Intermediate filaments and stress

Adaptive Müller cell responses to microglial activation mediate neuroprotection and coordinate inflammation in the retina

Purpose

Microglia and Müller cells are prominent participants in retinal responses to injury and disease that shape eventual tissue adaptation or damage. This investigation examined how microglia and Müller cells interact with each other following initial microglial activation.

Methods

Mouse Müller cells were cultured alone, or co-cultured with activated or unactivated retinal microglia, and their morphological, molecular, and functional responses were evaluated. Müller cell-feedback signaling to microglia was studied using Müller cell-conditioned media. Corroborative in vivo analyses of retinal microglia-Müller cell interactions in the mouse retina were also performed.

Results

Our results demonstrate that Müller cells exposed to activated microglia, relative to those cultured alone or with unactivated microglia, exhibit marked alterations in cell morphology and gene expression that differed from those seen in chronic gliosis. These Müller cells demonstrated in vitro (1) an upregulation of growth factors such as GDNF and LIF, and provide neuroprotection to photoreceptor cells, (2) increased pro-inflammatory factor production, which in turn increased microglial activation in a positive feedback loop, and (3) upregulated chemokine and adhesion protein expression, which allowed Müller cells to attract and adhere to microglia. In vivo activation of microglia by intravitreal injection of lipopolysaccharide (LPS) also induced increased Müller cell-microglia adhesion, indicating that activated microglia may translocate intraretinally in a radial direction using Müller cell processes as an adhesive scaffold.

Conclusion

Our findings demonstrate that activated microglia are able to influence Müller cells directly, and initiate a program of bidirectional microglia-Müller cell signaling that can mediate adaptive responses within the retina following injury. In the acute aftermath following initial microglia activation, Müller cell responses may serve to augment initial inflammatory responses across retinal lamina and to guide the intraretinal mobilization of migratory microglia using chemotactic cues and adhesive cell contacts. Understanding adaptive microglia-Müller cell interactions in injury responses can help discover therapeutic cellular targets for intervention in retinal disease.

Epithelial and stromal developmental patterns in a novel substitute of the human skin generated with fibrin-agarose biomaterials

Development of human skin substitutes by tissue engineering may offer new therapeutic alternatives to the use of autologous tissue grafts. For that reason, it is necessary to investigate and develop new biocompatible biomaterials that support the generation of a proper human skin construct. In this study, we generated a novel model of bioengineered human skin substitute using human cells obtained from skin biopsies and fibrin-agarose biomaterials and we evaluated this model both at the ex vivo and the in vivo levels. Once the dermal fibroblasts and the epithelial keratinocytes were isolated and expanded in culture, we used fibrin-agarose scaffolds for the development of a full-thickness human skin construct, which was evaluated after 1, 2, 3 and 4 weeks of development ex vivo. The skin substitutes were then grafted onto immune-deficient nude mice and analyzed at days 10, 20, 30 and 40 postimplantation using transmission electron microscopy, histochemistry and immunofluorescence. The results demonstrated that the fibrin-agarose artificial skin had adequate biocompatibility and proper biomechanical properties. A proper development of both the bioengineered dermis and epidermis was found after 30 days in vivo, although the tissues kept ex vivo and those implanted in the animal model for 10 or 20 days showed lower levels of differentiation. In summary, our model of fibrin-agarose skin equivalent was able to reproduce the structure and histological architecture of the native human skin, especially after long-term in vivo implantation, suggesting that these tissues could reproduce the native skin.

Immunohistochemical distribution of desmin in the human fetal heart

Desmin is a member of the intermediate filaments, which play crucial roles in the maturation, maintenance and recovery of muscle fibers. Its expression has been examined in human cardiac muscle, rat and chicken, but its spatial distribution in the human fetal heart has not been described. The present study investigated desmin expression in the human fetal heart and associated great vessels in 14 mid-term fetuses from 9 to 18 weeks of gestation. Immunoreactivity for myosin heavy chain (MHC) and alpha smooth muscle actin (α-SMA), as well as neuron-specific enolase (NSE), was also examined. Increased expression of desmin from 9 to 18 weeks was clearly localized in the atrial wall, the proximal portions of the pulmonary vein and vena cava, and around the atrioventricular node. Desmin-positive structures were also positive for MHC. Meanwhile, the great vessels were also positive for α-SMA. The distribution of desmin exhibited a pattern quite different from that described in previous studies of rat and chicken. Thus, desmin in the human fetal heart does not seem to play a general role in myocardial differentiation but rather a specific role closely related to the maturation of the α-isozyme of MHC. Desmin expression in the developing fetal heart also appeared to be induced by mechanical stress due to the involvement of venous walls against the atrium.

Regulation of keratin expression by retinoids

Vitamin A and its natural and synthetic metabolites (retinoids) affect growth and differentiation of human skin and among the genes affected by retinoids in epidermis are keratin genes. Keratins are intermediate filament proteins that have essential functions in maintaining the structural integrity of epidermis and its appendages. Their expressions are under strict control to produce keratins that are optimally adapted to their environment. In this article, retinoid regulation of keratin expression in cultured human epidermal keratinocytes and in human skin in vivo will be reviewed. The direct and indirect mechanisms involved will be discussed and novel therapeutic strategies will be proposed for utilizing retinoids in skin disorders due to keratin mutations (e.g., epidermolysis bullosa simplex and epidermolytic ichthyosis).

Nestin as a regulator of Cdk5 in differentiating myoblasts

Many types of progenitor cells are distinguished by the expression of the intermediate filament protein nestin, a frequently used stem cell marker, the physiological roles of which are still unknown. Whereas myogenesis is characterized by dynamically regulated nestin levels, we studied how altering nestin levels affects myoblast differentiation. Nestin determined both the onset and pace of differentiation. Whereas depletion of nestin by RNAi strikingly accelerated the process, overexpression of nestin completely inhibited differentiation. Nestin down-regulation augmented the early stages of differentiation, at the level of cell-cycle withdrawal and expression of myogenic markers, but did not affect proliferation of undifferentiated dividing myoblasts. Nestin regulated the cleavage of the Cdk5 activator protein p35 to its degradation-resistant form, p25. In this way, nestin has the capacity to halt myoblast differentiation by inhibiting sustained activation of Cdk5 by p25, which is critical for the progress of differentiation. Our results imply that nestin regulates the early stages of myogenesis rather than maintains the undifferentiated state of progenitor cells. In the bidirectional interrelationship between nestin and Cdk5, Cdk5 regulates the organization and stability of its own nestin scaffold, which in turn controls the effects of Cdk5. This nestin-Cdk5 cross-talk sets the pace of muscle differentiation.

Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis

The digastricus and omohyoideus muscles are known to carry two muscle bellies connected by an intermediate tendon. However, according to our histological observations of 22 fetuses (7-20 weeks of gestation), the mode of formation of the intermediate tendon was critically different between these two muscles. At 7-9 weeks, the posterior belly of the digastricus carried a definite intramuscular tendon continuous with a long descending tendon. The stylohyoideus, external carotid artery and hypoglossal nerve appeared to impede attachment of the tendon to the Reichert or hyoid cartilage. The digastricus anterior belly did not contain any intramuscular tendon, but desmin-positive muscle fibers consistently surrounded a bulb-like mesenchymal condensation at the caudal free end of the digastricus posterior tendon. Thus, most parts of the digastricus tendon were apparently derived from the posterior belly. In contrast, the omohyoideus always possessed a single long muscle belly until 15 weeks. The intermediate tendon first appeared at 15 weeks as a short plate-like connective structure along the medial margin of the muscle. Vimentin immunoreactivity suggested the presence of mechanical stress along the plate-like tissue, possibly due to bending of the long muscle. Muscle fibers were replaced by collagen fibers to form an intermediate tendon by 20 weeks.

Trichoplein controls microtubule anchoring at the centrosome by binding to Odf2 and ninein

The keratin cytoskeleton performs several functions in epithelial cells and provides regulated interaction sites for scaffold proteins, including trichoplein. Previously, we found that trichoplein was localized on keratin intermediate filaments and desmosomes in well-differentiated, non-dividing epithelia. Here, we report that trichoplein is widely expressed and has a major function in the correct localization of the centrosomal protein ninein in epithelial and non-epithelial cells. Immunocytochemical analysis also revealed that this protein is concentrated at the subdistal to medial zone of both mother and daughter centrioles. Trichoplein binds the centrosomal proteins Odf2 and ninein, which are localized at the distal to subdistal ends of the mother centriole. Trichoplein depletion abolished the recruitment of ninein, but not Odf2, specifically at the subdistal end. However, Odf2 depletion inhibited the recruitment of trichoplein to a mother centriole, whereas ninein depletion did not. In addition, the depletion of each molecule impaired MT anchoring at the centrosome. These results suggest that trichoplein has a crucial role in MT-anchoring activity at the centrosome in proliferating cells, probably through its complex formation with Odf2 and ninein.

Modulus-driven differentiation of marrow stromal cells in 3D scaffolds that is independent of myosin-based cytoskeletal tension

Proliferation and differentiation of cells are known to be influenced by the physical properties of the extracellular environment. Previous studies examining biophysics underlying cell response to matrix stiffness utilized a two-dimensional (2D) culture format, which is not representative of the three-dimensional (3D) tissue environment in vivo. We report on the effect of 3D matrix modulus on human bone marrow stromal cell (hBMSC) differentiation. hBMSCs underwent osteogenic differentiation in poly(ethylene glycol) hydrogels of all modulus (300-fold modulus range, from 0.2 kPa to 59 kPa) in the absence of osteogenic differentiation supplements. This osteogenic differentiation was modulus-dependent and was enhanced in stiffer gels. Osteogenesis in these matrices required integrin-protein ligation since osteogenesis was inhibited by soluble Arginine-Glycine-Aspartate-Serine peptide, which blocks integrin receptors. Immunostained images revealed lack of well-defined actin filaments and microtubules in the encapsulated cells. Disruption of mechanosensing elements downstream of integrin binding that have been identified from 2D culture such as actin filaments, myosin II contraction, and RhoA kinase did not abrogate hBMSC material-driven osteogenic differentiation in 3D. These data show that increased hydrogel modulus enhanced osteogenic differentiation of hBMSCs in 3D scaffolds but that hBMSCs did not use the same mechanosensing pathways that have been identified in 2D culture.

E1--E4-mediated keratin phosphorylation and ubiquitylation: a mechanism for keratin depletion in HPV16-infected epithelium

The keratin IF network of epidermal keratinocytes provides a protective barrier against mechanical insult, it is also a major player in absorbing stress in these cells. The human papilloma virus (HPV) type 16 E1--E4 protein accumulates in the upper layers of HPV16-infected epithelium and is known to associate with and reorganise the keratin IF network in cells in culture. Here, we show that this function is conserved amongst a number of HPV alpha-group E1--E4 proteins and that the differentiation-dependent keratins are also targeted. Using time-lapse microscopy, HPV16 E1--E4 was found to effect a dramatic cessation of keratin IF network dynamics by associating with both soluble and insoluble keratin. Network disruption was accompanied by keratin hyperphosphorylation at several sites, including K8 S73, which is typically phosphorylated in response to stress stimuli. Keratin immunoprecipitated from E1--E4-expressing cells was also found to be ubiquitylated, indicating that it is targeted for proteasomal degradation. Interestingly, the accumulation of hyperphosphorylated, ubiquitylated E1--E4-keratin structures was found to result in an impairment of proteasomal function. These observations shed new light on the mechanism of keratin IF network reorganisation mediated by HPV16 E1--E4 and provide an insight into the depletion of keratin co-incident with E1--E4 accumulation observed in HPV-infected epithelium.

Novel keratins identified by quantitative proteomic analysis as the major cytoskeletal proteins of equine (Equus caballus) hoof lamellar tissue

The dermo-epidermal interface that connects the equine distal phalanx to the cornified hoof wall withstands great biomechanical demands, but is also a region where structural failure often ensues as a result of laminitis. The cytoskeleton in this region maintains cell structure and facilitates intercellular adhesion, making it likely to be involved in laminitis pathogenesis, although it is poorly characterized in the equine hoof lamellae. The objective of the present study was to identify and quantify the cytoskeletal proteins present in the epidermal and dermal lamellae of the equine hoof by proteomic techniques. Protein was extracted from the mid-dorsal epidermal and dermal lamellae from the front feet of 5 Standardbred geldings and 1 Thoroughbred stallion. Mass spectrometry-based spectral counting techniques, PAGE, and immunoblotting were used to identify and quantify cytoskeletal proteins, and indirect immunofluorescence was used for cellular localization of K14 and K124 (where K refers to keratin). Proteins identified by spectral counting analysis included 3 actin microfilament proteins; 30 keratin proteins along with vimentin, desmin, peripherin, internexin, and 2 lamin intermediate filament proteins; and 6 tubulin microtubule proteins. Two novel keratins, K42 and K124, were identified as the most abundant cytoskeletal proteins (22.0 ± 3.2% and 23.3 ± 4.2% of cytoskeletal proteins, respectively) in equine hoof lamellae. Immunoreactivity to K14 was localized to the basal cell layer, and that to K124 was localized to basal and suprabasal cells in the secondary epidermal lamellae. Abundant proteins K124, K42, K14, K5, and α(1)-actin were identified on 1- and 2-dimensional polyacrylamide gels and aligned with the results of previous studies. Results of the present study provide the first comprehensive analysis of cytoskeletal proteins present in the equine lamellae by using mass spectrometry-based techniques for protein quantification and identification.

The keratin-filament cycle of assembly and disassembly

Continuous and regulated remodelling of the cytoskeleton is crucial for many basic cell functions. In contrast to actin filaments and microtubules, it is not understood how this is accomplished for the third major cytoskeletal filament system, which consists of intermediate-filament polypeptides. Using time-lapse fluorescence microscopy of living interphase cells, in combination with photobleaching, photoactivation and quantitative fluorescence measurements, we observed that epithelial keratin intermediate filaments constantly release non-filamentous subunits, which are reused in the cell periphery for filament assembly. This cycle is independent of protein biosynthesis. The different stages of the cycle occur in defined cellular subdomains: assembly takes place in the cell periphery and newly formed filaments are constantly transported toward the perinuclear region while disassembly occurs, giving rise to diffusible subunits for another round of peripheral assembly. Remaining juxtanuclear filaments stabilize and encage the nucleus. Our data suggest that the keratin-filament cycle of assembly and disassembly is a major mechanism of intermediate-filament network plasticity, allowing rapid adaptation to specific requirements, notably in migrating cells.

Vimentin is a novel anti-cancer therapeutic target; insights from in vitro and in vivo mice xenograft studies

BACKGROUND

Vimentin is a ubiquitous mesenchymal intermediate filament supporting mechano-structural integrity of quiescent cells while participating in adhesion, migration, survival, and cell signaling processes via dynamic assembly/disassembly in activated cells. Soft tissue sarcomas and some epithelial cancers exhibiting "epithelial to mesenchymal transition" phenotypes express vimentin. Withaferin-A, a naturally derived bioactive compound, may molecularly target vimentin, so we sought to evaluate its effects on tumor growth in vitro and in vivo thereby elucidating the role of vimentin in drug-induced responses.

METHODS AND FINDINGS

Withaferin-A elicited marked apoptosis and vimentin cleavage in vimentin-expressing tumor cells but significantly less in normal mesenchymal cells. This proapoptotic response was abrogated after vimentin knockdown or by blockade of caspase-induced vimentin degradation via caspase inhibitors or overexpression of mutated caspase-resistant vimentin. Pronounced anti-angiogenic effects of Withaferin-A were demonstrated, with only minimal effects seen in non-proliferating endothelial cells. Moreover, Withaferin-A significantly blocked soft tissue sarcoma growth, local recurrence, and metastasis in a panel of soft tissue sarcoma xenograft experiments. Apoptosis, decreased angiogenesis, and vimentin degradation were all seen in Withaferin-A treated specimens.

CONCLUSIONS

In light of these findings, evaluation of Withaferin-A, its analogs, or other anti-vimentin therapeutic approaches in soft tissue sarcoma and "epithelial to mesenchymal transition" clinical contexts is warranted.

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.

Intermediate filament carbonylation during acute acrolein toxicity in A549 lung cells: functional consequences, chaperone redistribution, and protection by bisulfite

Extensive protein carbonylation accompanies cellular exposure to acrolein, a ubiquitous smoke constituent implicated in life-threatening pulmonary edema in fire victims, a condition involving rapid erosion of the "watertight" properties of respiratory epithelium. Since the identities of lung epithelial proteins that sustain carbonylation by acrolein are unknown, we sought to identify significant targets in subcellular fractions from A549 cells after 30 min exposure to either subtoxic or acutely toxic acrolein concentrations (60 or 360 fmol acrolein/cell). The lower concentration mainly modified cytosolic proteins while the higher concentration also damaged nuclear, membrane, and cytoskeletal proteins. The multifunctional intermediate filament proteins vimentin, keratin-18, keratin-7 and keratin-8, were conspicuous targets. Consistent with their mechanical functions, a loss of cellular adhesive strength accompanied adduction of the two most abundant intermediate filaments in A549 cells, keratins-8 and -18. Acrolein also elicited redistribution of several chaperones (Hsp40, -70, -90, and -110) to intermediate filament fractions, suggesting chaperone-mediated autophagy contributes to the triage of acrolein-adducted proteins. The carbonyl scavenger bisulfite suppressed acrolein toxicity, intermediate filament adduction, vimentin cross-linking, Hsp90 redistribution, and loss of cellular adhesive strength, while also suppressing vimentin hyperphosphorylation. These novel observations identify intermediate filaments as key targets for the reactive smoke constituent acrolein.

Intermediate filaments take the heat as stress proteins

Intermediate filament (IF) proteins and heat shock proteins (HSPs) are large multimember families that share several features, including protein abundance, significant upregulation in response to a variety of stresses, cytoprotective functions, and the phenocopying of several human diseases after IF protein or HSP mutation. We are now coming to understand that these common elements point to IFs as important cellular stress proteins with some roles akin to those already well-characterized for HSPs. Unique functional roles for IFs include protection from mechanical stress, whereas HSPs are characteristically involved in protein folding and as chaperones. Shared IF and HSP cytoprotective roles include inhibition of apoptosis, organelle homeostasis, and scaffolding. In this report, we review data that corroborate the view that IFs function as highly specialized cytoskeletal stress proteins that promote cellular organization and homeostasis.

Fluorescence-activated cell sorting of PCK-26 antigen-positive cells enables selection of ovine esophageal epithelial cells with improved viability on scaffolds for esophagus tissue engineering

OBJECTIVE

For esophagus tissue engineering, isolation and proliferation of esophageal epithelial cells (EEC) is a pre-requisite for scaffold seeding to create constructs. The aim of this study was to sort EEC expressing cytokeratin markers and their proliferative subpopulations; also, to investigate the viability of differentiated EEC subpopulations on collagen scaffolds.

METHODS

Ovine esophageal epithelial cells (OEECs) from sheep esophagus were analyzed using flow cytometry for pan cytokeratin (PCK-26) and proliferation cell nuclear antigen (PCNA). Using fluorescent-activated cell sorting, OEEC were separated and analyzed for PCNA expression. The OEEC subpopulations were seeded on collagen scaffolds for a week in vitro culture.

RESULTS

Proliferation cell nuclear antigen was expressed in >45% of OEEC isolated. In flow cytometry, 30% OEEC were PCK-26 positive which exhibited a high-proliferative capacity of 80%. PCK-26-negative OECC exhibited a low-proliferative capability of 13%. Scanning electron microscopy demonstrated organized attachment and uniform scaffold coverage in PCK-26-positive cells.

CONCLUSION

Ovine esophageal epithelial cells can be divided into PCK-26-positive and negative subpopulations. PCK-26-positive OEEC constitute one-third of the isolated cells with high-proliferative capability. Seeding of PCK-26-positive OEEC on collagen scaffolds leads to uniform distribution of cells in vitro. In esophagus, tissue engineering PCK-26-positive OEEC subpopulation is important for optimal construct generation.

Intermediate filaments in Caenorhabditis elegans

Intermediate filaments (IFs) make up one of the three major fibrous cytoskeletal systems in metazoans. Numerous IF polypeptides are synthesized in cell type-specific combinations suggesting specialized functions. The review concentrates on IFs in the model organism Caenorhabditis elegans which carries great promise to elucidate the still unresolved mechanisms of IF assembly into complex networks and to determine IF function in a living organism. In contrast to Drosophila melanogaster, which lacks cytoplasmic IFs altogether, the nematode genome contains 11 genes coding for cytoplasmic IFs and only a single gene for a nuclear lamin. Its cytoplasmic IFs are expressed in developmentally and spatially defined patterns. As an example we present the case of the intestinal IFs which are abundant in the mechanically resilient endotube, a prominent feature of the C. elegans intestinal terminal web region. This IF-rich structure brings together all three cytoskeletal filaments that are integrated into a coherent entity by the C. elegans apical junction (CeAJ) thereby completely surrounding and stabilizing the intestinal lumen with its characteristic brush border. Concepts on the developmental establishment of the endotube in relation to polarization and its function for maintenance of epithelial integrity are discussed. Furthermore, possible connections of the cytoplasmic cytoskeleton to the nuclear lamin IFs and the importance of these links for nuclear positioning are summarized.

Low concentration of GA activates a preconditioning response in HepG2 cells during oxidative stress-roles of Hsp90 and vimentin

Oxidative stress can be a significant cause of cell death and apoptosis. We performed studies in HepG2 cells to explore whether prior exposure to oxidative stress ("oxidative preconditioning") and geldanamycin (GA) treatment can protect the cell from damage caused by subsequent oxidative insults. The cells were treated with 10 nM GA for 24 h before oxidative stress. Oxidative preconditioning was achieved by 2 h exposures to H(2)O(2) (50 microM) separated by a 10-h recovery period in normal culture medium. Oxidative stress was induced by exposure to 500 microM H(2)O(2) for 24 h. The effects of GA and oxidative preconditioning were investigated on the formation of Hsp90, vimentin, insoluble vimentin aggregates, and cleavage of vimentin in a cell culture model of oxidative stress. GA treatment leads to enhanced expression of Hsp90 and vimentin and to inhibition of vimentin protein aggregation. Similar results were obtained by oxidative preconditioning. It is confirmed that low concentrations of GA protected HepG2 cells from subsequent oxidative stress by increasing the levels of Hsp90 and by alleviating the extent of cell apoptosis induced by oxidative stress, which is similar to oxidative preconditioning. However, in contrast to preconditioning, GA treatment obviously changed binding activity of Hsp90 to vimentin cleavages. All the above indicated that low concentrations of GA treatment triggered cell protection from oxidative stress. Both the level of Hsp90 and its ability to bind with vimentin were changed by low concentrations of GA and might contribute to oxidative stress protection.

The identification and localization of two intermediate filament proteins in the tunic of Styela plicata (Tunicata, Styelidae)

The intermediate filament (IF) proteins Styela C and Styela D from the tunicate Styela (Urochordata) are co-expressed in all epidermal cells and they are thought to behave as type I and type II keratins. These two IF proteins, Styela C and Styela D, were identified in immunoblots of proteins isolated from the tunic of Styela plicata. The occurrence and distribution of these proteins within the tunic of this ascidian was examined by means of immunofluorescence and immunoperoxidase techniques, using anti-Styela C and anti-Styela D antibodies. In addition, immuno-electron microscopy of the tunic showed that the two proteins are located in the cuticle layer and in the tunic matrix. These results represent the first data about the presence of IF proteins in the tunic of adult ascidian S. plicata. The possible involvement of these IF proteins in reinforcing the integrity of the tunic, that represents the interface between the animal body and the external environment, is discussed.

Periplakin interactions with lens intermediate and beaded filaments

PURPOSE

The lens assembles two systems of intermediated filaments-vimentin intermediate filament (IF) and highly divergent, lens-specific beaded filament (BF)-sequentially as epithelial cells differentiate into fiber cells. The goal of this study was to identify linker proteins that integrate the different lens IF into the biology of the lens fiber cells.

METHODS

Antibodies to periplakin were used in coimmunoprecipitation studies to identify proteins that complex with BF and IF in detergent extracts of mouse lens. GST-periplakin fusion proteins were used to confirm coimmunoprecipitation

RESULTS

Yeast two-hybrid analysis was used to establish direct linkage between periplakin and BF/IF proteins and to narrow down binding domains. Immunocytochemistry was used to establish spatial and temporal coexpression of periplakin and BF/IF. results. Periplakin is found complexed to BF and IF in the lens. The COOH terminus of periplakin was shown to have a strong affinity for the CP49 rod 2 domain but not its head or rod 1 domains. Low-level affinity was seen between the filensin rod domain and periplakin. Periplakin localization in lens overlapped with BF and IF.

CONCLUSIONS

Despite divergence in primary sequence, predicted secondary structure, and filament structure, CP49 has conserved the capacity to bind a common IF linker protein, periplakin, and shares that binding capacity with the other major lens IF protein, vimentin. This suggests that mutations in periplakin have the potential to emulate the cataract seen in lenses with defective BF proteins.

Intermediate filament-like proteins in bacteria and a cytoskeletal function in Streptomyces

Actin and tubulin cytoskeletons are conserved and widespread in bacteria. A strikingly intermediate filament (IF)-like cytoskeleton, composed of crescentin, is also present in Caulobacter crescentus and determines its specific cell shape. However, the broader significance of this finding remained obscure, because crescentin appeared to be unique to Caulobacter. Here we demonstrate that IF-like function is probably a more widespread phenomenon in bacteria. First, we show that 21 genomes of 26 phylogenetically diverse species encoded uncharacterized proteins with a central segmented coiled coil rod domain, which we regarded as a key structural feature of IF proteins and crescentin. Experimental studies of three in silico predicted candidates from Mycobacterium and other actinomycetes revealed a common IF-like property to spontaneously assemble into filaments in vitro. Furthermore, the IF-like protein FilP formed cytoskeletal structures in the model actinomycete Streptomyces coelicolor and was needed for normal growth and morphogenesis. Atomic force microscopy of living cells revealed that the FilP cytoskeleton contributed to mechanical fitness of the hyphae, thus closely resembling the function of metazoan IF. Together, the bioinformatic and experimental data suggest that an IF-like protein architecture is a versatile design that is generally present in bacteria and utilized to perform diverse cytoskeletal tasks.

Keratin contribution to cellular mechanical stress response at focal adhesions as assayed by laser tweezers

The ability of adherent cells to sense and adapt to a mechanical stress generated at focal adhesions (FAs) largely occurs through the integrin-mediated interaction between the cytoskeleton, namely actin microfilaments, and extracellular matrix elements, like fibronectin. Here we assessed the contribution of keratin 8 and 18 (K8/K18) intermediate filaments (IFs) in simple epithelial cells in response to a mechanical stress applied on integrins at FAs. To this end, we used monolayer cultures of K8-knockdown H4-II-E-C3 (shK8b1) rat hepatoma cells and their K8/K18-containing counterparts (H4ev). The stress was generated with a laser tweezers mediated force applied on a fibronectin-coated polystyrene bead attached to integrins α5/β1 forming FAs. Measurement of the bead displacement allowed assessment of the viscoelastic response at FAs and the associated surface membrane stiffness. Notably, the loss of K8/K18 IFs in shK8b1 cells revealed an immediate reduction in bead displacements characteristic of a sudden increased in the FA elastic stiffness, incompatible with the K8/K18 IF intrinsic viscoelastic features, but in line with an induced perturbation of the mechanotransduction signals triggered at integrins. In addition, actin microfilament disruption, and to a lesser extent microtubule disruption, led to prominent decreases in the elastic stiffness of FAs, thus identifying actin-MFs and MTs as modulators of the time-dependent FA stiffening in both H4ev cells and shK8b1 cells, in response to mechanical stress. On technical ground, the laser tweezers offer a tool of choice to delineate the K8/K18 IF-mediated modulation of cytoskeletal versus signaling activities at FAs in epithelial cells in response to mechanical stress.

Maintenance of the intestinal tube in Caenorhabditis elegans: the role of the intermediate filament protein IFC-2

The Caenorhabditis elegans intestinal lumen is surrounded by a dense cytoplasmic network that is laterally attached to the junctional complex and is referred to as the endotube. It localizes to the terminal web region which anchors the microvillar actin filament bundles and is particularly rich in intermediate filaments. To examine their role in intestinal morphogenesis and function, C. elegans reporter strains were generated expressing intestine-specific CFP-tagged intermediate filament polypeptide IFB-2. When these animals were treated with dsRNA against intestinal intermediate filament polypeptide IFC-2, the endotube developed multiple bubble-shaped invaginations that protruded into the enterocytic cytoplasm. The irregularly widened lumen remained surrounded by a continuous IFB-2::CFP-labeled layer. Comparable but somewhat mitigated phenotypic changes were also noted in wild-type N2 worms treated with ifc-2 (RNAi). Junctional complexes were ultrastructurally and functionally normal and the apical domain of intestinal cells was also not altered. These observations demonstrate that IFC-2 is important for structural maintenance of the intestinal tube but is not needed for establishment of the endotube and epithelial cell polarity.

Intermediate filaments: versatile building blocks of cell structure

Cytoskeletal intermediate filaments (IF) are organized into a dynamic nanofibrillar complex that extends throughout mammalian cells. This organization is ideally suited to their roles as response elements in the subcellular transduction of mechanical perturbations initiated at cell surfaces. IF also provide a scaffold for other types of signal transduction that together with molecular motors ferries signaling molecules from the cell periphery to the nucleus. Recent insights into their assembly highlight the importance of co-translation of their precursors, the hierarchical organization of their subunits in the formation of unit-length filaments (ULF) and the linkage of ULF into mature apolar IF. Analyses by atomic force microscopy reveal that mature IF are flexible and can be stretched to over 300% of their length without breaking, suggesting that intrafilament subunits can slide past one another when exposed to mechanical stress and strain. IF also play a role in the organization of organelles by modulating their motility and providing anchorage sites within the cytoplasm.