other sex steroid receptor ligands. These results indicate that novel drug candidates to alleviate inflammatory conditions of the colon could be found among sex steroid receptor activating compounds.

Keratins regulate β-cell mitochondrial morphology, motility, and homeostasis

Loss of the epithelial intermediate filament protein keratin 8 (K8) in murine β cells leads to irregular insulin vesicles and decreased insulin levels. Because mitochondria are central in glucose-stimulated insulin secretion, the relationship between keratins and β-cell mitochondrial function and morphology was investigated. β cells in murine K8-knockout (K8^-/-) islets of Langerhans have increased numbers of mitochondria, which are rounder and have diffuse cristae, as seen by electron microscopy. The mitochondrial network in primary cultured K8^-/- β cells is more fragmented compared with K8^+/+ mitochondria, correlating with decreased levels of mitofusin 2 and the mitofusin 2- and keratin-binding protein trichoplein. K8^-/- β-cell mitochondria have decreased levels of total and mitochondrial cytochrome c, which correlates with a reduction in electron transport complexes I and IV. This provokes loss of mitochondrial membrane potential and reduction of ATP and insulin amount, as seen in K8^-/- β cells. Mitochondria in K8 wild-type β cells and MIN6 insulinoma cells overexpressing K8 and 18 are more stationary compared with mitochondria in keratin-deficient cells. In conclusion, keratins, likely through trichoplein-mitofusin interactions, regulate both structural and dynamic functions of β-cell mitochondria, which could have implications for downstream insulin secretion.-Silvander, J. S. G., Kvarnström, S. M., Kumari-Ilieva, A., Shrestha, A., Alam, C. M., Toivola, D. M. Keratins regulate β-cell mitochondrial morphology, motility, and homeostasis.

Keratin 8-deletion induced colitis predisposes to murine colorectal cancer enforced by the inflammasome and IL-22 pathway

Keratins (K) are intermediate filament proteins important in protection from cellular stress. K8, K18 and K19 are the main components of keratin filaments in colonic epithelia but their role in intestinal diseases remains ambiguous. A function for keratins in intestinal health is supported by the K8-knock-out (K8(-/-)) mouse which manifests an early chronic ulcerative colitis-like inflammatory bowel disease and epithelial hyperproliferation. We tested whether K8(-/-) mice are more susceptible to colorectal cancer (CRC) compared to K8 wild type (K8(+/+)), and K8 heterozygote (K8(+/-)) mice showing increased proliferation but no inflammation. K8(-/-) mice did not develop CRC spontaneously, but had dramatically increased numbers of tumors in the distal colon in the azoxymethane (AOM) and Apc(Min/+) CRC models while neither K8(+/+) nor K8(+/-) mice were susceptible. Upregulation of IL-22 in combination with a complete loss of its negative regulator IL-22BP, and increased downstream STAT3-signaling in K8(-/-) and K8(-/-)Apc(Min/+) colonic epithelia confirmed that the IL-22 pathway, important in inflammation, proliferation and tissue regeneration, was activated. The nearly total loss of IL-22BP correlated with an activated inflammasome leading to increased cleaved caspase-1, and the putative IL-22BP inhibitor, IL-18, as well as a decrease in ALDH1/2. Ablation of K8 in a colorectal cancer cell line similarly resulted in increased IL-18 and decreased ALDH1/2. K8/K18 co-immunoprecipitated with pro-caspase-1, a component of the inflammasome in the colon, which suggests that keratins modulate inflammasome activity and protect the colon from inflammation and tumorigenesis. The K8-null mouse models also provide novel epithelial-derived robust colon-specific CRC models.

Keratin 8 knockdown leads to loss of the chloride transporter DRA in the colon

Keratins (K) are intermediate filament proteins important in protection from stress. The roles of keratins in the intestine are not clear, but K8 knockout (K8(-/-)) mice develop a Th2-type colonic inflammation, epithelial hyperproliferation, and mild diarrhea caused by a keratin level-dependent decrease in short-circuit current and net sodium and chloride absorption in the distal colon. The lack of K8 leads to mistargeting or altered levels of membrane proteins in colonocytes; however, the main transporter responsible for the keratin-related ion transport defect is unknown. We here analyzed protein and mRNA levels of candidate ion transporters CFTR, PAT-1, NHE-3, and DRA in ileum, cecum, and proximal and distal colon. Although no differences were observed for CFTR, PAT-1, or NHE-3, DRA mRNA levels were decreased by three- to fourfold and DRA protein was almost entirely lost in K8(-/-) cecum and proximal and distal colon compared with K8(+/+), whereas the levels in ileum were normal. In K8(+/-) mice, DRA mRNA levels were unaltered, while decreased DRA protein levels were detected in the proximal colon. Immunofluorescence staining confirmed the loss of DRA in K8(-/-) distal colon, while K8(+/-) displayed a similar but more patchy apical DRA distribution compared with K8(+/+) DRA was similarly decreased when K8 was knocked down in Caco-2 cells, confirming that K8 levels modulate DRA levels in an inflammation-independent manner. Taken together, the loss of DRA in the K8(-/-) mouse colon and cecum explains the dramatic chloride transport defect and diarrheal phenotype after K8 inactivation and identifies K8 as a novel regulator of DRA.

Targeted modulation of cell differentiation in distinct regions of the gastrointestinal tract via oral administration of differently PEG-PEI functionalized mesoporous silica nanoparticles

Targeted delivery of drugs is required to efficiently treat intestinal diseases such as colon cancer and inflammation. Nanoparticles could overcome challenges in oral administration caused by drug degradation at low pH and poor permeability through mucus layers, and offer targeted delivery to diseased cells in order to avoid adverse effects. Here, we demonstrate that functionalization of mesoporous silica nanoparticles (MSNs) by polymeric surface grafts facilitates transport through the mucosal barrier and enhances cellular internalization. MSNs functionalized with poly(ethylene glycol) (PEG), poly(ethylene imine) (PEI), and the targeting ligand folic acid in different combinations are internalized by epithelial cells in vitro and in vivo after oral gavage. Functionalized MSNs loaded with γ-secretase inhibitors of the Notch pathway, a key regulator of intestinal progenitor cells, colon cancer, and inflammation, demonstrated enhanced intestinal goblet cell differentiation as compared to free drug. Drug-loaded MSNs thus remained intact in vivo, further confirmed by exposure to simulated gastric and intestinal fluids in vitro. Drug targeting and efficacy in different parts of the intestine could be tuned by MSN surface modifications, with PEI coating exhibiting higher affinity for the small intestine and PEI–PEG coating for the colon. The data highlight the potential of nanomedicines for targeted delivery to distinct regions of the tissue for strict therapeutic control.

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.

Keratin 8 absence down-regulates colonocyte HMGCS2 and modulates colonic ketogenesis and energy metabolism

Absence of colonic keratin 8 causes intestinal inflammation and decreased levels of the ketogenic enzyme HMGCS2. Upstream, the butyrate transporter MCT1 is decreased, leading to increased luminal butyrate. Ketogenic conditions fail to induce HMGCS2 in the keratin 8–knockout colon, suggesting a role for keratins in colonocyte energy homeostasis.

Simple-type epithelial keratins are intermediate filament proteins important for mechanical stability and stress protection. Keratin mutations predispose to human liver disorders, whereas their roles in intestinal diseases are unclear. Absence of keratin 8 (K8) in mice leads to colitis, decreased Na/Cl uptake, protein mistargeting, and longer crypts, suggesting that keratins contribute to intestinal homeostasis. We describe the rate-limiting enzyme of the ketogenic energy metabolism pathway, mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2), as a major down-regulated protein in the K8-knockout (K8^−/−) colon. K8 absence leads to decreased quantity and activity of HMGCS2, and the down-regulation is not dependent on the inflammatory state, since HMGCS2 is not decreased in dextran sulfate sodium-induced colitis. Peroxisome proliferator–activated receptor α, a transcriptional activator of HMGCS2, is similarly down-regulated. Ketogenic conditions—starvation or ketogenic diet—increase K8^+/+ HMGCS2, whereas this response is blunted in the K8^−/− colon. Microbiota-produced short-chain fatty acids (SCFAs), substrates in the colonic ketone body pathway, are increased in stool, which correlates with decreased levels of their main transporter, monocarboxylate transporter 1 (MCT1). Microbial populations, including the main SCFA-butyrate producers in the colon, were not altered in the K8^−/−. In summary, the regulation of the SCFA-MCT1-HMGCS2 axis is disrupted in K8^−/− colonocytes, suggesting a role for keratins in colonocyte energy metabolism and homeostasis.

Keratins in health and disease

The cytoprotective keratins (K) compose the intermediate filaments of epithelial cells and their inherited and spontaneous mutations give rise to keratinopathies. For example, mutations in K1/K5/K10/K14 cause epidermal skin diseases whereas simple epithelial K8/K18/K19 variants predispose to development of several liver disorders. Due to their abundance, tissue- and context-specific expression, keratins constitute excellent diagnostic markers of both neoplastic and non-neoplastic diseases. During injury and in disease, keratin expression levels, cellular localization or posttranslational modifications are altered. Accumulating evidence suggests that these changes modulate multiple processes including cell migration, tumor growth/metastasis and development of infections. Therefore, our understanding of keratins is shifting from diagnostic markers to active disease modifiers.

On the non-linear attachment characteristics of blood to bacterial cellulose/kaolin biomaterials

In this communication, we report a non-linear variation in the strength of blood attachment to bacterial cellulose/kaolin biomaterials as the fractions of bacterial cellulose to kaolin are increased. The changes observed for attachment strength are elucidated following both experimental and numerical investigations on both the biomaterial and the blood-biomaterial interface. Our research reveals that the non-linear strength of attachment of blood is related to topographical characteristics on the surface of the biomaterial, the maleability of the biomaterial and the intermolecular strength of attraction between clotted blood proteins (fibrinogen) with the cellulose/kaolin components of the biomaterial.

Keratin 8 modulates β-cell stress responses and normoglycaemia

Keratin intermediate filament (IF) proteins are epithelial cell cytoskeletal components that provide structural stability and protection from cell stress, among other cellular and tissue-specific functions. Numerous human diseases are associated with IF gene mutations, but the function of keratins in the endocrine pancreas and their potential significance for glycaemic control are unknown. The impact of keratins on β-cell organisation and systemic glucose control was assessed using keratin 8 (K8) wild-type (K8(+/+)) and K8 knockout (K8(-/-)) mice. Islet β-cell keratins were characterised under basal conditions, in streptozotocin (STZ)-induced diabetes and in non-obese diabetic (NOD) mice. STZ-induced diabetes incidence and islet damage was assessed in K8(+/+) and K8(-/-) mice. K8 and K18 were the predominant keratins in islet β-cells and K8(-/-) mice expressed only remnant K18 and K7. K8 deletion resulted in lower fasting glucose levels, increased glucose tolerance and insulin sensitivity, reduced glucose-stimulated insulin secretion and decreased pancreatic insulin content. GLUT2 localisation and insulin vesicle morphology were disrupted in K8(-/-) β-cells. The increased levels of cytoplasmic GLUT2 correlated with resistance to high-dose STZ-induced injury in K8(-/-) mice. However, K8 deletion conferred no long-term protection from STZ-induced diabetes and prolonged STZ-induced stress caused increased exocrine damage in K8(-/-) mice. β-cell keratin upregulation occurred 2 weeks after treatments with low-dose STZ in K8(+/+) mice and in diabetic NOD mice, suggesting a role for keratins, particularly in non-acute islet stress responses. These results demonstrate previously unrecognised functions for keratins in β-cell intracellular organisation, as well as for systemic blood glucose control under basal conditions and in diabetes-induced stress.

Casein hydrolysate diet controls intestinal T cell activation, free radical production and microbial colonisation in NOD mice

AIMS/HYPOTHESIS

Dietary and microbial factors and the gut immune system are important in autoimmune diabetes. We evaluated inflammatory activity in the whole gut in prediabetic NOD mice using ex vivo imaging of reactive oxygen and nitrogen species (RONS), and correlated this with the above-mentioned factors.

METHODS

NOD mice were fed a normal diet or an anti-diabetogenic casein hydrolysate (CH) diet. RONS activity was detected by chemiluminescence imaging of the whole gut. Proinflammatory and T cell cytokines were studied in the gut and islets, and dietary effects on gut microbiota and short-chain fatty acids were determined.

RESULTS

Prediabetic NOD mice displayed high RONS activity in the epithelial cells of the distal small intestine, in conjunction with a proinflammatory cytokine profile. RONS production was effectively reduced by the CH diet, which also controlled (1) the expression of proinflammatory cytokines and colonisation-dependent RegIIIγ (also known as Reg3g) in ileum; (2) intestinal T cell activation; and (3) islet cytokines. The CH diet diminished microbial colonisation, increased the Bacteroidetes:Firmicutes ratio, and reduced lactic acid and butyric acid production in the gut.

CONCLUSIONS/INTERPRETATION

Epithelial RONS production and proinflammatory T cell activation appears in the ileum of NOD mice after weaning to normal laboratory chow, but not after weaning to an anti-diabetogenic CH diet. Our data suggest a link between dietary factors, microbial colonisation and mucosal immune activation in NOD mice.

Shape engineering vs organic modification of inorganic nanoparticles as a tool for enhancing cellular internalization

In nanomedicine, physicochemical properties of the nanocarrier affect the nanoparticle's pharmacokinetics and biodistribution, which are also decisive for the passive targeting and nonspecific cellular uptake of nanoparticles. Size and surface charge are, consequently, two main determining factors in nanomedicine applications. Another important parameter which has received much less attention is the morphology (shape) of the nanocarrier. In order to investigate the morphology effect on the extent of cellular internalization, two similarly sized but differently shaped rod-like and spherical mesoporous silica nanoparticles were synthesized, characterized and functionalized to yield different surface charges. The uptake in two different cancer cell lines was investigated as a function of particle shape, coating (organic modification), surface charge and dose. According to the presented results, particle morphology is a decisive property regardless of both the different surface charges and doses tested, whereby rod-like particles internalized more efficiently in both cell lines. At lower doses whereby the shape-induced advantage is less dominant, charge-induced effects can, however, be used to fine-tune the cellular uptake as a prospective 'secondary' uptake regulator for tight dose control in nanoparticle-based drug formulations.

Adrenoceptor activity of muscarinic toxins identified from mamba venoms

BACKGROUND AND PURPOSE

Muscarinic toxins (MTs) are snake venom peptides named for their ability to interfere with ligand binding to muscarinic acetylcholine receptors (mAChRs). Recent data infer that these toxins may have other G-protein-coupled receptor targets than the mAChRs. The purpose of this study was to systematically investigate the interactions of MTs with the adrenoceptor family members.

EXPERIMENTAL APPROACH

We studied the interaction of four common MTs, MT1, MT3, MT7 and MTα, with cloned receptors expressed in insect cells by radioligand binding. Toxins showing modest to high-affinity interactions with adrenoceptors were additionally tested for effects on functional receptor responses by way of inhibition of agonist-induced Ca²⁺ increases.

KEY RESULTS

All MTs behaved non-competitively in radioligand displacement binding. MT1 displayed higher binding affinity for the human α(2B)-adrenoceptor (IC₅₀ = 2.3 nM) as compared with muscarinic receptors (IC₅₀ ≥ 100 nM). MT3 appeared to have a broad spectrum of targets showing high-affinity binding (IC₅₀ = 1-10 nM) to M₄ mAChR, α(1A)-, α(1D)- and α(2A)-adrenoceptors and lower affinity binding (IC₅₀ ≥ 25 nM) to α(1B)- and α(2C)-adrenoceptors and M₁ mAChR. MT7 did not detectably bind to other receptors than M₁, and MTα was specific for the α(2B)-adrenoceptor. None of the toxins showed effects on β₁- or β₂-adrenoceptors.

CONCLUSIONS AND IMPLICATIONS

Some of the MTs previously found to interact predominantly with mAChRs were shown to bind with high affinity to selected adrenoceptor subtypes. This renders these peptide toxins useful for engineering selective ligands to target various adrenoceptors.

Cytoskeletal keratin glycosylation protects epithelial tissue from injury

Keratins 8 and 18 (K8 and K18) are heteropolymeric intermediate filament phosphoglycoproteins of simple-type epithelia. Mutations in K8 and K18 predispose the affected individual to liver disease as they protect hepatocytes from apoptosis. K18 undergoes dynamic O-linked N-acetylglucosamine glycosylation at Ser 30, 31 and 49. We investigated the function of K18 glycosylation by generating mice that overexpress human K18 S30/31/49A substitution mutants that cannot be glycosylated (K18-Gly(-)), and compared the susceptibility of these mice to injury with wild-type and other keratin-mutant mice. K18-Gly(-) mice are more susceptible to liver and pancreatic injury and apoptosis induced by streptozotocin or to liver injury by combined N-acetyl-D-glucosaminidase inhibition and Fas administration. The enhanced apoptosis in the livers of mice that express K18-Gly(-) involves the inactivation of Akt1 and protein kinase Ctheta as a result of their site-specific hypophosphorylation. Akt1 binds to K8, which probably contributes to the reciprocal hyperglycosylation and hypophosphorylation of Akt1 that occurs on K18 hypoglycosylation, and leads to decreased Akt1 kinase activity. Therefore, K18 glycosylation provides a unique protective role in epithelial injury by promoting the phosphorylation and activation of cell-survival kinases.

Keratins modulate the shape and function of hepatocyte mitochondria: a mechanism for protection from apoptosis

Absence or mutation of keratins 8 (K8) or 18 (K18) cause predisposition to liver injury and apoptosis. We assessed the mechanisms of hepatocyte keratin-mediated cytoprotection by comparing the protein expression profiles of livers from wild-type and K8-null mice using two-dimensional differential-in-gel-electrophoresis (2D-DIGE) and mass spectrometry. Prominent among the alterations were those of mitochondrial proteins, which were confirmed using 2D-DIGE of purified mitochondria. Ultrastructural analysis showed that mitochondria of livers that lack or have disrupted keratins are significantly smaller than mitochondria of wild-type livers. Immunofluorescence staining showed irregular distribution of mitochondria in keratin-absent or keratin-mutant livers. K8-null livers have decreased ATP content; and K8-null mitochondria have less cytochrome c, increased release of cytochrome c after exposure to Ca(2+) and oxidative stimulation, and a higher sensitivity to Ca(2+)-induced permeability transition. Therefore, keratins play a direct or indirect role in regulating the shape and function of mitochondria. The effects of keratin mutation on mitochondria are likely to contribute to hepatocyte predisposition to apoptosis and oxidative injury, and to play a pathogenic role in keratin-mutation-related human liver disease.

Autophagy activation by rapamycin eliminates mouse Mallory-Denk bodies and blocks their proteasome inhibitor-mediated formation

The proteasomal and lysosomal/autophagy pathways in the liver and other tissues are involved in several biological processes including the degradation of misfolded proteins. Exposure of hepatocyte cell lines to proteasome inhibitors (PIs) results in the formation of inclusions that resemble Mallory-Denk bodies (MDBs). Keratins are essential for MDB formation and keratin 8 (K8)-overexpressing transgenic mice are predisposed to MDB formation. We tested the hypothesis that PIs induce MDBs in vivo and that autophagy participates in MDB turnover. The effect of the PI bortezomib (which is used to treat some malignancies) on MDB formation was tested in K8-overexpressing mice and in cultured cells. Inclusion formation was examined using immune and conventional electron microscopy (EM). Bortezomib induced MDB-like inclusions composed of keratins, ubiquitin, and p62 in cultured cells. Short-term exposure to bortezomib induced similar inclusions in K8-overexpressing but not in nontransgenic mice, without causing liver injury. In bortezomib-treated mice, autophagy was activated in hepatocytes as determined by EM and biochemical analysis. Further activation of autophagy by rapamycin (Rap) decreased the number of inclusions in bortezomib-treated K8 transgenic mice significantly. Rap also led to resorption of spontaneously formed MDBs in aging K8-overexpressing mice. Immune EM demonstrated K8-positive and ubiquitin-positive structures in autophagic vacuoles in the mouse liver.

CONCLUSION

PIs alone are sufficient to induce MDBs in susceptible animals, while Rap-mediated activation of autophagy prevents MDB formation and causes MDB resorption. These findings suggest that some patients treated with PIs may become predisposed to MDB formation. Autophagy provides a potential cellular mechanism for the resorption of cytoplasmic inclusions.

Keratin mutation predisposes to mouse liver fibrosis and unmasks differential effects of the carbon tetrachloride and thioacetamide models

BACKGROUND & AIMS

Keratins 8 and 18 (K8/K18) are important hepatoprotective proteins. Animals expressing K8/K18 mutants show a marked susceptibility to acute/subacute liver injury. K8/K18 variants predispose to human end-stage liver disease and associate with fibrosis progression during chronic hepatitis C infection. We sought direct evidence for a keratin mutation-related predisposition to liver fibrosis using transgenic mouse models because the relationship between keratin mutations and cirrhosis is based primarily on human association studies.

METHODS

Mouse hepatofibrosis was induced by carbon tetrachloride (CCl(4)) or thioacetamide. Nontransgenic mice, or mice that over express either human Arg89-to-Cys (R89C mice) or wild-type K18 (WT mice) were used. The extent of fibrosis was evaluated by quantitative real-time reverse-transcription polymerase chain reaction of fibrosis-related genes, liver hydroxyproline measurement, and Picro-Sirius red staining and collagen immunofluorescence staining.

RESULTS

Compared with control animals, CCl(4) led to similar liver fibrosis but increased injury in K18 R89C mice. In contrast, thioacetamide caused more severe liver injury and fibrosis in K18 R89C as compared with WT and nontransgenic mice and resulted in increased messenger RNA levels of collagen, tissue inhibitor of metalloproteinase 1, matrix metalloproteinase 2, and matrix metalloproteinase 13. Analysis in nontransgenic mice showed that thioacetamide and CCl(4) have dramatically different molecular expression responses involving cytoskeletal and chaperone proteins.

CONCLUSIONS

Over expression of K18 R89C predisposes transgenic mice to thioacetamide- but not CCl(4)-induced liver fibrosis. Differences in the keratin mutation-associated fibrosis response among the 2 models raise the hypothesis that keratin variants may preferentially predispose to fibrosis in unique human liver diseases. Findings herein highlight distinct differences in the 2 widely used fibrosis models.

Keratin overexpression levels correlate with the extent of spontaneous pancreatic injury

Mutation of the adult hepatocyte keratins K8 and K18 predisposes to liver disease. In contrast, exocrine pancreas K8 and K18 are dispensable and are co-expressed with limited levels of membrane-proximal K19 and K20. Overexpression of mutant K18 or genetic ablation of K8 in mouse pancreas is well tolerated whereas overexpression of K8 causes spontaneous chronic pancreatitis. To better understand the effect of exocrine pancreatic keratin overexpression, we compared transgenic mice that overexpress K18, K8, or K8/K18, associated with minimal, modest, or large increases in keratin expression, respectively, with nontransgenic wild-type (WT) mice. Overexpression of the type-II keratin K8 up-regulated type-I keratins K18, K19, and K20 and generated K19/K20-containing neocytoplasmic typical or short filaments; however, overexpression of K18 had no effect on K8 levels. K8- and K18-overexpressing pancreata were histologically similar to WT, whereas K8/K18 pancreata displayed age-enhanced vacuolization and atrophy of the exocrine pancreas and exhibited keratin hyperphosphorylation. Zymogen granules in K8/K18 pancreata were 50% smaller and more dispersed than their normal apical concentration but were twice as numerous as in WT controls. Therefore, modest keratin overexpression has minor effects on the exocrine pancreas whereas significant keratin overexpression alters zymogen granule organization and causes aging-associated exocrine atrophy. Keratin absence or mutation is well tolerated after pancreatic but not liver injury, whereas excessive overexpression is toxic to the pancreas but not the liver when induced under basal conditions.

Absence of keratin 19 in mice causes skeletal myopathy with mitochondrial and sarcolemmal reorganization

Intermediate filaments, composed of desmin and of keratins, play important roles in linking contractile elements to each other and to the sarcolemma in striated muscle. We examined the contractile properties and morphology of fast-twitch skeletal muscle from mice lacking keratin 19. Tibialis anterior muscles of keratin-19-null mice showed a small but significant decrease in mean fiber diameter and in the specific force of tetanic contraction, as well as increased plasma creatine kinase levels. Costameres at the sarcolemma of keratin-19-null muscle, visualized with antibodies against spectrin or dystrophin, were disrupted and the sarcolemma was separated from adjacent myofibrils by a large gap in which mitochondria accumulated. The costameric dystrophin-dystroglycan complex, which co-purified with gamma-actin, keratin 8 and keratin 19 from striated muscles of wild-type mice, co-purified with gamma-actin but not keratin 8 in the mutant. Our results suggest that keratin 19 in fast-twitch skeletal muscle helps organize costameres and links them to the contractile apparatus, and that the absence of keratin 19 disrupts these structures, resulting in loss of contractile force, altered distribution of mitochondria and mild myopathy. This is the first demonstration of a mammalian phenotype associated with a genetic perturbation of keratin 19.

Reg-II is an exocrine pancreas injury-response product that is up-regulated by keratin absence or mutation

The major keratins in the pancreas and liver are keratins 8 and 18 (K8/K18), but their function seemingly differs in that liver K8/K18 are essential cytoprotective proteins, whereas pancreatic K8/K18 are dispensable. This functional dichotomy raises the hypothesis that K8-null pancreata may undergo compensatory cytoprotective gene expression. We tested this hypothesis by comparing the gene expression profile in pancreata of wild-type and K8-null mice. Most prominent among the up-regulated genes in K8-null pancreas was mRNA for regenerating islet-derived (Reg)-II, which was confirmed by quantitative reverse transcription-polymerase chain reaction and by an anti-Reg-II peptide antibody we generated. Both K8-null and wild-type mice express Reg-II predominantly in acinar cells as determined by in situ hybridization and immunostaining. Analysis of Reg-II expression in various keratin-related transgenic mouse models showed that its induction also occurs in response to keratin cytoplasmic filament collapse, absence, or ablation of K18 Ser52 but not Ser33 phosphorylation via Ser-to-Ala mutation, which represent situations associated with predisposition to liver but not pancreatic injury. In wild-type mice, Reg-II is markedly up-regulated in two established pancreatitis models in response to injury and during the recovery phase. Thus, Reg-II is a likely mouse exocrine pancreas cytoprotective candidate protein whose expression is regulated by keratin filament organization and phosphorylation.

From Mallory to Mallory–Denk bodies: What, how and why?

Frank B. Mallory described cytoplasmic hyaline inclusions in hepatocytes of patients with alcoholic hepatitis in 1911. These inclusions became known as Mallory bodies (MBs) and have since been associated with a variety of other liver diseases including non-alcoholic fatty liver disease. Helmut Denk and colleagues described the first animal model of MBs in 1975 that involves feeding mice griseofulvin. Since then, mouse models have been instrumental in helping understand the pathogenesis of MBs. Given the tremendous contributions made by Denk to the field, we propose renaming MBs as Mallory-Denk bodies (MDBs). The major constituents of MDBs include keratins 8 and 18 (K8/18), ubiquitin, and p62. The relevant proteins and cellular processes that contribute to MDB formation and accumulation include the type of chronic stress, the extent of stress-induced protein misfolding and consequent proteasome overload, a K8-greater-than-K18 ratio, transamidation of K8 and other proteins, presence of p62 and autophagy. Although it remains unclear whether MDBs serve a bystander, protective or injury promoting function, they do serve an important role as histological and potential progression markers in several liver diseases.

"Heads and tails" of intermediate filament phosphorylation: multiple sites and functional insights

Intermediate filaments (IFs) are major components of the mammalian cytoskeleton. They are among the most abundant cellular phosphoproteins; their phosphorylation typically involves multiple sites at repeat or unique motifs, preferentially within the "head" or "tail" domains. Phosphorylation and dephosphorylation are essential for the regulation of IF dynamics by modulating the intrinsic properties of IFs: solubility, conformation and filament organization, and, in addition, for the regulation of other IF post-translational modifications. These phosphorylation-regulated properties dictate generalized and context-dependent IF functions that reflect their tissue-specific expression. Most important among IF phosphorylation-mediated functions are the regulation of IF cellular or subcellular compartmentalization, levels and turnover, binding with associated proteins, susceptibility to cell stresses (including apoptosis), tissue-specific functions and IF-associated disease pathogenesis (where IF hyperphosphorylation also serves as a tissue-injury marker).

Pharmacologic transglutaminase inhibition attenuates drug-primed liver hypertrophy but not Mallory body formation

Mallory bodies (MBs) are characteristic of several liver disorders, and consist primarily of keratins with transglutaminase-generated keratin crosslinks. We tested the effect of the transglutaminase-2 (TG2) inhibitor KCC009 on MB formation in a mouse model fed 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). KCC009 decreased DDC-induced liver enlargement without affecting MB formation or extent of liver injury. TG2 protein and activity increased after DDC feeding and localized within and outside hepatocytes. KCC009 inhibited DDC-induced hepatomegaly by affecting hepatocyte cell size rather than proliferation. Hence, TG2 is a potential mediator of injury-induced hepatomegaly via modulation of hepatocyte hypertrophy, and KCC009-mediated TG2 inhibition does not affect mouse MB formation.

Protein phosphatase-2A associates with and dephosphorylates keratin 8 after hyposmotic stress in a site- and cell-specific manner

Keratins 8 and 18 (K8 and K18) are regulated by site-specific phosphorylation in response to multiple stresses. We examined the effect and regulation of hyposmotic stress on keratin phosphorylation. K8 phospho-Ser431 (Ser431-P) becomes dephosphorylated in HT29 cells, but hyperphosphorylated on other K8 but not K18 sites in HRT18 and Caco2 cells and in normal human colonic ex vivo cultures. Hyposmosis-induced dephosphorylation involves K8 but not K18, K19 or K20, occurs preferentially in mitotically active cells, and peaks by 6-8 hours then returns to baseline by 12-16 hours. By contrast, hyperosmosis causes K8 Ser431 hyperphosphorylation in all tested cell lines. Hyposmosis-induced dephosphorylation of K8 Ser431-P is inhibited by okadaic acid but not by tautomycin or cyclosporine. The PP2A catalytic subunit co-immunoprecipitated with K8 and K18 after hyposmotic stress in HT29 cells, but not in HRT18 or Caco2 cells where K8 Ser431 becomes hyperphosphorylated. K8 Ser431-P dephosphorylation after hyposmosis was independent of PP2A levels but correlated with increased PP2A activity towards K8 Ser431-P. Therefore, hyposmotic stress alters K8 phosphorylation in a cell-dependent manner, and renders K8 Ser431-P a physiologic substrate for PP2A in HT29 cells as a result of PP2A activation and the physical association with K8 and K18. The divergent hyposmosis versus hyperosmosis K8 Ser431 phosphorylation changes in HT29 cells suggest that there are unique signaling responses to osmotic stress.

Keratin 8 overexpression promotes mouse Mallory body formation

Keratins 8 and 18 (K8/18) are major constituents of Mallory bodies (MBs), which are hepatocyte cytoplasmic inclusions seen in several liver diseases. K18-null but not K8-null or heterozygous mice form MBs, which indicates that K8 is important for MB formation. Early stages in MB genesis include K8/18 hyperphosphorylation and overexpression. We used transgenic mice that overexpress K8, K18, or K8/18 to test the importance of K8 and/or K18 in MB formation. MBs were induced by feeding 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). Livers of young K8 or K8/K18 overexpressors had no histological abnormalities despite increased keratin protein and phosphorylation. In aging mice, only K8-overexpressing livers spontaneously developed small “pre-MB” aggregates. Only K8-overexpressing young mice are highly susceptible to MB formation after short-term DDC feeding. Thus, the K8 to K18 ratio, rather than K8/18 overexpression by itself, plays an essential role in MB formation. K8 overexpression is sufficient to form pre-MB and primes animals to accumulate MBs upon DDC challenge, which may help explain MB formation in human liver diseases.

Cellular integrity plus: organelle-related and protein-targeting functions of intermediate filaments

Intermediate filament proteins (IFs) maintain cell and tissue integrity, based on evidence of their polymerization and mechanical properties, abundance and disease-associated phenotypes. This 'traditional' function is now augmented by organelle-related and protein-targeting roles. Mitochondrial location and function depend on intact IFs, as demonstrated for desmin, keratins and neurofilaments. Golgi positioning is regulated by several IFs, and endosomal/lysosomal protein distribution by vimentin. IFs dramatically affect nuclear function and shape and play a role in subcellular and membrane targeting of proteins. These functions have been noted in tissues but in some cases only in cell culture. The IF-related organelle-specific and protein-targeting roles, which are likely interrelated, provide functions beyond cell scaffolding and integrity and contribute to the cytoprotective and tissue-specific functions of IF proteins.

Keratin-8-deficient mice develop chronic spontaneous Th2 colitis amenable to antibiotic treatment

Keratin 8 (K8) is the major intermediate filament protein present in intestinal epithelia. Depending on the mouse genetic background, absence of K8 causes embryonic lethality or colonic hyperplasia and colitis. We studied disease progression, the inflammatory responses, and role of luminal bacteria in K8-null mice in order to characterize the intestinal pathology of K8-associated colitis. Colon lymphocytes were isolated for analysis of their phenotype and cytokine production, and vascular and lymphocyte adhesion molecule expression in K8-/- mice of varying ages. K8-/- mice had a marked increase in TCR(beta)-positive/CD4-positive T cells infiltrating the colon lamina propria, in association with enhanced Th2 cytokine (IL-4, IL-5 and IL-13) production. K8-/- mice show early signs of inflammation even prior to weaning, that increases with age, and their epithelial cells overexpress MHC class II antigens. The chronic colitis is related to increased CD4-positive infiltrating T cells displaying memory and naive phenotypes, and an altered vascular endothelium with aberrant expression of peripheral node addressin. Analysis of normal gut-specific homing molecules, reveals an increased number of alpha(4)beta(7)-positive cells and vascular mucosal addressin cell adhesion molecule-1 in K8-null colons. Antibiotic treatment markedly decreased colon inflammation and ion transporter AE1/2 mistargeting, indicating that luminal bacteria play an important role in the observed phenotype. Therefore, K8-null mice develop chronic spontaneous Th2-type colitis due to a primary epithelial rather than immune cell defect, which is amenable to antibiotic therapy. These mice provide a model to investigate epithelial-leukocyte and epithelial-microbial cross-talk.

Organ-specific stress induces mouse pancreatic keratin overexpression in association with NF-kappaB activation

Keratin polypeptides 8 and 18 (K8/K18) are the major intermediate filament proteins of pancreatic acinar cells and hepatocytes. Pancreatic keratin function is unknown, whereas hepatocyte keratins protect from mechanical and non-mechanical forms of stress. We characterized steady-state pancreatic keratin expression in Balb/c mice after caerulein and choline-deficient ethionine-supplemented diet (CDD), or on exposure to the generalized stresses of heat and water immersion. Keratins were studied at the protein, RNA and organizational levels. Isolated acini were used to study the role of nuclear factor (NF)-kappaB using selective inhibitors. Keratins were found to be abundant proteins making up 0.2%, 0.3% and 0.5% of the total cellular protein of pancreas, liver and small intestine, respectively. Caerulein and CDD caused a threefold transcription-mediated overall increase in K8/K18/K19/K20 proteins. Keratin overexpression begins on tissue recovery, peaks 2 days after caerulein injection, or 1 day after CDD discontinuation, and returns to basal levels after 10 days. K19/K20-containing cytoplasmic filaments are nearly absent pre-injury but form post-injury then return to their original membrane-proximal distribution after 10 days. By contrast, generalized stresses of heat or water-immersion stress do not alter keratin expression levels. Caerulein-induced keratin overexpression is associated with NF-kappaB activation when tested using ex vivo acinar cell cultures. In conclusion, keratins are abundant proteins that can behave as stress proteins in response to tissue-specific but not generalized forms of injury. Pancreatic keratin overexpression is associated with NF-kappaB activation and may serve unique functions in acinar or ductal cell response to injury.

Keratin 8 and 18 hyperphosphorylation is a marker of progression of human liver disease

Keratin 8 and 18 (K8/18) phosphorylation plays a significant and site-specific role in regulating keratin filament organization, association with binding proteins, and modulation of cell cycle progression. Keratin hyperphosphorylation correlates with exposure to a variety of stresses in cultured cells and in mouse models of liver, pancreatic, and gallbladder injury, and it is found in association with mouse and human Mallory bodies. We asked whether K8/18 phosphorylation correlates with human liver disease progression by analyzing liver explants and biopsies of patients with chronic noncirrhotic hepatitis C virus (HCV) or cirrhosis. We also examined the effect of HCV therapy with interleukin-10 on keratin phosphorylation. Using site-specific antiphosphokeratin antibodies we found keratin hyperphosphorylation on most K8/18 sites in all cirrhotic liver explants tested and in most liver biopsies from patients with chronic HCV infection. Immunofluorescence staining of precirrhotic HCV livers showed focal keratin hyperphosphorylation and limited reorganization of keratin filament networks. In cirrhotic livers, keratin hyperphosphorylation occurred preferentially in hepatic nodule cells adjacent to bridging fibrosis and associated with increased stress kinase activation and apoptosis. Histological and serological improvement after interleukin-10 therapy was accompanied by normalization of keratin hyperphosphorylation on some sites in 7 of 10 patients. In conclusion, site-specific keratin phosphorylation in liver disease is a progression marker when increased and a likely regression marker when decreased.

Keratin-8 null mice have different gallbladder and liver susceptibility to lithogenic diet-induced injury

Keratin transgenic mouse models and the association of human keratin mutations with liver disease highlight the importance of keratins in protecting the liver from environmental insults, but little is known regarding keratins and their function in the gallbladder. We characterized keratin expression pattern and filament organization in normal and keratin polypeptide-8 (K8)-null, K18-null and K19-null gallbladders, and examined susceptibility to liver and gallbladder injury induced by a high-fat lithogenic diet (LD) in K8-null mice. The major keratins of normal mouse gallbladder are K8>K19>K18 which become markedly depleted in K8-null mice with minor K18/K19 remnants and limited K7 over-expression. Compensatory K18/K20 protein and RNA overexpression occur in K19-null but not in K18-null gallbladders, probably because of the higher levels of K19 than K18 in normal gallbladder. LD challenge causes more severe liver injury in K8-null than wild-type mice without altering keratin protein levels. In contrast, wild-type and K8-null gallbladders are equally susceptible to LD-induced injury and stone formation, but wild-type gallbladders do overexpress keratins upon LD challenge. LD-induced injury triggers keratin hyperphosphorylation in wild-type livers and gallbladders. Hence, mouse gallbladder K8/K18/K19 expression is induced in response to cholelithiasis injury. A high-fat LD increases the susceptibility of K8-null mice to liver but not gallbladder injury, which suggests that keratin mutations may increase the risk of liver damage in patients with steatohepatitis. Differences between K8-null mouse gallbladder and hepatocyte susceptibility to injury may be related to their minimal versus absent keratin expression, respectively.

Keratins modulate colonocyte electrolyte transport via protein mistargeting

The function of intestinal keratins is unknown, although keratin 8 (K8)–null mice develop colitis, hyperplasia, diarrhea, and mistarget jejunal apical markers. We quantified the diarrhea in K8-null stool and examined its physiologic basis. Isolated crypt-units from K8-null and wild-type mice have similar viability. K8-null distal colon has normal tight junction permeability and paracellular transport but shows decreased short circuit current and net Na absorption associated with net Cl secretion, blunted intracellular Cl/HCO₃-dependent pH regulation, hyperproliferation and enlarged goblet cells, partial loss of the membrane-proximal markers H,K-ATPase-β and F-actin, increased and redistributed basolateral anion exchanger AE1/2 protein, and redistributed Na-transporter ENaC-γ. Diarrhea and protein mistargeting are observed 1–2 d after birth while hyperproliferation/inflammation occurs later. The AE1/2 changes and altered intracellular pH regulation likely account, at least in part, for the ion transport defects and hyperproliferation. Therefore, colonic keratins have a novel function in regulating electrolyte transport, likely by targeting ion transporters to their cellular compartments.

Keratin 20 helps maintain intermediate filament organization in intestinal epithelia

Of the >20 epithelial keratins, keratin 20 (K20) has an unusual distribution and is poorly studied. We began to address K20 function, by expressing human wild-type and Arg80-->His (R80H) genomic (18 kb) and cDNA K20 in cells and mice. Arg80 of K20 is conserved in most keratins, and its mutation in epidermal keratins causes several skin diseases. R80H but not wild-type K20 generates disrupted keratin filaments in transfected cells. Transgenic mice that overexpress K20 R80H have collapsed filaments in small intestinal villus regions, when expressed at moderate levels, whereas wild-type K20-overexpressing mice have normal keratin networks. Overexpressed K20 maintains its normal distribution in several tissues, but not in the pancreas and stomach, without causing any tissue abnormalities. Hence, K20 pancreatic and gastric expression is regulated outside the 18-kb region. Cross-breeding of wild-type or R80H K20 mice with mice that overexpress wild-type K18 or K18 that is mutated at the conserved K20 Arg80-equivalent residue show that K20 plays an additive and compensatory role with K18 in maintaining keratin filament organization in the intestine. Our data suggest the presence of unique regulatory domains for pancreatic and gastric K20 expression and support a significant role for K20 in maintaining keratin filaments in intestinal epithelia.

Type II keratins are phosphorylated on a unique motif during stress and mitosis in tissues and cultured cells

Epithelial cell keratins make up the type I (K9-K20) and type II (K1-K8) intermediate filament proteins. In glandular epithelia, K8 becomes phosphorylated on S73 ((71)LLpSPL) in human cultured cells and tissues during stress, apoptosis, and mitosis. Of all known proteins, the context of the K8 S73 motif (LLS/TPL) is unique to type II keratins and is conserved in epidermal K5/K6, esophageal K4, and type II hair keratins, except that serine is replaced by threonine. Because knowledge regarding epidermal and esophageal keratin regulation is limited, we tested whether K4-K6 are phosphorylated on the LLTPL motif. K5 and K6 become phosphorylated in vitro on threonine by the stress-activated kinase p38. Site-specific anti-phosphokeratin antibodies to LLpTPL were generated, which demonstrated negligible basal K4-K6 phosphorylation. In contrast, treatment of primary keratinocytes and other cultured cells, and ex vivo skin and esophagus cultures, with serine/threonine phosphatase inhibitors causes a dramatic increase in K4-K6 LLpTPL phosphorylation. This phosphorylation is accompanied by keratin solubilization, filament reorganization, and collapse. K5/K6 LLTPL phosphorylation occurs in vivo during mitosis and apoptosis induced by UV light or anisomycin, and in human psoriatic skin and squamous cell carcinoma. In conclusion, type II keratins of proliferating epithelia undergo phosphorylation at a unique and conserved motif as part of physiological mitotic and stress-related signals.

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.

Simple epithelial keratins are dispensable for cytoprotection in two pancreatitis models

Pancreatic acinar cells express keratins 8 and 18 (K8/18), which form cytoplasmic filament (CF) and apicolateral filament (ALF) pools. Hepatocyte K8/18 CF provide important protection from environmental stresses, but disruption of acinar cell CF has no significant impact. We asked whether acinar cell ALF are important in providing cytoprotective roles by studying keratin filaments in pancreata of K8- and K18-null mice. K8-null pancreas lacks both keratin pools, but K18-null pancreas lacks only CF. Mouse but not human acinar cells also express apicolateral keratin 19 (K19), which explains the presence of apicolateral keratins in K18-null pancreas. K8- and K18-null pancreata are histologically normal, and their acini respond similarly to stimulated secretion, although K8-null acini viability is reduced. Absence of total filaments (K8-null) or CF (K18-null) does not increase susceptibility to pancreatitis induced by caerulein or a choline-deficient diet. In normal and K18-null acini, K19 is upregulated after caerulein injury and, unexpectedly, forms CF. As in hepatocytes, acinar injury is also associated with keratin hyperphosphorylation. Hence, K19 forms ALF in mouse acinar cells and helps define two distinct ALF and CF pools. On injury, K19 forms CF that revert to ALF after healing. Acinar keratins appear to be dispensable for cytoprotection, in contrast to hepatocyte keratins, despite similar hyperphosphorylation patterns after injury.

Effects of keratin filament disruption on exocrine pancreas-stimulated secretion and susceptibility to injury

Disruption or absence of hepatocyte keratins 8 and 18 is associated with chronic hepatitis, marked hepatocyte fragility, and a significant predisposition to stress-induced liver injury. In contrast, pancreatic keratin disruption in transgenic mice that express keratin 18 Arg89 --> Cys (K18C) is not associated with an obvious pancreatic pathology. We compared the effects of keratin filament disruption on pancreatic acini or acinar cell viability, and on cholecystokinin (CCK)-stimulated secretion, in transgenic mice that overexpress wild-type keratin 18 and harbor normal extended keratin filaments (TG2) and K18C mice. We also compared the response of these mice to pancreatitis induced by a choline-deficient ethionine-supplemented diet or by caerulein. Despite extensive cytoplasmic keratin filament disruption, the apicolateral keratin filament bundles appear intact in the acinar pancreas of K18C mice, as determined ultrastructurally and by light microscopy. No significant pancreatitis-associated histologic, serologic, or F-actin/keratin apicolateral redistribution differences were noted between TG2 and K18C mice. Acinar cell viability and yield after collagenase digestion were lower in K18C than in TG2 mice, but the yields of intact acini and their (125)I-CCK uptake and responses to CCK-stimulated secretion were similar. Our results indicate that keratin filament reorganization is a normal physiologic response to pancreatic cell injury, but an intact keratin cytoplasmic filament network is not as essential in protection from cell injury as in the liver. These findings raise the possibility that the abundant apicolateral acinar keratin filaments, which are not as evident in hepatocytes, may play the cytoprotective role that is seen in liver and other tissues. Alternatively, identical keratins may function differently in different tissues.

The cytoskeleton of digestive epithelia in health and disease

The mammalian cell cytoskeleton consists of a diverse group of fibrillar elements that play a pivotal role in mediating a number of digestive and nondigestive cell functions, including secretion, absorption, motility, mechanical integrity, and mitosis. The cytoskeleton of higher-eukaryotic cells consists of three highly abundant major protein families: microfilaments (MF), microtubules (MT), and intermediate filaments (IF), as well as a growing number of associated proteins. Within digestive epithelia, the prototype members of these three protein families are actins, tubulins, and keratins, respectively. This review highlights the important structural, regulatory, functional, and unique features of the three major cytoskeletal protein groups in digestive epithelia. The emerging exciting biological aspects of these protein groups are their involvement in cell signaling via direct or indirect interaction with a growing list of associated proteins (MF, MT, IF), the identification of several disease-causing mutations (IF, MF), the functional role that they play in protection from environmental stresses (IF), and their functional integration via several linker proteins that bridge two or potentially all three of these groups together. The use of agents that target specific cytoskeletal elements as therapeutic modalities for digestive diseases offers potential unique areas of intervention that remain to be fully explored.