Intermediate filaments: a historical perspective

Resistance exercise and mechanical overload upregulate vimentin for skeletal muscle remodeling

We adopted a proteomic and follow-through approach to investigate how mechanical overload (MOV) potentially affects novel targets in skeletal muscle, and how a perturbation in this response could potentially affect the adaptive response. First, we determined that 10 wk of resistance training in 15 college-aged females increased sarcolemmal-associated protein content (+10.1%, P < 0.05). Sarcolemmal protein isolates were then queried using mass spectrometry-based proteomics, ∼10% (38/387) of proteins putatively associated with the sarcolemma or extracellular matrix (ECM) were upregulated (>1.5-fold, P < 0.05), and one target (intermediate filament vimentin; VIM) warranted further investigation due to its correlation to myofiber hypertrophy (r = 0.652, P = 0.009). VIM expression was then examined in 4-mo-old C57BL/6J mice following 10 and 20 days of plantaris MOV via synergist ablation. Relative to Sham (control) mice, VIM mRNA and protein content was significantly higher in MOV mice, and immunohistochemistry indicated that VIM predominantly resided in the ECM. MOV experiments were replicated in Pax7-DTA (satellite cell depleted) mice, which reduced VIM in the ECM by ∼74%. A third MOV experiment was performed in C57BL/6 mice intramuscularly injected with either AAV9-scrambled (control) or AAV9-VIM-shRNA. Although VIM-shRNA mice possessed lower VIM in the ECM (∼45%), plantaris masses in response to MOV were similar between groups. However, VIM-shRNA mice possessed smaller and more centrally nucleated MyHCemb-positive fibers in response to MOV. In summary, skeletal muscle VIM appears to be enriched in the ECM following MOV, satellite cells may regulate its expression, and a disruption in expression during MOV leads to an excessive regenerative phenotype.NEW & NOTEWORTHY Our highly integrative approach suggests that skeletal muscle vimentin seems to function as a mechanosensitive protein that becomes enriched in the extracellular matrix following MOV. Satellite cells may play a role in regulating their expression, and an exaggerated regenerative response occurs when vimentin expression becomes dysregulated during mechanical overload. Although these data implicate vimentin in aiding with tissue remodeling following MOV, more data are needed to determine the functional ramifications of VIM response deficiencies.

Mechanics of Single Cytoskeletal Filaments

The cytoskeleton comprises networks of different biopolymers, which serve various cellular functions. To accomplish these tasks, their mechanical properties are of particular importance. Understanding them requires detailed knowledge of the mechanical properties of the individual filaments that make up these networks, in particular, microtubules, actin filaments, and intermediate filaments. Far from being homogeneous beams, cytoskeletal filaments have complex mechanical properties, which are directly related to the specific structural arrangement of their subunits. They are also versatile, as the filaments' mechanics and biochemistry are tightly coupled, and their properties can vary with the cellular context. In this review, we summarize decades of research on cytoskeletal filament mechanics, highlighting their most salient features and discussing recent insights from this active field of research.

Investigating the role of keratin proteins and microbial associations in hereditary and pathogenic alopecia

The purpose of this research was to identify the role of keratin proteins in causing inherited as well as pathogenic alopecia, pinpoint deleterious SNPs, and predict structural changes affecting protein-protein interactions in hair disorders. To elucidate the role of keratin proteins and genetic mutations in alopecia by analyzing protein structures through bioinformatics and identifying a mutation in the LPAR6 gene. It sought to identify the microorganisms linked to alopecia and conducted a comprehensive bioinformatics analysis of proteins with unknown experimental structures and molecular simulation analysis. The study identified a genetic mutation (c.188 A > T, p.Asp63Val) in the LPAR6 gene associated with hereditary hair loss. Pathogenic alopecia was identified to be associated with S. aureus and two ic keratinophilic fungi namely M. canis, and T. violaceum. Additionally, among 14 proteins lacking prior structural information, four proteins namely Keratin, type II cuticular Hb3 (KR1), Keratin, type II cuticular Hb6 (KR2), Keratin, type II cytoskeletal 74 (KR3) and Keratin, type II cuticular Hb1 (KR4) exhibited common 'K-head' and 'F' domains. Docking analysis revealed five distinct binding sites (C1-C5) for each protein. The 'K-head' displayed the highest predicted binding affinities with Vina scores of -5.6 for KR2 and - 4.7 for KR4 whereas the 'F' domain showed Vina scores of -6.0 for KR3 and - 5.7 for KR2. This research underscores the crucial role of keratin proteins in both hereditary and pathogenic alopecia, emphasizing their significance for future investigations.

Septin 9 expression regulates 'don't eat me' signals and identifies an immune-epithelial class of intrahepatic cholangiocarcinoma

Intrahepatic cholangiocarcinoma (iCCA) is a highly heterogeneous and aggressive liver cancer with limited therapeutic options. Precise classification and immunotherapy are perspectives to improve the treatments. We reported the role of septin 9 in apico-basal polarity and epithelial-to-mesenchymal transition (EMT). Here, we aim to elucidate its role in iCCA. We analyzed single-cell transcriptomes from human iCCA tumor cells based on phenotype and cell state. Knockdown of the septin 9 gene (SEPT9) was done using small interfering RNA (siRNA); interferon-γ (IFN-γ) stimulation was performed using different CCA cells; gene expressions were analyzed by reverse transcription and real-time PCR analysis (RT-qPCR); and immunofluorescence, immunoblotting, and flow cytometry were performed to assess the expression of proteins. The differential distributions of SEPT9 and vimentin (VIM) gene expressions allowed us to define specific cellular trajectories of malignant cells and thus identified distinct clusters of iCCA cells. One cluster was enriched in VIM and extracellular-matrix (ECM) remodeling molecules, and another had high expression of SEPT9 and genes from the 'don't eat me' signal involved in immune escape. This antagonism between SEPT9 and VIM was confirmed by in vitro experiments. Notably, SEPT9 and 'don't eat me' gene expressions were inversely correlated to those of vimentin and the EMT markers. SEPT9 expression was upregulated by IFN-γ and SEPT9 knockdown decreased expression of 'don't eat me' signal genes and increased expression of mesenchymal markers. Cancer Cell Line Encyclopedia (CCLE) transcriptome database analyses confirmed that iCCA cells enriched in septin 9 exhibit epithelial-like features. This study revealed septin 9 as a cytoskeleton element of iCCA epithelial-like cells and a regulator of the immune system response. It also brings new insights into the enigmatic relationship between EMT and immune response. Notably, we decoded a potential mechanism that could sensitize patients to immunotherapies.

Intermediate filaments at a glance

Intermediate filaments (IFs) comprise a large family of versatile cytoskeletal proteins, divided into six subtypes with tissue-specific expression patterns. IFs have a wide repertoire of cellular functions, including providing structural support to cells, as well as active roles in mechanical support and signaling pathways. Consequently, defects in IFs are associated with more than 100 diseases. In this Cell Science at a Glance article, we discuss the established classes of IFs and their general features, their functions beyond structural support, and recent advances in the field. We also highlight their involvement in disease and potential use as clinical markers of pathological conditions. Finally, we provide our view on current knowledge gaps and the future directions of the IF field.

Deciphering vimentin assembly: Bridging theoretical models and experimental approaches

The intricate assembly process of vimentin intermediate filaments (IFs), key components of the eukaryotic cytoskeleton, has yet to be elucidated. In this work, we investigated the transition from soluble tetrameric vimentin units to mature 11-nm tubular filaments, addressing a significant gap in the understanding of IF assembly. Through a combination of theoretical modeling and analysis of experimental data, we propose a novel assembly sequence, emphasizing the role of helical turns and gap filling by soluble tetramers. Our findings shed light on the unique structural dynamics of vimentin and suggest broader implications for the general principles of IF formation.

Keratin isoform shifts modulate motility signals during wound healing

Keratin intermediate filaments form strong mechanical scaffolds that confer structural stability to epithelial tissues, but the reason this function requires a protein family with 54 isoforms is not understood. During skin wound healing, a shift in keratin isoform expression alters the composition of keratin filaments. How this change modulates cellular function to support epidermal remodeling remains unclear. We report an unexpected effect of keratin isoform variation on kinase signal transduction. Increased expression of wound-associated keratin 6A, but not of steady-state keratin 5, potentiated keratinocyte migration and wound closure without compromising epidermal stability by activating myosin motors. This pathway depended on isoform-specific interaction between intrinsically disordered keratin head domains and non-filamentous vimentin shuttling myosin-activating kinases. These results substantially expand the functional repertoire of intermediate filaments from their canonical role as mechanical scaffolds to include roles as isoform-tuned signaling scaffolds that organize signal transduction cascades in space and time to influence epithelial cell state.

Dynamic changes in protein concentrations of keratins in crop milk and related gene expression in pigeon crops during different incubation and chick rearing stages

1. The objective of this study was to examine the keratin composition of crop milk, the variation of epithelial thickness and keratin (K) gene expression in samples from young pigeon during incubation and chick rearing.2. Crop milk was collected from 1-, 3- and 5-day-old squab crops for keratin content analysis. Results showed that K4 accounted for the highest proportion of all detected keratins.3. In total, 42 pairs of adult pigeons were allocated to seven groups according to different stages to collect crop samples. Gene expression studies showed that the K3 gene expression was maximised at rearing Day 15 (15) and R1 in males and females, respectively. K6a gene level was the greatest at R15 in females, whereas it peaked at incubation Day 4 (I4) in males. The K12, K13, K23 and K80 gene levels were inhibited at the peak period of crop milk formation in comparison with I4. In females, K cochleal expression peaked at I10, whereas it was the greatest at R25 in males. K4 and K14 gene expression was the highest at I10 in females, while K4 and K14 were minimised at I17 and R7 in males, respectively. Gene expressions of K5, K8, K19 and K20 in males and K19 in females were maximised at R1. The K5, K20 and K75 gene levels in females peaked at R7. K75 and K8 expressions in males and females reached a maximum value at R25 and I17, respectively.4. The epithelial thickness of male and female crops reached their greatest levels at R1 and had the highest correlation with K19.5. These results emphasised the importance of keratinisation in crop milk formation, and different keratins probably play various roles during this period. The K19 was probably a marker for pigeon crop epithelium development. The sex of the parent pigeon affected keratin gene expression profiles.

Effect of the Rho-Kinase/ROCK Signaling Pathway on Cytoskeleton Components

The mechanical properties of cells are important in tissue homeostasis and enable cell growth, division, migration and the epithelial-mesenchymal transition. Mechanical properties are determined to a large extent by the cytoskeleton. The cytoskeleton is a complex and dynamic network composed of microfilaments, intermediate filaments and microtubules. These cellular structures confer both cell shape and mechanical properties. The architecture of the networks formed by the cytoskeleton is regulated by several pathways, a key one being the Rho-kinase/ROCK signaling pathway. This review describes the role of ROCK (Rho-associated coiled-coil forming kinase) and how it mediates effects on the key components of the cytoskeleton that are critical for cell behaviour.

A cytoplasmic escapee: desmin is going nuclear

It has been a long time since researchers have focused on the cytoskeletal proteins' unconventional functions in the nucleus. Subcellular localization of a protein not only affects its functions but also determines the accessibility for cellular processes. Desmin is a muscle-specific, cytoplasmic intermediate filament protein, the cytoplasmic roles of which are defined. Yet, there is some evidence pointing out nuclear functions for desmin. In silico and wet lab analysis shows that desmin can enter and function in the nucleus. Furthermore, the candidate nuclear partners of desmin support the notion that desmin can serve as a transcriptional regulator inside the nucleus. Uncovering the nuclear functions and partners of desmin will provide a new insight into the biological significance of desmin.

Revealing transcriptional and post-transcriptional regulatory mechanisms of γ-glutamyl transferase and keratin isoforms as novel cooperative biomarkers in low-grade glioma and glioblastoma multiforme

Gamma-glutamyltransferase (GGT) and keratins (KRT) are key factors in regulating tumor progression rely on emerging evidence. However, the prognostic values of GGT and KRT isoforms and their regulation patterns at transcriptional and post-transcriptional levels have been rarely studied. In this study, we aimed to identify cooperative prognostic biomarker signature conducted by GGT and KRT genes for overall survival prediction and discrimination in patients with low-grade glioma (LGG) and glioblastoma multiforme (GBM). To this end, we employed a differential expression network analysis on LGG-NORMAL, GBM-NORMAL, and LGG-GBM datasets. Then, all the differentially expressed genes related to a GO term "GGT activity" were excluded. After that, for obtained potential biomarkers genes, differentially expressed lncRNAs were used to detect cis-regulatory elements (CREs) and trans-regulatory elements (TREs). To scrutinize the regulation on the cytoplasm, potential interactions between these biomarker genes and DElncRNAs were predicted. Our analysis, for the first time, revealed that GGT6, KRT33B, and KRT75 in LGG, GGT2, and KRT75 in GBM and KRT75 for LGG to GBM transformation tumors can be novel cooperative prognostic biomarkers that may be applicable for early detection of LGG, GBM, and LGG to GBM transformation tumors. Consequently, KRT75 was the most important gene being regulated at both transcriptional and post-transcriptional levels significantly. Furthermore, CREs and their relative genes were coordinative up-regulated or down-regulated suggesting CREs as regulation points of these genes. In the end, up-regulation of most DElncRNAs that had physical interaction with target genes pints out that the transcripted genes may have obstacles for translation process.

Cytoskeletal proteins as glioblastoma biomarkers and targets for therapy: A systematic review

Glioblastoma, the most common primary brain malignancy, is an exceptionally fatal cancer. Lack of suitable biomarkers and efficient treatment largely contribute to the therapy failure. Cytoskeletal proteins are crucial proteins in glioblastoma pathogenesis and can potentially serve as biomarkers and therapeutic targets. Among them, GFAP, has gained most attention as potential diagnostic biomarker, while vimentin and microtubules are considered as prospective therapeutic targets. Microtubules represent one of the best anti-cancer targets due to their critical role in cell proliferation. Despite testing in clinical trials, the efficiency of taxanes, epothilones, vinca-domain binding drugs, colchicine-domain binding drugs and γ-tubulin binding drugs remains to be confirmed. Moreover, tumor treating field that disrupts microtubules draw attention because of its high efficiency and is called "the fourth cancer treatment modality". Thereby, because of the involvement of cytoskeleton in key physiological and pathological processes, its therapeutic potential in glioblastoma is currently extensively investigated.

Seasonal morphology and immunoreactivity of cytokeratin and atrial natriuretic peptide in dromedary camel poll glands

Poll glands are characteristic feature of dromedary camels; although they yield a yellowish offensive secretion, especially during rutting (breeding) season, their function is not yet exactly specified. The present study shows the seasonal morphology and immunoreactivity of cytokeratin (CK) and atrial natriuretic peptide (ANP) in the camel poll gland; the result could clearly specify the role of the gland in camel reproduction. Poll glands are compound tubulo-alveolar in structure. During rutting season, the secretory units showed wide lumina and simple squamous epithelium surrounded by myoepithelial cells; the lumina became narrower and the epithelium changed to simple cuboidal during non-rutting season. Many glandular lobules showed abundant interlobular connective tissue with fewer and smaller secretory units during non-rutting season compared to rutting season. Positive CK and ANP immunoreactivity was detected in the cytoplasm of epithelial cells of secretory units and ducts together with the myoepithelial cells and blood vessels. Although CK immunoreactivity was more intense during rutting season in comparison to non-rutting season, there was no seasonal variation in ANP immunoreactivity. During both seasons, while the glandular capsule, connective tissue septa, interstitium showed negative CK immunoreactions, they reacted moderately to ANP. In conclusion, the poll gland undergoes annual structural and functional changes which are suggested to correlate with the male seasonal sexual behaviour. Further, the biological role of CK and ANP proteins together with their immunohistochemical expression in the camel poll gland suggests a stimulatory effect in the glandular secretory cells, and hence, they might modify camel sexual activity.

Generation of Rat Monoclonal Antibody for Cytokeratin 18 by Immunization of Three-Dimensional-Cultured Cancer Cells

Cytokeratin (CK) 18 is an intermediate filament protein that plays a major functional role in the integrity and mechanical stability of cells. Since both CK8 and CK18 are major components of simple epithelia, in the context of tumors, they are expressed in most carcinomas, and have been studied as diagnostic and prognostic markers in tumor pathology. CK18 is also cleaved by some caspases during apoptosis. Three-dimensional (3D)-cultured cancer cells are useful for cancer research as an intermediate model between in vitro cancer cell line cultures and in vivo tumors. In this study, we produced rat monoclonal antibodies (mAbs) through immunization of the lysate from 3D-cultured DLD-1 cells to elucidate a characteristic feature of a tumor, and our results showed that mAb 2H7 recognized human CK18. Furthermore, we indicated that mAb 2H7 was useful for immunoblotting, immunoprecipitation, and immunofluorescence staining. Therefore, it may be useful as a diagnostic tool for evaluating malignancy.

Scaling up single-cell mechanics to multicellular tissues - the role of the intermediate filament-desmosome network

Cells and tissues sense, respond to and translate mechanical forces into biochemical signals through mechanotransduction, which governs individual cell responses that drive gene expression, metabolic pathways and cell motility, and determines how cells work together in tissues. Mechanotransduction often depends on cytoskeletal networks and their attachment sites that physically couple cells to each other and to the extracellular matrix. One way that cells associate with each other is through Ca2+-dependent adhesion molecules called cadherins, which mediate cell-cell interactions through adherens junctions, thereby anchoring and organizing the cortical actin cytoskeleton. This actin-based network confers dynamic properties to cell sheets and developing organisms. However, these contractile networks do not work alone but in concert with other cytoarchitectural elements, including a diverse network of intermediate filaments. This Review takes a close look at the intermediate filament network and its associated intercellular junctions, desmosomes. We provide evidence that this system not only ensures tissue integrity, but also cooperates with other networks to create more complex tissues with emerging properties in sensing and responding to increasingly stressful environments. We will also draw attention to how defects in intermediate filament and desmosome networks result in both chronic and acquired diseases.

Intermediate Filaments as Effectors of Cancer Development and Metastasis: A Focus on Keratins, Vimentin, and Nestin

Intermediate filament (IF) proteins make up the largest family of cytoskeletal proteins in metazoans, and are traditionally known for their roles in fostering structural integrity in cells and tissues. Remarkably, individual IF genes are tightly regulated in a fashion that reflects the type of tissue, its developmental and differentiation stages, and biological context. In cancer, IF proteins serve as diagnostic markers, as tumor cells partially retain their original signature expression of IF proteins. However, there are also characteristic alterations in IF gene expression and protein regulation. The use of high throughput analytics suggests that tumor-associated alterations in IF gene expression have prognostic value. Parallel research is also showing that IF proteins directly and significantly impact several key cellular properties, including proliferation, death, migration, and invasiveness, with a demonstrated impact on the development, progression, and characteristics of various tumors. In this review, we draw from recent studies focused on three IF proteins most associated with cancer (keratins, vimentin, and nestin) to highlight how several “hallmarks of cancer” described by Hanahan and Weinberg are impacted by IF proteins. The evidence already in hand establishes that IF proteins function beyond their classical roles as markers and serve as effectors of tumorigenesis.

Distribution patterns of the zebrafish neuronal intermediate filaments inaa and inab

It has been reported that the neuronal intermediate filament (IF) α-internexin may plays a role in the formation of the neuronal cytoskeleton during mammalian development. From a phylogenetic viewpoint, zebrafish express inaa and inab as homologs of mammalian α-internexin. However, the distribution patterns of the inaa and inab proteins throughout zebrafish development have not been well-characterized. We generated antibodies specific for zebrafish inaa and inab and analyzed the distribution of these two proteins in developing zebrafish. Inaa was identified in the major subdivisions of embryonic and larval brains as early as 1 day postfertilization (dpf), including the telencephalon, optic tectum, and cerebellum, and inab was also detected in the same regions from 3 dpf to the adult stage. Moreover, we demonstrated for the first time that inaa was distinctively expressed in the photoreceptor-like cells of the pineal gland, where inab was sparsely detected. Besides, the expression of inaa in male adult fish was found to be stable under different photoperiod conditions. Thus, we suggest that inaa is one of useful markers for studies of zebrafish cone photoreceptors not only in the retina but also in the pineal gland. In conclusion, we report that the distribution patterns of inaa and inab are phylogenetically conserved in the telencephalon, optic tectum, and cerebellum. Moreover, inaa and inab had different expression patterns in the pineal gland and retina during zebrafish development. Both inaa and inab are neuronal IFs and their functional roles may be different in various aspects of zebrafish neuronal development.

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

Histological and Ultrastructure Study of the Testes of Acrylamide Exposed Adult Male Albino Rat and Evaluation of the Possible Protective Effect of Vitamin E Intake

Acrylamide (AA) is a hazardous unavoidable gonadal toxin. Hence, the aim of this study is to clarify its harmful effects on the testis of adult albino rat by light and electron microscope and to evaluate the possible role of Vitamin E (Vit E) in the prevention of such effects. Thirty-five adult male albino rats were enrolled in this study. They were divided into three groups: Group I (control); Group II (AA exposed), and Group III (AA and concomitant Vit E treated group). Animals of Groups II and III were further subdivided into two equal subgroups (each subgroup included five rats): (a) rats were sacrificed after 4 weeks and (b) rats were sacrificed after 6 weeks. The testes of each rat were dissected out, processed, and examined by Hematoxylin and Eosin, Periodic acid-Schiff and Mallory's trichrome stains as well as electron microscopic study. The study revealed that AA induces testicular damage at the histological and ultrastructural level in the form of degeneration and arrested spermatogenesis. Moreover, decreased seminiferous tubules diameters and epithelial height were detected. These changes are maximally improved in Vit E treated group. Hence, we could conclude that AA causes degenerative changes of the testes of albino rats and arrest of spermatogenesis. The AA-induced histological and ultrastructural changes of the testes could be explained by oxidative stress. These effects changes are proportional to the duration of exposure. Moreover, it could be concluded that Vitamin E has a protective role against AA-induced testicular damage by its antioxidant and anti-apoptotic effects.

A curated catalog of canine and equine keratin genes

Keratins represent a large protein family with essential structural and functional roles in epithelial cells of skin, hair follicles, and other organs. During evolution the genes encoding keratins have undergone multiple rounds of duplication and humans have two clusters with a total of 55 functional keratin genes in their genomes. Due to the high similarity between different keratin paralogs and species-specific differences in gene content, the currently available keratin gene annotation in species with draft genome assemblies such as dog and horse is still imperfect. We compared the National Center for Biotechnology Information (NCBI) (dog annotation release 103, horse annotation release 101) and Ensembl (release 87) gene predictions for the canine and equine keratin gene clusters to RNA-seq data that were generated from adult skin of five dogs and two horses and from adult hair follicle tissue of one dog. Taking into consideration the knowledge on the conserved exon/intron structure of keratin genes, we annotated 61 putatively functional keratin genes in both the dog and horse, respectively. Subsequently, curators in the RefSeq group at NCBI reviewed their annotation of keratin genes in the dog and horse genomes (Annotation Release 104 and Annotation Release 102, respectively) and updated annotation and gene nomenclature of several keratin genes. The updates are now available in the NCBI Gene database (https://www.ncbi.nlm.nih.gov/gene).

A Seasonal and Age-Related Study of Interstitial Cells in the Pineal Gland of Male Viscacha (Lagostomus maximus maximus)

The pineal gland of viscacha exhibits histophysiological variations throughout the year, with periods of maximal activity in winter and minimal activity in summer. The aim of this work is to analyze the interstitial cells (IC) in the pineal gland of male viscachas in relation to season and age. The S-100 protein, glio-fibrillary acidic protein (GFAP), and vimentin were detected in adult and immature animals by immunohistochemistry (IHC). Double-IHC was also performed. The S-100 protein was localized within both, IC nucleus and cytoplasm. GFAP was present only in the cytoplasm. Vimentin was expressed in some IC, besides endothelial cells, and perivascular spaces. In the adult males, the morphometric parameters analyzed for the S-100 protein and GFAP exhibited seasonal variations with higher values of immunopositive area percentage in winter and lower values in summer, whereas the immature ones showed the lowest values for all the adult animals studied. Colocalization of S-100 protein and GFAP was observed. The IC exhibited differential expression for the proteins studied, supporting the hypothesis of the neuroectodermal origin. The IC generate an intraglandular communication network, suggesting its participation in the glandular activity regulation processes. The results of double-IHC might indicate the presence of IC in different functional stages, probably related to the needs of the cellular microenvironment. The morphometric variations in the proteins analyzed between immature and adult viscachas probed to be more salient in the latter, suggesting a direct relationship between the expression of the S-100 protein and GFAP, and animal age. Anat Rec, 2017. © 2017 Wiley Periodicals Inc. Anat Rec, 300:1847-1857, 2017. © 2017 Wiley Periodicals, Inc.

Isolation of Intermediate Filament Proteins from Multiple Mouse Tissues to Study Aging-associated Post-translational Modifications

Intermediate filaments (IFs), together with actin filaments and microtubules, form the cytoskeleton - a critical structural element of every cell. Normal functioning IFs provide cells with mechanical and stress resilience, while a dysfunctional IF cytoskeleton compromises cellular health and has been associated with many human diseases. Post-translational modifications (PTMs) critically regulate IF dynamics in response to physiological changes and under stress conditions. Therefore, the ability to monitor changes in the PTM signature of IFs can contribute to a better functional understanding, and ultimately conditioning, of the IF system as a stress responder during cellular injury. However, the large number of IF proteins, which are encoded by over 70 individual genes and expressed in a tissue-dependent manner, is a major challenge in sorting out the relative importance of different PTMs. To that end, methods that enable monitoring of PTMs on IF proteins on an organism-wide level, rather than for isolated members of the family, can accelerate research progress in this area. Here, we present biochemical methods for the isolation of the total, detergent-soluble, and detergent-resistant fraction of IF proteins from 9 different mouse tissues (brain, heart, lung, liver, small intestine, large intestine, pancreas, kidney, and spleen). We further demonstrate an optimized protocol for rapid isolation of IF proteins by using lysing matrix and automated homogenization of different mouse tissues. The automated protocol is useful for profiling IFs in experiments with high sample volume (such as in disease models involving multiple animals and experimental groups). The resulting samples can be utilized for various downstream analyses, including mass spectrometry-based PTM profiling. Utilizing these methods, we provide new data to show that IF proteins in different mouse tissues (brain and liver) undergo parallel changes with respect to their expression levels and PTMs during aging.

Cysteine Prevents the Reduction in Keratin Synthesis Induced by Iron Deficiency in Human Keratinocytes

L-cysteine is currently recognized as a conditionally essential sulphur amino acid. Besides contributing to many biological pathways, cysteine is a key component of the keratin protein by its ability to form disulfide bridges that confer strength and rigidity to the protein. In addition to cysteine, iron represents another critical factor in regulating keratins expression in epidermal tissues, as well as in hair follicle growth and maturation. By focusing on human keratinocytes, the aim of this study was to evaluate the effect of cysteine supplementation as nutraceutical on keratin biosynthesis, as well as to get an insight on the interplay of cysteine availability and cellular iron status in regulating keratins expression in vitro. Herein we demonstrate that cysteine promotes a significant up-regulation of keratins expression as a result of de novo protein synthesis, while the lack of iron impairs keratin expression. Interestingly, cysteine supplementation counteracts the adverse effect of iron deficiency on cellular keratin expression. This effect was likely mediated by the up-regulation of transferrin receptor and ferritin, the main cellular proteins involved in iron homeostasis, at last affecting the labile iron pool. In this manner, cysteine may also enhance the metabolic iron availability for DNA synthesis without creating a detrimental condition of iron overload. To the best of our knowledge, this is one of the first study in an in vitro keratinocyte model providing evidence that cysteine and iron cooperate for keratins expression, indicative of their central role in maintaining healthy epithelia.

Discovery of keratin function and role in genetic diseases: the year that 1991 was

In 1991, a set of transgenic mouse studies took the fields of cell biology and dermatology by storm in providing the first credible evidence that keratin intermediate filaments play a unique and essential role in the structural and mechanical support in keratinocytes of the epidermis. Moreover, these studies intimated that mutations altering the primary structure and function of keratin filaments underlie genetic diseases typified by cellular fragility. This Retrospective on how these studies came to be is offered as a means to highlight the 25th anniversary of these discoveries.

Desmin enters the nucleus of cardiac stem cells and modulates Nkx2.5 expression by participating in transcription factor complexes that interact with the nkx2.5 gene

The transcription factor Nkx2.5 and the intermediate filament protein desmin are simultaneously expressed in cardiac progenitor cells during commitment of primitive mesoderm to the cardiomyogenic lineage. Up-regulation of Nkx2.5 expression by desmin suggests that desmin may contribute to cardiogenic commitment and myocardial differentiation by directly influencing the transcription of the nkx2.5 gene in cardiac progenitor cells. Here, we demonstrate that desmin activates transcription of nkx2.5 reporter genes, rescues nkx2.5 haploinsufficiency in cardiac progenitor cells, and is responsible for the proper expression of Nkx2.5 in adult cardiac side population stem cells. These effects are consistent with the temporary presence of desmin in the nuclei of differentiating cardiac progenitor cells and its physical interaction with transcription factor complexes bound to the enhancer and promoter elements of the nkx2.5 gene. These findings introduce desmin as a newly discovered and unexpected player in the regulatory network guiding cardiomyogenesis in cardiac stem cells.

Topographical mapping of α- and β-keratins on developing chicken skin integuments: Functional interaction and evolutionary perspectives

Avian integumentary organs include feathers, scales, claws, and beaks. They cover the body surface and play various functions to help adapt birds to diverse environments. These keratinized structures are mainly composed of corneous materials made of α-keratins, which exist in all vertebrates, and β-keratins, which only exist in birds and reptiles. Here, members of the keratin gene families were used to study how gene family evolution contributes to novelty and adaptation, focusing on tissue morphogenesis. Using chicken as a model, we applied RNA-seq and in situ hybridization to map α- and β-keratin genes in various skin appendages at embryonic developmental stages. The data demonstrate that temporal and spatial α- and β-keratin expression is involved in establishing the diversity of skin appendage phenotypes. Embryonic feathers express a higher proportion of β-keratin genes than other skin regions. In feather filament morphogenesis, β-keratins show intricate complexity in diverse substructures of feather branches. To explore functional interactions, we used a retrovirus transgenic system to ectopically express mutant α- or antisense β-keratin forms. α- and β-keratins show mutual dependence and mutations in either keratin type results in disrupted keratin networks and failure to form proper feather branches. Our data suggest that combinations of α- and β-keratin genes contribute to the morphological and structural diversity of different avian skin appendages, with feather-β-keratins conferring more possible composites in building intrafeather architecture complexity, setting up a platform of morphological evolution of functional forms in feathers.

Impact of keratin intermediate filaments on insulin-mediated glucose metabolism regulation in the liver and disease association

In all cells, a tight regulation exists between glucose uptake and utilization to prevent diseases related to its perturbed metabolism. In insulin-targeted cells, such as hepatocytes, proper glucose utilization requires an elaborate interplay between the insulin receptor, the glucose transporter, and mitochondria that involves the participation of actin microfilaments and microtubules. In addition, there is increasing evidence of an involvement of the third cytoskeletal network provided by intermediate filaments (IFs). Keratins belong to the multigene family of IF proteins, coordinately expressed as distinct pairs within the context of epithelial cell differentiation. Hepatocyte IFs are made up of the [keratin (K)8/K18] pair only, whereas pancreatic β-cell IFs additionally include small amounts of K7. There are accumulating examples of K8/K18 involvement in the glucose-insulin cross-talk, including the modulation of plasma glucose levels, insulin release from pancreatic β-cells, and insulin-mediated glucose uptake and glycogen production in hepatocytes after a K8/K18 loss. This review integrates the mechanistic features that support such an impact of K8/K18 IFs on insulin-dependent glucose metabolism regulation in liver and its implication in glucose- or insulin-associated diseases.

Order and disorder in intermediate filament proteins

Intermediate filaments (IFs), important components of the cytoskeleton, provide a versatile, tunable network of self-assembled proteins. IF proteins contain three distinct domains: an α-helical structured rod domain, flanked by intrinsically disordered head and tail domains. Recent studies demonstrated the functional importance of the disordered domains, which differ in length and amino-acid sequence among the 70 different human IF genes. Here, we investigate the biophysical properties of the disordered domains, and review recent findings on the interactions between them. Our analysis highlights key components governing IF functional roles in the cytoskeleton, where the intrinsically disordered domains dictate protein-protein interactions, supramolecular assembly, and macro-scale order.

Evolutionary aspects in intermediate filament proteins

Intermediate filament (IF) proteins, together with tubulins and actins, constitute the majority of cytoskeletal proteins in metazoans. Proteins of the IF family fulfil increasingly diverse functions but share common structural features. Phylogenetic analysis within the metazoan lineage traces back their origin to a common lamin-like ancestor. Major steps in lamin evolution occurred at the base of the vertebrate radiation, while cytoplasmic IF protein subfamilies evolved independently in the major metazoan lineages.

Keratin pearl degradation in oral squamous cell carcinoma: reciprocal roles of neutrophils and macrophages

BACKGROUND

We have reported that neutrophilic infiltration was associated with round-shaped dyskeratosis foci, a kind of keratin pearl, of oral carcinoma in situ and that those inflammatory cells are recruited from intra-epithelially entrapped blood vessels. Based on these lines of evidence, we have formulated a hypothesis that keratin pearls are terminally degraded by neutrophils. To confirm this hypothesis, we investigated immunohistochemically stepwise degradation of keratin pearls in oral squamous cell carcinoma (SCC) to clarify any other type scavenger cells in addition to neutrophils are involved in this particular degradation process.

METHODS

Neutrophils (neutrophil elastase) and macrophage subpopulations (CD68, CD163 and CD204) were immunohistochemically localized in 30 cases of oral SCC with typical round-shaped keratin pearls. SCC cells were revealed by immunohistochemistry for keratin (K) 17, and blood vessels were demonstrated by CD31.

RESULTS

Keratin pearl degradation process was divided into four steps: (i) intact stage: no macrophage infiltration but minimal neutrophils were found in keratin pearls; (ii) neutrophil recruit stage: no macrophage infiltration but focal neutrophilic infiltration within the pearls; (iii) neutrophil predominant stage: dense neutrophil infiltration with minimal macrophages and segregated keratinized cancer cells strongly positive for K17; and (iv) macrophage predominant stage: dense infiltration of CD68-, CD163 (mononuclear)- and CD204 (multinucleated)-positive macrophages engulfing detached keratinized SCC cells.

CONCLUSION

Keratin pearl degradation in oral SCC is strictly regulated by two types of scavenger cells: neutrophils, which perform initial tasks, and macrophages, which reciprocally take over from neutrophils the role to finalize the degradation processes.

Hair curvature: a natural dialectic and review

Although hair forms (straight, curly, wavy, etc.) are present in apparently infinite variations, each fibre can be reduced to a finite sequence of tandem segments of just three types: straight, bent/curly, or twisted. Hair forms can thus be regarded as resulting from genetic pathways that induce, reverse or modulate these basic curvature modes. However, physical interconversions between twists and curls demonstrate that strict one-to-one correspondences between them and their genetic causes do not exist. Current hair-curvature theories do not distinguish between bending and twisting mechanisms. We here introduce a multiple papillary centres (MPC) model which is particularly suitable to explain twisting. The model combines previously known features of hair cross-sectional morphology with partially/completely separated dermal papillae within single follicles, and requires such papillae to induce differential growth rates of hair cortical material in their immediate neighbourhoods. The MPC model can further help to explain other, poorly understood, aspects of hair growth and morphology. Separate bending and twisting mechanisms would be preferentially affected at the major or minor ellipsoidal sides of fibres, respectively, and together they exhaust the possibilities for influencing hair-form phenotypes. As such they suggest dialectic for hair-curvature development. We define a natural-dialectic (ND) which could take advantage of speculative aspects of dialectic, but would verify its input data and results by experimental methods. We use this as a top-down approach to first define routes by which hair bending or twisting may be brought about and then review evidence in support of such routes. In particular we consider the wingless (Wnt) and mammalian target of rapamycin (mTOR) pathways as paradigm pathways for molecular hair bending and twisting mechanisms, respectively. In addition to the Wnt canonical pathway, the Wnt/Ca(2+) and planar cell polarity (PCP) pathways, and others, can explain many alternatives and specific variations of hair bending phenotypes. Mechanisms for hair papilla budding or its division by bisection or fission can explain MPC formation. Epithelial-to-mesenchymal (EMT) and mesenchymal-to-epithelial (MET) transitions, acting in collaboration with epithelial-mesenchymal communications are also considered as mechanisms affecting hair growth and its bending and twisting. These may be treated as sub-mechanisms of an overall development from neural-crest stem cell (NCSC) lineages to differentiated hair follicle (HF) cell types, thus providing a unified framework for hair growth and development.

Oral cytokeratins in health and disease

The dynamics of oral mucosa is known by its inherent defensive nature. Certain areas demand tough shield when subjected to mechanical insults. This is met by structural scaffolding material referred as cytoskeleton comprised of intracellular protein filaments called cytokeratins in the surface squames of oral epithelia. They also equally contribute towards the architecture of odontogenic apparatus and salivary gland. Differentiation of epithelial cells within stratified epithelia regulates the expression of specific keratin gene. Any mutation in, or autoantibodies to keratins, desmosomal and cornified envelope proteins is translated into genetic and acquired human disorders. Sound knowledge of structural proteins, their expression, distribution and function plays a vital role in acquainting with these disorders and their application as differentiation markers. Thus, they form an integral aid in diagnostic pathology and may be instrumental in the future interventions by gene therapy. This review focuses on basics to current updates on oral cytokeratins with an emphasis on the genetic and acquired disorders of cytokeratins with oral implications.

Harvesting the potential of the human umbilical cord: isolation and characterisation of four cell types for tissue engineering applications

The human umbilical cord (UC) has attracted interest as a source of cells for many research applications. UC solid tissues contain four cell types: epithelial, stromal, smooth muscle and endothelial cells. We have developed a unique protocol for the sequential extraction of all four cell types from a single UC, allowing tissue reconstruction using multiple cell types from the same source. By combining perfusion, immersion and explant techniques, all four cell types have been successfully expanded in monolayer cultures. We have also characterised epithelial and Wharton's jelly cells (WJC) by immunolabelling of specific proteins. Epithelial cell yields averaged at 2.3 × 10(5) cells per centimetre UC, and the cells expressed an unusual combination of keratins typical of simple, mucous and stratified epithelia. Stromal cells in the Wharton's jelly expressed desmin, α-smooth muscle actin, elastin, keratins (K12, K16, K18 and K19), vimentin and collagens. Expression patterns in cultured cells resembled those found in situ except for basement membrane components and type III collagen. These stromal cells featured a sustained proliferation rate up to passage 12 after thawing. The mesenchymal stem cell (MSC) character of the WJC was confirmed by their expression of typical MSC surface markers and by adipogenic and osteogenic differentiation assays. To emphasise and demonstrate their potential for regenerative medicine, UC cell types were successfully used to produce human tissue-engineered constructs. Both bilayered stromal/epithelial and vascular substitutes were produced, establishing the versatility and importance of these cells for research and therapeutic applications.

Dynamic expression of synemin isoforms in mouse embryonic stem cells and neural derivatives

Background

Intermediate filaments (IFs) are major components of the mammalian cytoskeleton and expressed in cell-type-specific patterns. Morphological changes during cell differentiation are linked to IF network remodeling. However, little is known concerning the presence and the role of IFs in embryonic stem (ES) cells and during their differentiation.

Results

We have examined the expression profile of synemin isoforms in mouse pluripotent ES cells and during their neural differentiation induced by retinoic acid. Using RT-PCR, Western blotting and immunostaining, we show that synemin M is present at both mRNA and protein levels in undifferentiated ES cells as early as pluripotency factor Oct-3/4 and IF keratin 8. Synemin H was produced only in neural precursors when neural differentiation started, concurrently with synemin M, nestin and glial fibrillary acidic protein. However, both synemin H and M were restricted to the progenitor line during the neural differentiation program. Our in vivo analysis also confirmed the expression of synemins H/M in multipotent neural stem cells in the subventricular zone of the adult brain, a neurogenic germinal niche of the mice. Knocking down synemin in ES cells by shRNA lentiviral particles transduction has no influence on expression of Oct4, Nanog and SOX2, but decreased keratin 8 expression.

Conclusions

Our study shows a developmental stage specific regulation of synemin isoforms in ES cells and its neural derivatives. These findings represent the first evidence that synemins could potentially be useful markers for distinguishing multipotent ES cells from undifferentiated neural stem cells and more committed progenitor cells.

[Intermediate-filament-associated diseases]

Intracellular protein filaments intermediate in size between actin filaments and microtubules are composed of a variety of tissue specific proteins. The sequence conservation of the coiled-coil alpha-helical structure responsible for polymerization into individual 10 nm filaments defines a large gene family. Intermediate filaments (IFs) include the nuclear lamins, which are universal in Metazoans, and the cytoplasmic intermediate filaments, which are more varied and form cell type specific networks in animal cells. IFs all share a common tripartite structure consisting of a highly conserved central helical rod domain and variable N-head and C-tail domains. In contrast to actin and tubulin, IFs do not require nucleoside triphosphates such as ATP or GTP for polymerization but they self assemble. According to sequences, the IFs proteins are grouped into seven classes, including five cytoplasmic, one nuclear and one sub-cortical localizations. The search for functions of IFs has led to discoveries of roles in the skin, heart, muscle, liver and brain, in premature aging and of involvement in several degenerative disorders. Mutations in IFs cause or predispose to more than 80 human tissue-specific diseases. Mouse models and gene invalidation have been extremely helpful in eliciting IF role in physiopathology. Besides mechanical role in cell plasticity and stress absorbers, IF functions are related to the capacity to interact with signaling molecules and cell kinases, controlling gene regulatory networks. The reviews herein include a historical perspective about IFs, describe how mutations affect IF structure and assembly properties in desminopathies, inclusion formation in the neurodegenerative Alexander disease, and how they induce multiple disorders in laminopathies.

Insights into intermediate filament regulation from development to ageing

Intermediate filament (IF) proteins comprise a large family with more than 70 members. Initially, IFs were assumed to provide only structural reinforcement for the cell. However, IFs are now known to be dynamic structures that are involved in a wide range of cellular processes during all stages of life, from development to ageing, and during homeostasis and stress. This Commentary discusses some lesser-known functional and regulatory aspects of IFs. We specifically address the emerging roles of nestin in myogenesis and cancer cell migration, and examine exciting evidence on the regulation of nestin and lamin A by the notch signalling pathway, which could have repercussions for our understanding of the roles of IF proteins in development and ageing. In addition, we discuss the modulation of the post-translational modifications of neuronally expressed IFs and their protein-protein interactions, as well as IF glycosylation, which not only has a role in stress and ageing, but might also regulate IFs during development. Although many of these recent findings are still preliminary, they nevertheless open new doors to explore the functionality of the IF family of proteins.

Keratin 8 expression in colon cancer associates with low faecal butyrate levels

BACKGROUND

Butyrate has been implicated in the mechanistic basis of the prevention of colorectal cancer by dietary fibre. Numerous in vitro studies have shown that butyrate regulates cell cycle and cell death. More recently we have shown that butyrate also regulates the integrity of the intermediate filament (IF) cytoskeleton in vitro. These and other data suggest a link between the role of diet and the implication of a central role for the keratin 8 (K8) as guardian of the colorectal epithelium.

METHODS

In this cross-sectional study possible links between butyrate levels, field effects and keratin expression in cancer were addressed directly by analysing how levels of expression of the IF protein K8 in tumours, in adjacent fields and at a distant landmark site may be affected by the level of butyrate in the colon microenvironment. An immunohistochemical scoring protocol for K8 was developed and applied to samples, findings were further tested by immunoblotting.

RESULTS

Levels of K8 in colorectal tumours are lower in subjects with higher levels of faecal butyrate. Immunoblotting supported this finding.Although there were no significant relationships with butyrate on the non-tumour tissues, there was a consistent trend in all measures of extent or intensity of staining towards a reduction in expression with elevated butyrate, consistent with the inverse association in tumours.

CONCLUSIONS

The data suggest that butyrate may associate with down-regulation of the expression of K8 in the cancerized colon. If further validated these findings may suggest the chemopreventive value of butyrate is limited to early stage carcinogenesis as low K8 expression is associated with a poor prognosis.

Oridonin suppresses the proliferation of human pancreatic carcinoma SW1990 cells

AIM: To investigate whether oridonin has antiproliferation effects against human pancreatic carcinoma SW1990 cells and to explore possible mechanisms involved.

METHODS: SW1990 cells were treated with different concentrations of oridonin. Cell proliferation was measured by MTT assay. Cell morphological changes were observed by laser confocal microscopy after double immunofluorescence staining. Cell apoptosis was detected by flow cytometry (FCM).

RESULTS: Treatment with oridonin significantly inhibited the growth of SW1990 cells. After treatment with oridonin, apoptosis-related morphological changes, such as condensation of chromatin and nuclear fragmentation, were observed in SW1990 cells. The fluorescence intensity of β-actin in the cytoplasm was increased, especially in the perinuclear area. The early apoptotic rates of SW1990 cells treated with 12.5, 25.0 and 50.0 µmol/L oridonin were significantly higher than those of control cells (2.17% ± 0.57%, 5.83% ± 0.57%, 4.43% ± 0.78% vs 0.30% ± 0.17%, all P < 0.05). The late apoptotic and cell necrosis rates were also significantly higher in oridonin-treated cells than in control cells (5.10% ± 0.98%, 7.03% ± 1.52%, 10.13% ± 0. 70% vs 2.63% ± 0.06%, all P < 0.05).

CONCLUSION: Oridonin can significantly inhibit proliferation and induce apoptosis of SW1990 cells perhaps by altering β-actin depolymerization/polymerization.

Conformational dynamics of actin: effectors and implications for biological function

Actin is a protein abundant in many cell types. Decades of investigations have provided evidence that it has many functions in living cells. The diverse morphology and dynamics of actin structures adapted to versatile cellular functions is established by a large repertoire of actin-binding proteins. The proper interactions with these proteins assume effective molecular adaptations from actin, in which its conformational transitions play essential role. This review attempts to summarise our current knowledge regarding the coupling between the conformational states of actin and its biological function.

Seven kinds of intermediate filament networks in the cytoplasm of polarized cells: structure and function

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.