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The conduction system in the human heart at midgestation--immunohistochemical demonstration of the intermediate filament protein skeletin.

The intermediate filaments have during recent years increasingly attracted attention and new information on the distribution of the subunits of the filaments has become available by the use of specific antibodies. In the present study human fetal hearts at midgestation were studied with immunofluorescence microscopy for a demonstration of the intermediate filament subunit skeletin. In the ordinary ventricular and atrial myocytes the fluorescence was moderate, mainly concentrated to the Z disk levels. The proximal parts of the conduction system also showed a moderate fluorescence, while the fluorescence was intense in the cells in the peripheral parts. The shift from moderate to intense fluorescence occurred in the very proximal part of the left bundle branch and in the distal part of the intramural right bundle branch. We conclude that the conduction cells attain Purkinje fibre-like characteristics at these levels, as evidenced by the content of skeletin. Furthermore, the human fetal Purkinje fibres are more easily distinguished with the technique used herein than with conventional histological techniques. The observed differences in content of skeletin are discussed in terms of the cytoskeletal function of intermediate filaments and the embryology of the conduction system.

Cholinesterases↗

Cytoskeleton-associated plectin: in situ localization, in vitro reconstitution, and binding to immobilized intermediate filament proteins.

The association and interaction of plectin (Mr 300,000) with intermediate filaments and filament subunit proteins were studied. Immunoelectron microscopy of whole mount cytoskeletons from various cultured cell lines (rat glioma C6, mouse BALB/c 3T3, and Chinese hamster ovary) and quick-frozen, deep-etched replicas of Triton X-100-extracted rat embryo fibroblast cells revealed that plectin was primarily located at junction sites and branching points of intermediate filaments. These results were corroborated by in vitro recombination studies using vimentin and plectin purified from C6 cells. Filaments assembled from mixtures of both proteins were extensively crosslinked by oligomeric plectin structures, as demonstrated by electron microscopy of negatively stained and rotary-shadowed specimens as well as by immunoelectron microscopy; the binding of plectin structures on the surface of filaments and cross-link formation occurred without apparent periodicity. Plectin's cross-linking of reconstituted filaments was also shown by ultracentrifugation experiments. As revealed by the rotary-shadowing technique, filament-bound plectin structures were oligomeric and predominantly consisted of a central globular core region of 30-50 nm with extending filaments or filamentous loops. Solid-phase binding to proteolytically degraded vimentin fragments suggested that plectin interacts with the helical rod domain of vimentin, a highly conserved structural element of all intermediate filament proteins. Accordingly, plectin was found to bind to the glial fibrillar acidic protein, the three neurofilament polypeptides, and skin keratins. These results suggest that plectin is a cross-linker of vimentin filaments and possibly also of other intermediate filament types.

Animals↗

Intermediate filament associated proteins.

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Animals↗

The relationship of bovine intermediate filament proteins. A comparative analysis of glial fibrillary acidic protein, desmin and the neurofilament 70 kDa protein.

Three bovine intermediate filament proteins, glial fibrillary acidic protein, desmin and the 70 kDa component of the neurofilament are compared by cleavage at cysteine and tryptophan. The results of these experiments show that the difference in molecular weight between the glial fibrillary acidic protein and desmin is due to a longer portion of the desmin amino terminal to the tryptophan. On the other hand, the 70 kDa protein contains a carboxy terminal addition. The tryptophan and cysteine contents of these proteins are also determined by amino-acid analysis. Differences in the apparent amount of cysteine determined by these methods in the glial fibrillary acidic protein and 70 kDa proteins are discussed. Interchain disulfide bonds result in the formation of dimers in glial fibrillary acidic protein. The bovine 70 kDa neurofilament protein and desmin also form dimers under nonreducing conditions. This emphasizes the structural similarity of these intermediate filament proteins.

Animals↗

Aggregation of wool keratin intermediate filament proteins.

The wool keratin intermediate filament proteins were isolated as their S-carboxymethyl derivatives (S-carboxymethylkerateine A, SCMKA) and purified by gel filtration to remove residual non-helical protein of low molecular weight. The alpha-helix content of purified SCMKA was approximately 62% in agreement with that predicted for the alpha-helical coiled-coil segments from the amino acid sequences of the subunits. In aqueous buffer at pH 11 or in n-propanol (20% v/v) at pH 9.2 very large aggregates are dissociated and SCMKA exists largely as a mixture of the dimer (two-chain coiled-coil of Mr approximately 103,000) and the tetramer. The protein species are not in rapidly reversible equilibrium as judged from gel filtration and sedimentation equilibrium. It is probable that species with a range of association constants are present. The equilibrium is shifted towards the dimer with change of pH from 9.2 to 11 or by the addition of 20% (v/v) n-propanol. The tetrameric proteolytic digestion product which is derived from the 1B segment of the alpha-helical rod section of the keratin molecule dissociates in a similar way to intact SCMKA with increase of pH and in the presence of n-propanol. This indicates the importance of this region of the rod domain in the initial stages of the assembly of the filament. Electrostatic and hydrophobic interactions are implicated in the association of the two-chain coiled-coil to the tetramer both in intact SCMKA and the 1B segment tetramer. The results are discussed in relation to the intact dimeric and tetrameric complexes obtained from other intermediate filament types.

Amino Acids↗

A nuclear matrix protein related to intermediate filaments proteins is a member of the complex binding alphoid DNA in vitro.

A complex of three proteins (of 80, 70, 58 kDa-p80, p70, and p58, respectively) with the ability to bind alphoid DNA (alpha-satDNA) was revealed by gel mobility shift assay (GMSA) in human nuclear matrix. The probes of the alpha-satDNA bound in the GMSA with the greatest specificity, but the complex was capable of binding human satellite 3 fragment. According to ion exchange and affinity chromatography, the complex includes two DNA-binding proteins, p70 and p80, and a non-DNA-binding one, p58, which enhances the specificity of binding to the alpha-satDNA. GMSA, SDS-PAGE and immunoblotting showed that the lamins, as well as constitutive centromeric proteins (CENP-A, CENP-B, CENP-C, CENP-G), were not incorporated into the complex. It was demonstrated by immunoprecipitation assay that p70 and, probably p58, share a common antigen determinant with the rod domain of intermediate filaments (IF) proteins. The results obtained indicate that the nuclear matrix contains at least one IF-related protein that is able to bind specifically to alpha-satDNA in vitro and that this protein is distinct from the lamins.

Animals↗

Expression of intermediate filament proteins in fetal and adult human kidney: modulations of intermediate filament patterns during development and in damaged tissue.

The expression of intermediate filament proteins, particularly individual cytokeratins (CKs), vimentin, and glial filament protein, was immunohistochemically investigated using frozen sections and Carnoy-fixed, paraffin-embedded tissue from normal fetal and adult human kidneys as well as from pathologically altered kidneys. In fetal kidneys, the co-expression of CKs and vimentin was detected in the visceral and parietal epithelium of the glomerulus, the proximal tubules, the thin loops of Henle, and the collecting ducts. In contrast, in the tubules of normal adult kidneys, the presence of vimentin and CKs was nearly always mutually exclusive. While CKs 8 and 18 were present in all tubular epithelia, CKs 19 and 7 each exhibited a distinctive distribution pattern, there being a striking alteration between positive and negative segments and, not infrequently, intratubular heterogeneities. In certain segments, particular cell types (e.g., "plica cells," intercalated cells) could thus be recognized. In tubular epithelia altered by various injurious conditions, novel or enhanced expression of vimentin, CK 19 and CK 7, and, less frequently, CK 17 and glial filament protein was noted in certain segments. The increase in intermediate filament protein expression in altered (particularly proximal) tubules appeared to parallel the reduction in the degree of differentiation. Vimentin was never detected in distal tubules. The present results reveal a considerable similarity between the intermediate filament patterns in non-neoplastic proximal tubules of fetal and damaged kidney tissue and those in clear-cell and chromophilic renal cell carcinomas. They also serve to illustrate that the analysis of both fetal development and reactive cell changes may significantly contribute to our understanding of differentiation phenomena in malignant tumors.

Antibodies, Monoclonal↗

Survey of intermediate filament proteins in optic nerve and spinal cord: evidence for differential expression.

The distribution of intermediate filament proteins in optic nerve and spinal cord from rat, hamster, goldfish, frog, and newt were analyzed by two-dimensional gel electrophoresis. General as well as specific monoclonal and polyclonal antibodies were reacted against putative intermediate filament proteins. In vitro incubations of excised optic nerve in the presence of [35S]methionine distinguished between neuronal and nonneuronal intermediate filament proteins. The proteins of the intermediate filament complex in the two tissues for rat and hamster were similar. The typical neurofilament triplet and glial fibrillary acidic protein (GFAP) were observed. Vimentin was more concentrated in the optic nerve than in the spinal cord. The goldfish, newt, and frog contained neurofilament proteins in the 145-150K range and in the 70-85K range. In addition, predominant neurofilament proteins in the 58-62K molecular-weight range were found in all three species. In contrast to mammalian species, the goldfish, newt, and frog displayed extensive heterogeneity between optic nerve and spinal cord in the expression of both neuronal and nonneuronal intermediate filament proteins. The distinctive presence of low-molecular-weight intermediate filament proteins and their high concentration in the optic nerve and spinal cord of these nonmammalian vertebrates is discussed in terms of neuronal development and regeneration.

Amphibians↗

Purified glial fibrillary acidic protein and desmin are distinct intermediate filament proteins exhibiting similar properties.

Glial fibrillary acidic (GFA) protein and desmin were purified from bovine brain and large intestine, respectively, and used in a comparison of the major protein components of two classes of intermediate filaments, the glial and smooth muscle filaments. The proteins are similar in size, charge, and amino acid composition, but clearly distinct. By sodium dodecyl sulfate-gel electrophoresis, GFA protein is about 5,000 daltons smaller than desmin. GFA protein is composed of three isoelectric variants which are all slightly more basic than the two variants observed for desmin. One-dimensional peptide mapping following limited proteolysis under denaturing conditions or following cyanogen bromide cleavage demonstrates that the proteins are not closely related in primary structure. Assembly-disassembly experiments reveal that the proteins share solubility properties and that negatively stained preparations of in vitro polymerized filaments are very similar. Limited proteolysis under native conditions demonstrates substructural similarities; comparative peptide mapping following digestion with chymotrypsin and trypsin suggests related core polypeptides of about 37,000 and 21,000 daltons. We conclude that GFA protein and desmin are distinct with respect to primary structure, but probably represent two of the more closely related classes of intermediate filament proteins.

Amino Acids↗

Characterization of mammalian synemin, an intermediate filament protein present in all four classes of muscle cells and some neuroglial cells: co-localization and interaction with type III intermediate filament proteins and keratins.

Using a monoclonal antibody, we have detected a high molecular weight muscle protein, co-localized and co-isolating with desmin. Searching a human cDNA database with partial amino acid sequences of the protein, we found a cDNA clone encoding a 1565-amino-acid polypeptide, identified as a mammalian (human) synemin, a member of the intermediate filament (IF) protein family. Immunoblotting showed the presence of a 180-kDa polypeptide in skeletal muscle and 180- and 200-kDa polypeptides in cardiac and smooth muscles. Interestingly, synemin was also found in myoepithelial cells, which have keratin filaments instead of desmin. Moreover, synemin was also found in astrocytes of optic nerves and non-myelin-forming Schwann cells, together with glial fibrillary acidic protein (GFAP) and vimentin. Blot overlays pointed to molecular interactions of synemin with desmin, vimentin, GFAP and keratin 5 and 6, but not with keratin 14. The experimental data also suggested a possible link with nebulin, a skeletal muscle protein. Purified synemin was coassembled with desmin in different molar ratios, and at 1:25, as typically found in vivo, IFs were formed which were comparable in length to desmin filaments. However, at molar ratios of 3:25 and 6:25, much shorter and irregular shaped filamentous polymers were generated. The fact that synemin is present in all four classes of muscle cells and a specific type of glial cells is indicative of important functions. Its incorporation may give structural and functional versatility to the IF cytoskeleton.

Animals↗

Distribution of specific intermediate-filament proteins in the goldfish retina.

The intermediate-filament proteins expressed in the goldfish retina were investigated by immunohistochemistry and by immunoblotting. Polyclonal antibodies that previously had been raised against the goldfish optic nerve neurofilament (ON1 and ON2) and glial filament (ON3 and ON4) proteins were used in this study. Anti-ON1/ON2 antiserum reacted on a retinal immunoblot with two proteins having molecular weights and isoelectric points corresponding to those of ON1 and ON2. Histologically, the most pronounced anti-ON1/ON2 reactivity was observed in the ganglion cell layer of the goldfish retina. The anti-ON3/ON4 antiserum reacted with a single protein on a retinal immunoblot. This protein had a molecular weight and isoelectric point which corresponded to the goldfish optic nerve glial filament proteins. This anti-serum labeled horizontal cells in retina sections. Three previously unidentified goldfish visual-pathway intermediate-filament proteins sharing a molecular weight of 60K were observed on two-dimensional gels of retinal cytoskeletal proteins and on retinal immunoblots which were probed with a monoclonal antibody which recognizes an epitope common to all intermediate filament proteins. The possible existence of homologs of mammalian GFAP and vimentin in the goldfish retina was also explored. Antibodies directed against mammalian GFAP and vimentin labeled the Müller fibers and the cone horizontal cells, respectively. However, immunoblot analysis and a comparison of the two-dimensional gel electrophoresis patterns of goldfish retinal and rat spinal cord cytoskeletal proteins demonstrated a lack of goldfish proteins identical to the mammalian intermediate-filament proteins.

Animals↗

Monoclonal antibodies to gel-excised glial filament protein and their reactivities with other intermediate filament proteins.

A series of 14 monoclonal antibodies (MAs) has been obtained from a single rat-mouse fusion using gel-excised bovine glial filament (GF) proteins as immunogens. These MAs were characterized by two separate immunochemical assays and by two different immunohistochemical methods. Nine MAs demonstrated specificity for GF proteins. One MA also recognized an epitope shared by intermediate filaments (IF) of the vimentin class (VF). Using the enzyme-linked immunosorbent assay, four of the MAs recognized 200,000, 150,000, and 51,000 dalton proteins, suggesting that these MAs were specific for GF proteins (the 51,000 dalton protein) and neurofilament (NF) proteins (the two high-molecular-weight proteins). However, in both of the immunohistochemical assay systems, these MAs stained neurons and their processes but not astroglial cells. These observations strongly suggest that the 51,000-dalton protein recognized by these four MAs was not derived from GF proteins but instead represents derivatives of NF protein subunits comigrating in gels with GF proteins. These data provide additional information concerning the unique and shared antigenic determinants of the three classes of IF (NF, GF, and VF) of the CNS. In addition, they draw attention to the fact that proteins of certain IF may undergo degradation and comigrate in gels with the proteins of unrelated IF. This emphasizes the need for the use of independent immunochemical and immunohistochemical assays in the characterization of the specificity of MAs.

Animals↗

Molecular characteristics and interactions of the intermediate filament protein synemin. Interactions with alpha-actinin may anchor synemin-containing heterofilaments.

Synemin is a cytoskeletal protein originally identified as an intermediate filament (IF)-associated protein because of its colocalization and copurification with the IF proteins desmin and vimentin in muscle cells. Our sequencing studies have shown that synemin is an unusually large member (1,604 residues, 182,187 Da) of the IF protein superfamily, with the majority of the molecule consisting of a long C-terminal tail domain. Molecular interaction studies demonstrate that purified synemin interacts with desmin, the major IF protein in mature muscle cells, and with alpha-actinin, an integral myofibrillar Z-line protein. Furthermore, expressed synemin rod and tail domains interact, respectively, with desmin and alpha-actinin. Analysis of endogenous protein expression in SW13 clonal lines reveals that synemin is coexpressed and colocalized with vimentin IFs in SW13.C1 vim+ cells but is absent in SW13.C2 vim- cells. Transfection studies indicate that synemin requires the presence of another IF protein, such as vimentin, in order to assemble into IFs. Taken in toto, our results suggest synemin functions as a component of heteropolymeric IFs and plays an important cytoskeletal cross-linking role by linking these IFs to other components of the cytoskeleton. Synemin in striated muscle cells may enable these heterofilaments to help link Z-lines of adjacent myofibrils and, thereby, play an important role in cytoskeletal integrity.

Actinin↗

Chicken filensin: a lens fiber cell protein that exhibits sequence similarity to intermediate filament proteins.

Filensin, a 100 kDa, membrane-associated, cytoskeletal protein, is uniquely expressed in the lens fiber cell (Merdes, A., Brunkener, M., Horstmann, H., and Georgatos, S. D. (1991) J. Cell Biol. 115, 397-410). I cloned and sequenced a full-length chicken lens cDNA encoding filensin, also known as CP95 (Ireland, M. and Maisel, H. (1989) Lens and Eye Toxicity Research 6, 623-638). The deduced amino acid sequence of 657 residues contained an internal 280 residue heptad repeat domain with sequence similarities to the rod domain of intermediate filament proteins. The putative filensin rod domain could be divided into three alpha-helical segments (1A, 1B and 2) separated by short, non-helical linkers. The sequence of the amino-terminal end of the filensin rod domain contained the highly conserved intermediate filament segment 1A motif (Conway, J. F. and Parry, D. A. D. (1988) Int. J. Biol. Macromol. 10, 79-98). Allowing conservative amino acid substitutions, the sequence of the carboxy-terminal end of the filensin rod domain was similar to that of the highly conserved intermediate filament rod carboxy terminus. The alpha-helical segments of the shorter filensin rod domain aligned with the corresponding segments of intermediate filament proteins by allowing a gap of four heptad repeats in the amino-terminal half of filensin segment 2. Filensin rod segment 2 contained the characteristic stutter in heptad repeat phasing, nine heptads from the end of the intermediate filament rod. The overall sequence identity between the rod domains of filensin and individual intermediate filament proteins was 20 to 25%, approximately the level of sequence identity observed between intermediate filament proteins of different types. The open reading frame of chicken filensin predicted a 657 amino acid protein with molecular mass of 76 kDa. Embryonic chicken filensin migrated in SDS-PAGE as a triplet of 102, 105 and 109 kDa, while rooster filensin migrated as a 105 and 109 kDa doublet. Antibodies to filensin labeled lens fiber cells but not lens epithelial cells. By immunofluorescence methods filensin was localized to the fiber cell plasma membranes, including the ends of elongated fiber cells.

Amino Acid Sequence↗

The 53kDa polypeptide component of the bovine fibre cell cytoskeleton is derived from the 115kDa beaded filament protein: evidence for a fibre cell specific intermediate filament protein.

The 115kDa protein found enriched in the PMCC (plasma membrane-cytoskeleton complex) fraction of the cortex in bovine lens fibre cells is proteolytically processed to a stable 53kDa product. The 115 kDa protein and the 53kDa polypeptide have been purified by a combination of ion exchange and hydroxyapatite chromatography. Tryptic peptide mapping using reverse phase HPLC and subsequent peptide sequencing confirmed that the 53kDa polypeptide is derived from the 115kDa protein. The 53kDa fragment is also a component of the PMCC as well as being a major component of the urea soluble fraction of lens plasma membranes which have been extracted with buffers containing 1M KC1. The 53kDa polypeptide has escaped identification as a breakdown product of the 115kDa protein because it is not recognised by a commonly used monoclonal antibody, R2D2, specific for the bovine 115kDa protein. This result suggests that proteolysis is important in determining the function(s) of the 115kDa protein, and that part of this function is satisfied by the 53kDa protein core. Both the purified 115kDa protein and the 53kDa polypeptide were unable to form either beaded or intermediate filaments on their own but they were able to form short 10nm rods indicative of an intermediate stage in intermediate filament assembly. Comparison ot the assembly properties of the 53 and 115kDa proteins indicate that there are sequences in the 115kDa protein which inhibit in vitro assembly. This is similar to the situation with neurofilament proteins. We suggest that the 115kDa protein is a lens-specific intermediate filament protein.

Amino Acid Sequence↗

Malnutrition induces an increase in intermediate filament protein content of rat cerebral cortex.

Wistar rats were fed a normal protein (25% casein) or an isoenergetic low protein (8% casein) diet from the day of giving birth until pups were weaned. Some litters were killed at weaning; others (both normal and malnourished animals) received the 25% protein diet until d 90 when they were killed. Intermediate filament (IF) preparations were obtained by extraction of the cerebral cortex with a high salt PBS solution containing 1% Triton X-100. The pellet contained the bulk of the cytoskeleton proteins from tissue, identified as the 150- and 68-kDa subunits of neurofilaments (NF-M and NF-L, respectively), the 66-kDa associated protein, the 57-kDa intermediate filament-like protein, and the 50-kDa glial fibrillary acidic protein. Intermediate filament-enriched fractions from control and malnourished rats at both d 21 and 90 were scanned following two-dimensional gel electrophoresis to determine the effects of postnatal malnutrition on the intermediate filament protein content. The results indicated that postnatal malnutrition imposed during the brain growth spurt period did not alter the expression of IF proteins of the cerebral cortex in 21-d-old rats, but increased the expression of NF-L and NF-M proteins in adult rats.

Animals↗

Phosphorylation of intermediate filament proteins by cAMP-dependent protein kinases.

The intermediate filament proteins, desmin and vimentin, are phosphorylated in skeletal muscle cells in vivo. Desmin kinase activities have been purified from mature chicken skeletal muscle and identified as the cAMP-dependent kinases. Chicken skeletal muscle contains two cAMP-dependent protein kinases which are similar to those of other tissues in their subunit composition and chromatographic behavior. The catalytic subunits purified from the two chicken cAMP-dependent kinases phosphorylate both desmin and vimentin in vitro, using cytoskeletal residues prepared from cultured myogenic cells as a substrate. Likewise, the purified catalytic subunits of the rabbit skeletal muscle and bovine heart cAMP-dependent protein kinases phosphorylate desmin and vimentin in vitro. Desmin and vimentin phosphorylation by the rabbit skeletal muscle catalytic subunit is inhibited by the addition of its regulatory subunit. This inhibition is reversed by the presence of cAMP in the reaction mixture. A small fraction of the vimentin phosphorylation is cAmP-independent. Tryptic peptide analysis of desmin phosphorylated in vivo shows two major phosphopeptides. Serine is the phosphorylated amino acid in both peptides. The same two peptides are phosphorylated in vitro by the bovine heart catalytic subunit, but additional peptides are also phosphorylated.

Animals↗

Characterisation of wool intermediate filament proteins separated by micropreparative two-dimensional electrophoresis.

Wool intermediate filament proteins (IFP) are a subclass of the cytokeratins, a group of structural proteins which form intermediate filaments in many cell types. Post-translational modifications, such as phosphorylation, play an important role in the control of intermediate filament assembly. Two-dimensional electrophoresis has previously been used to study the IFP distribution in wools with different physical characteristics. Charge heterogeneity has been observed in Type I and Type II IFP. In a previous study, two-dimensional electrophoresis of alkaline phosphatase-treated wool protein extracts was used to show that Type II IFP are phosphorylated. To facilitate post-separation analysis, micropreparative two-dimensional electrophoresis was used to separate milligram quantities of wool protein. Direct phosphoamino acid analysis has confirmed the presence of phosphorylation on serine residues on Type II IFP, whose identity was confirmed by amino acid compositional analysis. The isoelectric points of Type I IFP are very similar and they do not separate completely on the commercially available pH 4-7 immobilized pH gradients (IPG) used in this study. In situ tryptic digestion followed by automated Edman sequencing of the high performance liquid chromatography (HPLC)-separated peptides was used to confirm the identity of this group as Type I IFP. To improve the separation of the Type I IFP it will be necessary to use narrow range IPGs such as Immobiline DryPlates which are available from Pharmacia Biotech, in the pH ranges 4.2-4.9, 4.5-5.4, 5.0-6.0 and 5.6-6.6.

Animals↗