A conserved rod domain phosphotyrosine that is targeted by the phosphatase PTP1B promotes keratin 8 protein insolubility and filament organization

Type III intermediate filaments in redox interplay: key role of the conserved cysteine residue

Intermediate filaments (IFs) are cytoskeletal elements involved in mechanotransduction and in the integration of cellular responses. They are versatile structures and their assembly and organization are finely tuned by posttranslational modifications. Among them, type III IFs, mainly vimentin, have been identified as targets of multiple oxidative and electrophilic modifications. A characteristic of most type III IF proteins is the presence in their sequence of a single, conserved cysteine residue (C328 in vimentin), that is a hot spot for these modifications and appears to play a key role in the ability of the filament network to respond to oxidative stress. Current structural models and experimental evidence indicate that this cysteine residue may occupy a strategic position in the filaments in such a way that perturbations at this site, due to chemical modification or mutation, impact filament assembly or organization in a structure-dependent manner. Cysteine-dependent regulation of vimentin can be modulated by interaction with divalent cations, such as zinc, and by pH. Importantly, vimentin remodeling induced by C328 modification may affect its interaction with cellular organelles, as well as the cross-talk between cytoskeletal networks, as seems to be the case for the reorganization of actin filaments in response to oxidants and electrophiles. In summary, the evidence herein reviewed delineates a complex interplay in which type III IFs emerge both as targets and modulators of redox signaling.

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Tyrosine phosphorylation of lamin A by Src promotes disassembly of nuclear lamina in interphase

Lamins form the nuclear lamina, which is important for nuclear structure and activity. Although posttranslational modifications, in particular serine phosphorylation, have been shown to be important for structural properties and functions of lamins, little is known about the role of tyrosine phosphorylation in this regard. In this study, we found that the constitutively active Src Y527F mutant caused the disassembly of lamin A/C. We demonstrate that Src directly phosphorylates lamin A mainly at Tyr45 both in vitro and in intact cells. The phosphomimetic Y45D mutant was diffusively distributed in the nucleoplasm and failed to assemble into the nuclear lamina. Depletion of lamin A/C in HeLa cells induced nuclear dysmorphia and genomic instability as well as increased nuclear plasticity for cell migration, all of which were partially restored by re-expression of lamin A, but further promoted by the Y45D mutant. Together, our results reveal a novel mechanism for regulating the assembly of nuclear lamina through Src and suggest that aberrant phosphorylation of lamin A by Src may contribute to nuclear dysmorphia, genomic instability, and nuclear plasticity.

Sequence and Properties of Cagein, a Coiled-Coil Scaffold Protein Linking Basal Bodies in the Polykinetids of the Ciliate Euplotes aediculatus

In the hypotrich ciliate Euplotes, many individual basal bodies are grouped together in tightly packed clusters, forming ventral polykinetids. These groups of basal bodies (which produce compound ciliary organelles such as cirri and oral membranelles) are cross-linked into ordered arrays by scaffold structures known as "basal-body cages." The major protein comprising Euplotes cages has been previously identified and termed "cagein." Screening a E. aediculatus cDNA expression library with anti-cagein antisera identified a DNA insert containing most of a putative cagein gene; standard PCR techniques were used to complete the sequence. Probes designed from this gene identified a macronuclear "nanochromosome" of ca. 1.5 kb in Southern blots against whole-cell DNA. The protein derived from this sequence (463 residues) is predicted to be hydrophilic and highly charged; however, the native cage structures are highly resistant to salt/detergent extraction. This insolubility could be explained by the coiled-coil regions predicted to extend over much of the length of the derived cagein polypeptide. One frameshift sequence is found within the gene, as well as a short intron. BLAST searches find many ciliates with evident homologues to cagein within their derived genomic sequences.

The Cytoskeleton as Regulator of Cell Signaling Pathways

During interphase, filamentous actin, microtubules, and intermediate filaments regulate cell shape, motility, transport, and interactions with the environment. These activities rely on signaling events that control cytoskeleton properties. Recent studies uncovered mechanisms that go far beyond this one-directional flow of information. Thus, the three branches of the cytoskeleton impinge on signaling pathways to determine their activities. We propose that this regulatory role of the cytoskeleton provides sophisticated mechanisms to control the spatiotemporal output and the intensity of signaling events. Specific examples emphasize these emerging contributions of the cytoskeleton to cell physiology. In our opinion, further exploration of these pathways will uncover new concepts of cellular communication that originate from the cytoskeleton.

Epithelial keratins: Biology and implications as diagnostic markers for liquid biopsies

Keratins are essential elements of the cytoskeleton of normal and malignant epithelial cells. Because carcinomas commonly maintain their specific keratin expression pattern during malignant transformation, keratins are extensively used as tumor markers in cancer diagnosis including the detection of circulating tumor cells in blood of carcinoma patients. Interestingly, recent biological insights demonstrate that epithelial keratins should not only be considered as mere tumor markers. Emerging evidence suggests an active biological role of keratins in tumor cell dissemination and metastasis. In this review, we illustrate the family of keratin proteins, summarize the latest biological insights into keratin function related to cancer metastasis and discuss the current use of keratins for detection of CTCs and other blood biomarkers used in oncology.

Src and SHP2 coordinately regulate the dynamics and organization of vimentin filaments during cell migration

Vimentin intermediate filaments (VIFs), expressed in most mesenchymal and cancer cells, undergo dramatic reorganization during cell migration; however, the mechanism remains obscure. This study demonstrates that upon growth-factor stimulation, Src directly phosphorylates vimentin at Tyr117, leading to VIF disassembly into squiggles and particles at the cell edge during lamellipodia formation. The protein tyrosine phosphatase SHP2 counteracted the Src effects on VIF tyrosine phosphorylation and organization. VIFs formed by vimentin Y117D mutant were more soluble and dynamic than those formed by the wild-type and Y117F mutant. Increased expression of vimentin promoted growth-factor induced lamellipodia formation and cell migration, whereas the mutants suppressed both. The vimentin-induced increase in lamellipodia formation correlated with the activation of Rac and Vav2, with the latter associated with VIFs and recruited to the plasma membrane upon growth-factor stimulation. These results reveal a novel mechanism for regulating VIF dynamics through Src and SHP2 and demonstrate that proper VIF dynamics are important for Rac activation and cell migration.

Intermediate filaments and IF-associated proteins: from cell architecture to cell proliferation

Intermediate filaments (IFs), in coordination with microfilaments and microtubules, form the structural framework of the cytoskeleton and nucleus, thereby providing mechanical support against cellular stresses and anchoring intracellular organelles in place. The assembly and disassembly of IFs are mainly regulated by the phosphorylation of IF proteins. These phosphorylation states can be tracked using antibodies raised against phosphopeptides in the target proteins. IFs exert their functions through interactions with not only structural proteins, but also non-structural proteins involved in cell signaling, such as stress responses, apoptosis, and cell proliferation. This review highlights findings related to how IFs regulate cell division through phosphorylation cascades and how trichoplein, a centriolar protein originally identified as a keratin-associated protein, regulates the cell cycle through primary cilium formation.

Vimentin on the move: new developments in cell migration

The vimentin gene ( VIM) encodes one of the 71 human intermediate filament (IF) proteins, which are the building blocks of highly ordered, dynamic, and cell type-specific fiber networks. Vimentin is a multi-functional 466 amino acid protein with a high degree of evolutionary conservation among vertebrates. Vim ^−/− mice, though viable, exhibit systemic defects related to development and wound repair, which may have implications for understanding human disease pathogenesis. Vimentin IFs are required for the plasticity of mesenchymal cells under normal physiological conditions and for the migration of cancer cells that have undergone epithelial–mesenchymal transition. Although it was observed years ago that vimentin promotes cell migration, the molecular mechanisms were not completely understood. Recent advances in microscopic techniques, combined with computational image analysis, have helped illuminate vimentin dynamics and function in migrating cells on a precise scale. This review includes a brief historical account of early studies that unveiled vimentin as a unique component of the cell cytoskeleton followed by an overview of the physiological vimentin functions documented in studies on Vim ^−/− mice. The primary focus of the discussion is on novel mechanisms related to how vimentin coordinates cell migration. The current hypothesis is that vimentin promotes cell migration by integrating mechanical input from the environment and modulating the dynamics of microtubules and the actomyosin network. These new findings undoubtedly will open up multiple avenues to study the broader function of vimentin and other IF proteins in cell biology and will lead to critical insights into the relevance of different vimentin levels for the invasive behaviors of metastatic cancer cells.

Alexander disease

Alexander disease is a rare and generally fatal disorder of the central nervous system, originally defined by the distinctive neuropathology consisting of abundant Rosenthal fibers within the cytoplasm and processes of astrocytes. More recently, mutations in GFAP, encoding glial fibrillary acidic protein, the major intermediate filament protein of astrocytes, have been identified in nearly all patients. No other genetic causes have yet been identified. The precise mechanisms by which mutations lead to disease are poorly understood. Despite the genetic homogeneity, there are a wide range of clinical phenotypes. The genetic issues and the approach to diagnosis are the prime consideration in this chapter.

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

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

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.

Functional Implications of O-GlcNAcylation-dependent Phosphorylation at a Proximal Site on Keratin 18

Keratins 8/18 (K8/18) are phosphoglycoproteins and form the major intermediate filament network of simple epithelia. The three O-GlcNAcylation (Ser(29), Ser(30), and Ser(48)) and two phosphorylation (Ser(33) and Ser(52)) serine sites on K18 are well characterized. Both of these modifications have been reported to increase K18 solubility and regulate its filament organization. In this report, we investigated the site-specific interplay between these two modifications in regulating the functional properties of K18, like solubility, stability, and filament organization. An immortalized hepatocyte cell line (HHL-17) stably expressing site-specific single, double, and triple O-GlcNAc and phosphomutants of K18 were used to identify the site(s) critical for regulating these functions. Keratin 18 mutants where O-GlcNAcylation at Ser(30) was abolished (K18-S30A) exhibited reduced phosphorylation induced solubility, increased stability, defective filament architecture, and slower migration. Interestingly, K18-S30A mutants also showed loss of phosphorylation at Ser(33), a modification known to regulate the solubility of K18. Further to this, the K18 phosphomutant (K18-S33A) mimicked K18-S30A in its stability, filament organization, and cell migration. These results indicate that O-GlcNAcylation at Ser(30) promotes phosphorylation at Ser(33) to regulate the functional properties of K18 and also impact cellular processes like migration. O-GlcNAcylation and phosphorylation on the same or adjacent sites on most proteins antagonize each other in regulating protein functions. Here we report a novel, positive interplay between O-GlcNAcylation and phosphorylation at adjacent sites on K18 to regulate its fundamental properties.

Assays for Posttranslational Modifications of Intermediate Filament Proteins

Intermediate filament (IF) proteins are known to be regulated by a number of posttranslational modifications (PTMs). Phosphorylation is the best-studied IF PTM, whereas ubiquitination, sumoylation, acetylation, glycosylation, ADP-ribosylation, farnesylation, and transamidation are less understood in functional terms but are known to regulate specific IFs under various contexts. The number and diversity of IF PTMs is certain to grow along with rapid advances in proteomic technologies. Therefore, the need for a greater understanding of the implications of PTMs to the structure, organization, and function of the IF cytoskeleton has become more apparent with the increased availability of data from global profiling studies of normal and diseased specimens. This chapter will provide information on established methods for the isolation and monitoring of IF PTMs along with the key reagents that are necessary to carry out these experiments.

Phosphorylation and Reorganization of Keratin Networks: Implications for Carcinogenesis and Epithelial Mesenchymal Transition

Metastasis is one of hallmarks of cancer and a major cause of cancer death. Combatting metastasis is highly challenging. To overcome these difficulties, researchers have focused on physical properties of metastatic cancer cells. Metastatic cancer cells from patients are softer than benign cancer or normal cells. Changes of viscoelasticity of cancer cells are related to the keratin network. Unexpectedly, keratin network is dynamic and regulation of keratin network is important to the metastasis of cancer. Keratin is composed of heteropolymer of type I and II. Keratin connects from the plasma membrane to nucleus. Several proteins including kinases, and protein phosphatases bind to keratin intermediate filaments. Several endogenous compounds or toxic compounds induce phosphorylation and reorganization of keratin network in cancer cells, leading to increased migration. Continuous phosphorylation of keratin results in loss of keratin, which is one of the features of epithelial mesenchymal transition (EMT). Therefore, several proteins involved in phosphorylation and reorganization of keratin also have a role in EMT. It is likely that compounds controlling phosphorylation and reorganization of keratin are potential candidates for combating EMT and metastasis.

Regulation of keratin network organization

Keratins form the major intermediate filament cytoskeleton of epithelia and are assembled from heterodimers of 28 type I and 26 type II keratins in cell- and differentiation-dependent patterns. By virtue of their primary sequence composition, interactions with cell adhesion complexes and components of major signaling cascades, keratins act as targets and effectors of mechanical force and chemical signals to determine cell mechanics, epithelial cohesion and modulate signaling in keratin isotype-specific manners. Therefore, cell-specific keratin expression and organization impact on cell growth, migration and invasion. Here, we review the recent literature, focusing on the question how keratin networks are regulated and how the interplay of keratins with adhesion complexes affects these processes and provides a framework to understand keratins contribution to blistering and inflammatory disorders and to tumor metastasis.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Post-translational modifications of intermediate filament proteins: mechanisms and functions

Intermediate filaments (IFs) are cytoskeletal and nucleoskeletal structures that provide mechanical and stress-coping resilience to cells, contribute to subcellular and tissue-specific biological functions, and facilitate intracellular communication. IFs, including nuclear lamins and those in the cytoplasm (keratins, vimentin, desmin, neurofilaments and glial fibrillary acidic protein, among others), are functionally regulated by post-translational modifications (PTMs). Proteomic advances highlight the enormous complexity and regulatory potential of IF protein PTMs, which include phosphorylation, glycosylation, sumoylation, acetylation and prenylation, with novel modifications becoming increasingly appreciated. Future studies will need to characterize their on-off mechanisms, crosstalk and utility as biomarkers and targets for diseases involving the IF cytoskeleton.