Microinjection of intermediate filament proteins into living cells with and without preexisting intermediate filament network

Human cells grown in monolayer culture were microinjected with intermediate filament subunit proteins. In fibroblasts with a preexisting vimentin network, injected porcine glial fibrillary acidic protein (GFAP) co-localized with the vimentin network within 24 hours. Phosphorylated GFAP variants were found to become dephosphorylated concomitantly with their incorporation into filamentous structures. After microinjection of either porcine GFAP or murine vimentin into human carcinoma cells lacking cytoplasmic intermediate filaments, we observed that different types of filament networks developed. Whereas vimentin was incorporated into short filaments immediately after injection, GFAP was found to aggregate into rodlike structures. This may indicate a differential filament forming ability of these intermediate filament proteins in vivo.     

Identification of an epithelial protein related to the desmosome and intermediate filament network

Using a mAb, referred to as 08L, we have identified a protein, of M(r) approximately 140,000, associated with desmosomes of epithelial cells. The 08L antibody stained the intracellular side of lateral cell margins of monolayer epithelial cells but did not stain cell margins free of cell contact. Immunoelectron microscopy revealed that the 08L antigen was localized to the cytosolic surface of the desmosomal plaque near points of intermediate filament convergence with apparently little staining of the desmosomal plaque proper. Western blots revealed the 08L antigen to be a protein, of M(r) approximately 140,000, found in the Triton-X 100 insoluble pellet. High salt-containing buffers extracted the 08L antigen from the insoluble material. Examination of the assembly of 08L to the desmosome complex, in cells grown in low confluent culture or in calcium-switch assays, by double immunofluorescence with 08L and anti-desmoplakin antibody, revealed that 08L was recruited to morphologically identifiable desmosomes. 08L antigen may exist in a cytosolic pool prior to assembly to the cell surface. The solubility of 08L in low calcium and normal calcium conditions, however, was similar. 08L association to the desmosome was correlated with increased organization of the intermediate filament network. We suggest that the 08L antigen may be involved in the organization and stabilization of the desmosome-IF complexes of epithelia.     

Evidence for an intermediate in tau filament formation

Alzheimer's disease is defined in part by the intraneuronal accumulation of filaments comprised of the microtubule-associated protein tau. In vitro, fibrillization of full-length, unphosphorylated recombinant tau can be induced under near-physiological conditions by treatment with various agents, including anionic surfactants. Here we examine the pathway through which anionic surfactants promote tau fibrillization using a combination of electron microscopy and fluorescence spectroscopy. Protein and surfactant first interacted in solution to form micelles, which then provided negatively charged surfaces that accumulated tau aggregates. Surface aggregation of tau protein was followed by the time-dependent appearance of a thioflavin S reactive intermediate that accumulated over a period of hours. The intermediate was unstable in the absence of anionic surfaces, suggesting it was not filamentous. Fibrillization proceeded after intermediate formation with classic nucleation-dependent kinetics, consisting of lag phase followed by the exponential increase in filament lengths, followed by an equilibrium phase reached in approximately 24 h. The pathway did not require protein insertion into the micelle hydrophobic core or conformational change arising from mixed micelle formation, because anionic microspheres constructed from impermeable polystyrene were capable of qualitatively reproducing all aspects of the fibrillization reaction. It is proposed that the progression from amorphous aggregation through intermediate formation and fibrillization may underlie the activity of other inducers such as hyperphosphorylation and may be operative in vivo.     

The dynamic phosphorylation of the human intermediate filament keratin 1 chain

The major sites of phosphorylation have been determined on the human keratin intermediate filament keratin 1 (type II) chain expressed in terminally differentiating epidermis. A total of nine phosphate sites were found, involving 1 threonine and 8 serine residues, and were localized to end domain sequences. The sites identified corresponded to major sites of phosphorylation as determined by direct quantitation of O-phosphoserine. Since the tissue was cultured with [32 P] orthophosphate only briefly, labeling occurred primarily by turnover, so that information on the dynamics of phosphorylation was also obtained. The degrees and specific activities (that is, turnover rates) of phosphorylation of these sites varied widely between different isoelectric variants (phosphate isomers) of keratin 1 chains and correlated with their locations on the chain: those sites on the more exposed E1 and E2 subdomains were fully phosphorylated and turning over at high rates, while a site near the end of the rod domain in a presumably more confined location was only slightly phosphorylated and turning over at low rate. The nature of the sequences around the phosphorylated residues indicates that cAMP-dependent and probably other protein kinase activities operate simultaneously in intact normal epidermal tissue. The correlation between the degrees and rates of turnover of phosphorylation with the locations on the chain may have an important bearing on the functional role of phosphorylation of the keratin intermediate filaments in this tissue.     

The intermediate filament network in cultured human keratinocytes is remarkably extensible and resilient

The prevailing model of the mechanical function of intermediate filaments in cells assumes that these 10 nm diameter filaments make up networks that behave as entropic gels, with individual intermediate filaments never experiencing direct loading in tension. However, recent work has shown that single intermediate filaments and bundles are remarkably extensible and elastic in vitro, and therefore well-suited to bearing tensional loads. Here we tested the hypothesis that the intermediate filament network in keratinocytes is extensible and elastic as predicted by the available in vitro data. To do this, we monitored the morphology of fluorescently-tagged intermediate filament networks in cultured human keratinocytes as they were subjected to uniaxial cell strains as high as 133%. We found that keratinocytes not only survived these high strains, but their intermediate filament networks sustained only minor damage at cell strains as high as 100%. Electron microscopy of stretched cells suggests that intermediate filaments are straightened at high cell strains, and therefore likely to be loaded in tension. Furthermore, the buckling behavior of intermediate filament bundles in cells after stretching is consistent with the emerging view that intermediate filaments are far less stiff than the two other major cytoskeletal components F-actin and microtubules. These insights into the mechanical behavior of keratinocytes and the cytokeratin network provide important baseline information for current attempts to understand the biophysical basis of genetic diseases caused by mutations in intermediate filament genes.     

Molecular mechanism of intermediate filament recognition by plakin proteins

The plakin family of cytolinkers interacts with intermediate filaments (IFs) through plakin repeat domain (PRD) and linker modules. Recent structure/function studies have established the molecular basis of envoplakin-PRD and periplakin-linker interactions with vimentin. Both plakin modules share a broad basic groove which recognizes acidic rod elements on IFs, a mechanism that is applicable to other plakin family members. This review postulates a universal IF engagement mechanism that illuminates the specific effects of pathogenic mutations associated with diseases including arrhythmogenic right ventricular cardiomyopathy, and reveals how diverse plakin proteins offer tailored IF tethering to ensure stable, dynamic and regulated cellular structures.     

Isolation of the intermediate filament protein vimentin by chromatofocusing

A novel, simple and relatively rapid method is described for the isolation of the intermediate-sized filament protein vimentin from eye lens tissue. Chromatofocusing is applied as the sole purification step. The apparent isoelectric point of the protein in 6 M urea and at 22°C is 4.9. Electrophoretic mobility on one- and two-dimensional polyacrylamide gels, solubility in 6 M urea and amino acid composition were used for identification     

Assembly and exchange of intermediate filament proteins of neurons: neurofilaments are dynamic structures

We have explored the dynamics of intermediate filament assembly and subunit exchange using fluorescently labeled neurofilament proteins and a fluorescence resonance energy transfer assay. Neurofilaments (NFs) are assembled from three highly phosphorylated proteins with molecular masses of 180 (NF-H), 130 (NF-M), and 66 kD (NF-L) of which NF-L forms the structural core. The core component, NF-L, was stoichiometrically labeled at cysteine 321 with fluorescein, coumarin, or biotin-maleimide to produce assembly-competent fluorescent or biotinylated derivatives, respectively. Using coumarin-labeled NF-L as fluorescence donor and fluorescein-labeled NF-L as the fluorescence acceptor, assembly of NF filaments was induced by rapidly raising the NaCl concentration to 170 mM, and the kinetics was followed by the decrease in the donor fluorescence. Assembly of NF-L subunits into filaments does not require nucleotide binding or hydrolysis but is strongly dependent on ionic strength, pH, and temperature. The critical concentration of NF-L, that concentration that remains unassembled at equilibrium with fully formed filaments, is 38 micrograms/ml or 0.6 microM. Under physiological salt conditions NF-L filaments also undergo extensive subunit exchange. Kinetic analysis and evaluation of several possible mechanisms indicate that subunit exchange is preceded by dissociation of subunits from the filament and generation of a kinetically active pool of soluble subunits. Given the concentration of NF-L found in nerve cells and the possibility of regulating this pool, these results provide the first information that intermediate filaments are dynamic structures and that NF-L within the NF complex is in dynamic equilibrium with a small but kinetically active pool of unassembled NF-L units.     

Syncoilin is an intermediate filament protein in activated hepatic stellate cells

Hepatic stellate cells (HSCs) play an important role in several (patho)physiologic conditions in the liver. In response to chronic injury, HSCs are activated and change from quiescent to myofibroblast-like cells with contractile properties. This shift in phenotype is accompanied by a change in expression of intermediate filament (IF) proteins. HSCs express a broad, but variable spectrum of IF proteins. In muscle, syncoilin was identified as an alpha-dystrobrevin binding protein with sequence homology to IF proteins. We investigated the expression of syncoilin in mouse and human HSCs. Syncoilin expression in isolated and cultured HSCs was studied by qPCR, Western blotting, and fluorescence immunocytochemistry. Syncoilin expression was also evaluated in other primary liver cell types and in in vivo-activated HSCs as well as total liver samples from fibrotic mice and cirrhotic patients. Syncoilin mRNA was present in human and mouse HSCs and was highly expressed in in vitro- and in vivo-activated HSCs. Syncoilin protein was strongly upregulated during in vitro activation of HSCs and undetectable in hepatocytes and liver sinusoidal endothelial cells. Syncoilin mRNA levels were elevated in both CCl₄- and common bile duct ligation-treated mice. Syncoilin immunocytochemistry revealed filamentous staining in activated mouse HSCs that partially colocalized with α-smooth muscle actin, β-actin, desmin, and α-tubulin. We show that in the liver, syncoilin is predominantly expressed by activated HSCs and displays very low-expression levels in other liver cell types, making it a good marker of activated HSCs. During in vitro activation of mouse HSCs, syncoilin is able to form filamentous structures or at least to closely interact with existing cellular filaments.     

Chlamydial inclusions: Accumulation of fibers bearing an intermediate filament epitope

Chlamydia trachomatis is an obligate intracellular bacterium that replicates in a phagosomal inclusion within a eukaryotic host. Light and transmission electron microscopy studies with monoclonal antibody TIB. 131 detected an intermediate filament (IF) epitope, demonstrating that it is present and co-accumulates in the inclusion along with chlamydia-specific antigens. Light microscope studies established that the material constituted a distinctive mass at the inclusion site and remained associated with elementary bodies when cells lysed to release the infectious particles. Transmission electron microscopy demonstrated that material binding the IF-specific monoclonal antibody lies within the inclusion; it is filamentous and entwines maturing reticulate bodies. An enzyme immunoassay, with mature chlamydial elementary bodies isolated and purified from infected embryonated eggs, examined the reactivity of the anti-IF monoclonal antibody; its binding to elementary bodies was readily titrated, indicating that the intermediate filament-positive epitope, or one cross-reactive with it, was present. Thus, the epitope is retained during purification, and its presence is not a function of growth in McCoy cells.     

Relationship between tektins and intermediate filament proteins: an immunological study

Affinity-purified antibodies raised against three flagellar tektins (tektin A, B, and C) from each of two sea urchin species (Lytechinus pictus and Strongylocentrotus purpuratus) were used to study the immunological relationship between tektins and intermediate filament proteins. By immunofluorescence microscopy, several antitektins revealed a staining of intermediate filament-like arrays in three vertebrate cell lines tested. Immunoelectron microscopy substantiated the cross reaction of antitektins with intermediate filaments. When the cells were treated with cytochalasin B, the arrangement of the filaments recognized by anti-(Lp)-tektin B was altered; the alteration observed is typical for keratin filaments. By immunoblot, it was found that anti-(Lp)-tektin B cross reacted with two isoforms or different proteins of approximately 54 kD with pIs of 6.1 and 6.2 in human carcinoma epithelia (HeLa) cells and with two isoforms or different proteins of approximately 55 kD with pIs of 6.1 and 6.3 in pig kidney epithelia (LLC-PK1) cells. Furthermore, when antitektin antibodies were affinity purified with the 54 kD HeLa keratin, these keratin-specific antibodies again restained the original tektins on immunoblots. From these observations, it can be concluded that tektins and keratins are to a certain extent immunologically related. To determine the degree of the immunological relationship, tektin filaments and purified intermediate filaments from HeLa cells were cleaved with alpha-chymotrypsin and examined by quantitative immunoblot analysis. On immunoblots of digested tektins from L. pictus, anti-(Lp)-tektin B recognized several cleavage products in the range of 20 kD to 46 kD. However, when immunoblots of digested intermediate filaments from HeLa cells were probed, the cross reaction of anti-(Lp)-tektin B with HeLa keratins was eliminated by more than 98% within 2 min, suggesting that tektins have epitopes in common with the end domains of certain keratins.     

A requirement for cytoplasmic dynein and dynactin in intermediate filament network assembly and organization

We present evidence that vimentin intermediate filament (IF) motility in vivo is associated with cytoplasmic dynein. Immunofluorescence reveals that subunits of dynein and dynactin are associated with all structural forms of vimentin in baby hamster kidney-21 cells. This relationship is also supported by the presence of numerous components of dynein and dynactin in IF-enriched cytoskeletal preparations. Overexpression of dynamitin biases IF motility toward the cell surface, leading to a perinuclear clearance of IFs and their redistribution to the cell surface. IF-enriched cytoskeletal preparations from dynamitin-overexpressing cells contain decreased amounts of dynein, actin-related protein-1, and p150Glued relative to controls. In contrast, the amount of dynamitin is unaltered in these preparations, indicating that it is involved in linking vimentin cargo to dynactin. The results demonstrate that dynein and dynactin are required for the normal organization of vimentin IF networks in vivo. These results together with those of previous studies also suggest that a balance among the microtubule (MT) minus and plus end-directed motors, cytoplasmic dynein, and kinesin are required for the assembly and maintenance of type III IF networks in interphase cells. Furthermore, these motors are to a large extent responsible for the long recognized relationships between vimentin IFs and MTs.     

Dual regulation of intermediate filament phosphorylation

Intermediate filament proteins have been isolated from ME-180, cells of a human cervical carcinoma. Eight of these proteins have been identified as keratins by immunologic cross-reactivity to antibodies raised against authentic human epidermal keratins. The ME-180 keratin proteins consist of two major subunits designated MEK-1 and MEK-2 with approximate molecular weights of 58,000 and 53,000, respectively, and six minor subunits of 59, 57, 52.5, 50.5, 45, and 40 kilodaltons. When ME-180 cells were incubated for 2-24 h in the presence of [32P]orthophosphate, MEK-1 and MEK-2 as well as the 52.5- and 40- kilodalton keratins were phosphorylated at their serine residues. V8 protease digests revealed that phosphorylation of MEK-2 is restricted to one peptide representing approximately half the molecule. Regulation of MEK-1 and MEK-2 phosphorylation has been studied by prelabeling the cells for 2 h in 32P-labeled medium. This was followed by up to 2 h of continued incubation in the same medium after the addition of a variety of perturbing agents. The phosphorylation of MEK-2 increased in the presence of 10(-4) M dibutyryl cyclic AMP (twofold), 1 mM methylisobutylxanthine (2.5-fold), 10(-5) M isoproterenol (fivefold), and 10(-9) M cholera toxin (sevenfold). In contrast, MEK-1 phosphorylation was unaffected by these agents. Neither cyclic GMP, Ca++, hydrocortisone, nor epidermal growth factor had any effect on the phosphorylation of MEK-1 or MEK-2. The results indicate that the phosphorylation of these two keratins is independently controlled by cyclic AMP-dependent kinase for MEK-2 and by cyclic nucleotide- independent kinase for MEK-1. The observed differences in control suggest distinct functions for MEK-1 and MEK-2 within the cytoskeletal network.     

Recent insight into intermediate filament structure

Intermediate filaments (IFs) are key players in multiple cellular processes throughout human tissues. Their biochemical and structural properties are important for understanding filament assembly mechanisms, for interactions between IFs and binding partners, and for developing pharmacological agents that target IFs. IF proteins share a conserved coiled-coil central-rod domain flanked by variable N-terminal ‘head’ and C-terminal ‘tail’ domains. There have been several recent advances in our understanding of IF structure from the study of keratins, glial fibrillary acidic protein, and lamin. These include discoveries of (i) a knob–pocket tetramer assembly mechanism in coil 1B; (ii) a lamin-specific coil 1B insert providing a one-half superhelix turn; (iii) helical, yet flexible, linkers within the rod domain; and (iv) the identification of coil 2B residues required for mature filament assembly. Furthermore, the head and tail domains of some IFs contain low-complexity aromatic-rich kinked segments, and structures of IFs with binding partners show electrostatic surfaces are a major contributor to complex formation. These new data advance the connection between IF structure, pathologic mutations, and clinical diseases in humans.     

The beaded filament of the eye lens: an unexpected key to intermediate filament structure and function

In 1959, an unusual filamentous polymer, now called the beaded filament, was described in the lens of the eye. The constituent proteins, assembly properties and functions of the beaded filament have been elusive. The recent publication of the sequences for two major lens filament proteins (CP49 and filensin) and the reconstitution in vitro of structures closely resembling beaded filaments, suggests that the beaded filament is related structurally to intermediate filaments (IFs). The association of the lenticular chaperones, the alpha-crystallins, with the filament contributes to the characteristic beaded morphology, as well as giving important clues to the function of this unusual filament in the lens. These recent results have several implications for IF function and assembly.     

Modulation of desmin intermediate filament assembly by a monoclonal antibody

We have used a monoclonal antibody against desmin to examine the assembly of intermediate filaments (IF) from their building blocks, the tetrameric protofilaments. The antibody, designated D76, does not cross react with any other IF proteins (Danto, S.I., and D.A. Fischman. 1984. J. Cell Biol. 98:2179-2191). It binds to a region amino-terminal to cys-324 of avian desmin that is resistant to chymotrypsin and trypsin digestion, and in the electron microscope appears to bind to the ends of tetrameric protofilaments. In combination, these findings suggest that the epitope of the antibody resides at the amino-terminal end of the alpha-helical rod domain. Preincubation of desmin protofilaments with an excess of D76 antibodies blocks their subsequent assembly into IF. In the presence of sub-stoichiometric amounts of antibodies, IF are assembled from protofilaments but they are morphologically aberrant in that (a) they are capped by IgG molecules at one or both ends; (b) they are unraveled to varying degree, revealing a characteristic right-handed helical arrangement of sub-filamentous strands of different diameters. The antibody binds only to the ends but not along the length of desmin IF. The most straightforward explanation for this is that the epitope resides in a part of the desmin molecule that becomes buried within the core of the filament upon polymerization and is therefore inaccessible to the antibody.     

Fluorescently-labeled fimbrin decorates a dynamic actin filament network in live plant cells

Recently it has been established, through a detailed biochemical analysis, that recombinant Arabidopsis thaliana fimbrin 1 (AtFim1) is a member of the fimbrin/plastin family of actin filament bundling or cross-linking proteins [D.R. Kovar et al. (2000) Plant J 24:625-636]. To determine whether AtFim1 can function as an F-actin-binding protein in the complex environment of the plant cell cytoplasm, we created a fluorescent protein analog and introduced it by microinjection into live Tradescantia virginiana L. stamen hair cells. AtFim1 derivatized with Oregon Green 488 had biochemical properties similar to unlabeled fimbrin, including the Kd value for binding to plant F-actin and the ability to cross-link filaments into higher-order structures. Fluorescent-fimbrin decorated an array of fine actin filaments in the cortical cytoplasm of stamen hair cells, which were shown with time-course studies to be highly dynamic. These data establish AtFim1 as a bona fide member of the fimbrin/plastin family, and represent the first use of a plant actin-binding protein as a powerful cytological tool for tracking the spatial and temporal redistribution of actin filaments in individual cells.