Purification of smooth-muscle desmin and a protein-chemical comparison of desmins from chicken gizzard and hog stomach

Direct observation of oriented behavior of actin filaments interacting with desmin intermediate filaments

BACKGROUND

Associations between actin filaments (AFs) and intermediate filaments (IFs) are frequently observed in living cells. The crosstalk between these cytoskeletal components underpins cellular organization and dynamics; however, the molecular basis of filamentous interactions is not fully understood. Here, we describe the mode of interaction between AFs and desmin IFs (DIFs) in a reconstituted in vitro system.

METHODS

AFs (rabbit skeletal muscle) and DIFs (chicken gizzard) were labeled with fluorescent dyes. DIFs were immobilized on a heavy meromyosin (HMM)-coated collodion surface. HMM-driven AFs with ATP hydrolysis was assessed in the presence of DIFs. Images of single filaments were obtained using fluorescence microscopy. Vector changes in the trajectories of single AFs were calculated from microscopy images.

RESULTS

AF speed transiently decreased upon contact with DIF. The difference between the incoming and outgoing angles of a moving AF broadened upon contact with a DIF. A smaller incoming angle tended to result in a smaller outgoing angle in a nematic manner. The percentage of moving AFs decreased with an increasing DIF density, but the speed of the moving AFs was similar to that in the no-desmin control. An abundance of DIFs tended to exclude AFs from the HMM-coated surfaces.

CONCLUSIONS

DIFs agitate the movement of AFs with the orientation. DIFs can bind to HMMs and weaken actin-myosin interactions.

GENERAL SIGNIFICANCE

The study indicates that apart from the binding strength, the accumulation of weak interactions characteristic of filamentous structures may affect the dynamic organization of cell architecture.

Selective association of desmin intermediate filaments with a phospholipid layer in droplets

Desmin, an intermediate filament protein expressed in muscle cells, plays a key role in the integrity and regulation of the contractile system. Furthermore, the distribution of desmin in cells and its interplay with plasma and organelle membranes are crucial for cell functions; however, the fundamental properties of lipid-desmin interactions remain unknown. Using a water-in-oil method for a limited space system in vitro, we examined the distribution of desmin in three types of phospholipid droplets: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and 1,2-dioleoyl-sn-glycero-3-phosphoserine (DOPS). When fluorescent-labeled desmin was observed for 60 min after desmin assembly was initiated by adding 25 mM KCl, desmin accumulated on both the DOPE and DOPS layers; however, it did not accumulate on the DOPC layer of droplets. An increase in salt concentration did not moderate the accumulation. The initial form of either oligomer or mature filament affected the accumulation on each lipid layer. When liposomes were included in the droplets, desmin was associated with DOPE but not on DOPC liposomes. These results suggest that desmin has the potential for association with phospholipids concerning desmin form and lipid shape. The behavior and composition of living membranes may affect the distribution of desmin networks.

Stretching desmin filaments with receding meniscus reveals large axial tensile strength

Desmin forms the intermediate filament system of muscle cells where it plays important role in maintaining mechanical integrity and elasticity. Although the importance of intermediate-filament elasticity in cellular mechanics is being increasingly recognized, the molecular basis of desmin's elasticity is not fully understood. We explored desmin elasticity by molecular combing with forces calculated to be as large as 4nN. Average filament contour length increased 1.55-fold axial on average. Molecular combing together with EGTA-treatment caused the fragmentation of the filament into short, 60 to 120-nm-long and 4-nm-wide structures. The fragments display a surface periodicity of 38nm, suggesting that they are composed of laterally attached desmin dimers. The axis of the fragments may deviate significantly from that of the overstretched filament, indicating that they have a large orientational freedom in spite of being axially interconnected. The emergence of protofibril fragments thus suggests that the interconnecting head or tail domains of coiled-coil desmin dimers are load-bearing elements during axial stretch.

The specificity of the interaction between αB-crystallin and desmin filaments and its impact on filament aggregation and cell viability

CRYAB (αB-crystallin) is expressed in many tissues and yet the R120G mutation in CRYAB causes tissue-specific pathologies, namely cardiomyopathy and cataract. Here, we present evidence to demonstrate that there is a specific functional interaction of CRYAB with desmin intermediate filaments that predisposes myocytes to disease caused by the R120G mutation. We use a variety of biochemical and biophysical techniques to show that plant, animal and ascidian small heat-shock proteins (sHSPs) can interact with intermediate filaments. Nevertheless, the mutation R120G in CRYAB does specifically change that interaction when compared with equivalent substitutions in HSP27 (R140G) and into the Caenorhabditis elegans HSP16.2 (R95G). By transient transfection, we show that R120G CRYAB specifically promotes intermediate filament aggregation in MCF7 cells. The transient transfection of R120G CRYAB alone has no significant effect upon cell viability, although bundling of the endogenous intermediate filament network occurs and the mitochondria are concentrated into the perinuclear region. The combination of R120G CRYAB co-transfected with wild-type desmin, however, causes a significant reduction in cell viability. Therefore, we suggest that while there is an innate ability of sHSPs to interact with and to bind to intermediate filaments, it is the specific combination of desmin and CRYAB that compromises cell viability and this is potentially the key to the muscle pathology caused by the R120G CRYAB.

Cleavage of desmin by cysteine proteases: Calpains and cathepsin B

The intermediate filament protein, desmin, was purified from pork longissimus dorsi and incubated with either μ-calpain, m-calpain or cathepsin B. Proteolysis of desmin was followed using SDS-PAGE and Western blotting. After incubation of desmin with the proteases, cleavage sites on the desmin molecule were identified by N-terminal sequencing of the different proteolytic fragments. Desmin incubated with either m-calpain or μ-calpain was primarily cleaved in the head and tail region leaving the rod domain relatively intact even after prolonged incubation. Incubation with cathepsin B produces a sequential C-terminal degradation pattern characteristic of this dipeptylpeptidase. The substrate primary structure was not found to be essential for regulation of the proteolytic activity of the cysteine peptidases studied. However, the degradation patterns obtained imply that calpains are involved in degradation of desmin early post-mortem, targeting the non-helical region of the desmin molecule and resulting in depolymerisation and initial disorganisation of the intermediate filament structures of the muscle cell.

Flow birefringence property of desmin filaments

We have investigated the flow birefringence property and assembly process of desmin, a muscle specific intermediate protein. Solution of non-polar desmin filaments showed birefringence when aligned in the sheared flow. The amount of birefringence of desmin filaments was considerably lower when compared with that of F-actin solution. Assembly of desmin from soluble state was followed by the birefringence measurements. At any desmin concentrations examined, the degree of flow birefringence increased rapidly just after the addition of the assembly buffer and reached a saturated level within 30 min. The time to reach half-maximal values of flow birefringence slightly but definitely depended on the initial soluble desmin concentrations. The plotting of the initial velocity of the assembly against the soluble desmin concentrations showed a slope of 1.4. This result suggested that the assembly process detected by flow birefringence measurements followed second-order kinetics, and the process corresponded to the second step of the three stage model for type III intermediate filament assembly proposed by Herrmann and his colleagues; the annealing of unit length filaments into filaments.

Isolation of intermediate filaments

Intermediate filaments (IFs) are found in most eukaryotic cells and are made up of various IF proteins. IFs are highly insoluble in conventional extraction buffers and are therefore commonly purified under denaturing condition. Purified IF proteins can be reassembled into filaments by dialysis. At least 65 IF proteins are found in humans, and the procedures for the purification of each subunit vary somewhat, although many basic steps are similar. To illustrate the isolation of IFs, a detailed protocol is described for purifying neurofilament proteins (NFL, NFM, and NFH subunits) from bovine spinal cord. These three proteins form the predominant IF network in mature neurons. An alternative method for the purification of NFL from a prokaryotic expression system is also included. The isolation of recombinant proteins from bacteria is quite straightforward and may therefore be the method of choice for producing and purifying IFs. Finally, there is a discussion of the purification methods of other IF proteins.

Nanomechanical properties of desmin intermediate filaments

Desmin intermediate filaments play important role in the mechanical integrity and elasticity of muscle cells. The mechanisms of how desmin contributes to cellular mechanics are little understood. Here, we explored the nanomechanics of desmin by manipulating individual filaments with atomic force microscopy. In complex, hierarchical force responses we identified recurring features which likely correspond to distinct properties and structural transitions related to desmin's extensibility and elasticity. The most frequently observed feature is an initial unbinding transition that corresponds to the removal of approximately 45-nm-long coiled-coil dimers from the filament surface with 20-60 pN forces in usually two discrete steps. In tethers longer than 60 nm we most often observed force plateaus studded with bumps spaced approximately 16 nm apart, which are likely caused by a combination of protofilament unzipping, dimer-dimer sliding and coiled-coil-domain unfolding events. At high stresses and strains non-linear, entropic elasticity was dominant, and sometimes repetitive sawtooth force transitions were seen which might arise because of slippage within the desmin protofilament. A model is proposed in which mechanical yielding is caused by coiled-coil domain unfolding and dimer-dimer sliding/slippage, and strain hardening by the entropic elasticity of partially unfolded protofilaments.

Desmin aggregate formation by R120G alphaB-crystallin is caused by altered filament interactions and is dependent upon network status in cells

The R120G mutation in alphaB-crystallin causes desmin-related myopathy. There have been a number of mechanisms proposed to explain the disease process, from altered protein processing to loss of chaperone function. Here, we show that the mutation alters the in vitro binding characteristics of alphaB-crystallin for desmin filaments. The apparent dissociation constant of R120G alphaB-crystallin was decreased while the binding capacity was increased significantly and as a result, desmin filaments aggregated. These data suggest that the characteristic desmin aggregates seen as part of the disease histopathology can be caused by a direct, but altered interaction of R120G alphaB-crystallin with desmin filaments. Transfection studies show that desmin networks in different cell backgrounds are not equally affected. Desmin networks are most vulnerable when they are being made de novo and not when they are already established. Our data also clearly demonstrate the beneficial role of wild-type alphaB-crystallin in the formation of desmin filament networks. Collectively, our data suggest that R120G alphaB-crystallin directly promotes desmin filament aggregation, although this gain of a function can be repressed by some cell situations. Such circumstances in muscle could explain the late onset characteristic of the myopathies caused by mutations in alphaB-crystallin.

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.

Desmin filaments studied by quasi-elastic light scattering

We studied polymers of desmin, a muscle-specific type III intermediate filament protein, using quasi-elastic light scattering. Desmin was purified from chicken gizzard. Polymerization was induced either by 2 mM MgCl(2) or 150 mM NaCl. The polymer solutions were in the semidilute regime. We concluded that the persistence length of the filaments is between 0.1 and 1 microm. In all cases, we found a hydrodynamic diameter of desmin filaments of 16-18 nm. The filament dynamics exhibits a characteristic frequency in the sense that correlation functions measured on one sample but at different scattering vectors collapse onto a single master curve when time is normalized by the experimentally determined initial decay rate.

Characterization of distinct early assembly units of different intermediate filament proteins

We have determined the mass-per-length (MPL) composition of distinct early assembly products of recombinant intermediate filament (IF) proteins from the four cytoplasmic sequence homology classes, and compared these values with those of the corresponding mature filaments. After two seconds under standard assembly conditions (i.e. 25 mM Tris-HCl (pH 7.5), 50 mM NaCl, 37 degrees C), vimentin, desmin and the neurofilament triplet protein NF-L aggregated into similar types of "unit-length filaments" (ULFs), whereas cytokeratins (CKs) 8/18 already yielded long IFs at this time point, so the ionic strength had to be reduced. The number of molecules per filament cross-section, as deduced from the MPL values, was lowest for CK8/18, i.e. 16 and 25 at two seconds compared to 16 and 21 at one hour. NF-L exhibited corresponding values of 26 and 30. Vimentin ULFs yielded a pronounced heterogeneity, with major peak values of 32 and 45 at two seconds and 30, 37 and 44 after one hour. Desmin formed filaments of distinctly higher mass with 47 molecules per cross-section, at two seconds and after one hour of assembly. This indicates that individual types of IF proteins generate filaments with distinctly different numbers of molecules per cross-section. Also, the observed significant reduction of apparent filament diameter of ULFs compared to the corresponding mature IFs is the result of a "conservative" radial compaction-type reorganization within the filament, as concluded from the fact that both the immature and mature filaments contain very similar numbers of subunits per cross-section. Moreover, the MPL composition of filaments is strikingly dependent on the assembly conditions employed. For example, vimentin fibers formed in 0.7 mM phosphate (pH 7.5), 2.5 mM MgCl2, yield a significantly increased number of molecules per cross-section (56 and 84) compared to assembly under standard conditions. Temperature also strongly influences assembly: above a certain threshold temperature "pathological" ULFs form that are arrested in this state, indicating that the system is forced into strong but unproductive interactions between subunits. Similar "dead-end" structures were obtained with vimentins mutated to introduce principal alterations in subdomains presumed to be of general structural importance, indicating that these sequence changes led to new modes of intermolecular interactions.

Assembly and architecture of invertebrate cytoplasmic intermediate filaments reconcile features of vertebrate cytoplasmic and nuclear lamin-type intermediate filaments

The two major intermediate filament (IF) proteins from the esophagus epithelium of the snail Helix pomatia and the two major IF proteins from muscle tissue of the nematode Ascaris suum were investigated under a variety of assembly conditions. The lowest-order complexes from each of the four protostomic invertebrate (p-INV) IF proteins are parallel, unstaggered dimers involving two-stranded alpha-helical coiled coil formation of their approximately 350 amino acid residue central rod domain (i.e. long-rod). In the electron microscope these are readily recognized by their distinct approximately 56 nm long rod with two globular domains (i.e. representing the non-helical carboxy-terminal tail domain of the p-INV IF proteins) attached at one end, closely resembling vertebrate lamin dimers. The next-higher-order oligomers are tetramers, which are easily recognized by their two pairs of globular tail domains attached at either end of a approximately 72 nm long central rod portion. According to their size and shape, these tetramers are built from two dimers associated laterally in an antiparallel, approximately half-staggered fashion via the amino-terminal halves of their rod domains. This is similar to the NN-type tetramers found as the most abundant oligomer species in all types of vertebrate cytoplasmic IF proteins, which contain a approximately 310 amino acid residue central rod domain (i.e. short-rod). As a first step toward filament formation, the p-INV IF tetramers anneal longitudinally into protofilaments by antiparallel CC-type association of the carboxy-terminal halves of their dimer rods. The next step involves radial growth, occurring initially through lateral association of two four-chain protofilaments into octameric subfibrils, which then further associate into mature, full-width filaments. Head-to-tail polymers of dimers and paracrystalline fibers commonly observed with vertebrate lamins were only rarely seen with p-INV IF proteins. The globular domains residing at the carboxy-terminal end of p-INV IF dimers were studding the surface of the filaments at regular, approximately 24.5 nm intervals, thereby giving them a "beaded" appearance with an axial periodicity of about 24.5 nm, which is approximately 3 nm longer than the corresponding approximately 21.5 nm repeat pattern exhibited by short-rod vertebrate IFs.

Annexin VI binds S100A1 and S100B and blocks the ability of S100A1 and S100B to inhibit desmin and GFAP assemblies into intermediate filaments

Annexin VI, a member of a family of Ca(2+)-dependent phospholipid- and membrane-binding proteins, interacts with the Ca(2+)-regulated EF-hand proteins, S100A1 and S100B, and blocks the ability of these two proteins to inhibit the assembly of desmin and glial fibrillary acidic protein (GFAP) into intermediate filaments in a Ca(2+)- and dose-dependent manner. S100A1 and S100B each possess one annexin VI binding site, characterized by an affinity for annexin VI in the submicromolar range. Binding of annexin VI to either S100 protein occurs at a site that appears to differ in some parts from that recognizing desmin and GFAP. As S100A1 and S100B exist in solution as homodimers in which the two monomers are related by a 2-fold symmetry axis, each of the above S100 homodimers likely crosslinks two annexin VI molecules, a situation that appears typical of all the annexin-S100 protein complexes described thus far. However, whereas in the cases of other annexin-S100 complexes the C-terminal extension of the S100 molecule appears indispensable for annexin binding, the annexin VI binding site cannot be restricted to the S100A1 and S100B C-terminal extension. We speculate that the annexin VI site on S100A1/B may only partially overlap to the desmin/GFAP site. In contrast, no effects of annexin V on the ability of S100A1 or S100B to affect the desmin and GFAP assemblies could be documented, although binding of annexin V to S100A1 and S100B could be detected at relatively high Ca2+ concentrations. The present data suggest that annexin VI might regulate S100A1 and S100B activities and vice versa.

Copurification of vimentin, energy metabolism enzymes, and a MER5 homolog with nucleoside diphosphate kinase. Identification of tissue-specific interactions

Chromatography on immobilized antibodies specific to nucleoside diphosphate (NDP) kinase was utilized for affinity purification of this enzyme from detergent extracts of frog heart post-mitochondrial fractions. SDS-polyacrylamide gel electrophoresis analysis of eluates from these supports shows that five polypeptides co-purify with nucleoside diphosphate (NDP) kinase. Tryptic digests of each band were analyzed by mass spectrometric microsequencing. Data base searches by peptide mass matching and sequence homology led to the identification of these proteins as glyceraldehyde-3-phosphate dehydrogenase (40 kDa), creatine kinase (45 kDa), vimentin (55 kDa), pyruvate kinase (60 kDa), and a putative member of the antioxidant protein family (28 kDa). Distinct protein compositions were found in eluates of lung and liver extracts processed in a like manner. The 28-kDa band and vimentin were associated with NDP kinase from all tissues, but co-purification of pyruvate kinase was seen only in liver, while creatine kinase and glyceraldehyde-3-phosphate dehydrogenase were absent from eluates from lung and liver. The results suggest that while NDP kinase is associated with vimentin intermediate filaments and an antioxidant protein in most tissues, it interacts with energy metabolism enzymes in a tissue-specific manner.

Characterization of type III intermediate filament regulatory protein target epitopes: S-100 (beta and/or alpha) binds the N-terminal head domain; annexin II2-p11(2) binds the rod domain

We have investigated the interaction of S-100 proteins (beta and/or alpha) and annexin II2-p11(2) with glial fibrillary acidic protein (GFAP) and desmin to have further information on the mechanisms whereby S-100 proteins and annexin II2-p11(2) affect assembly/disassembly of GFAP and desmin intermediate filaments (IFs). Analyses were conducted on either native IF subunits, GFAP or desmin rod domain, or headless GFAP or desmin. Our data indicate that: (i) S-100 proteins bind to GFAP and desmin N-terminal head domain; (ii) annexin II2-p11(2) binds to GFAP rod domain; (iii) annexin II2-p11(2) does not interact with desmin nor affects desmin assembly. The present data suggest that the ability of S-100 proteins to inhibit GFAP and desmin assemblies and to promote the disassembly of preformed GFAP and desmin IFs depends on occupation of a site on the N-terminal head domain of these IF subunit. It is known that the N-terminal head domain is critical for the progression from the stage of GFAP and desmin dimers/tetramers to that of large oligomers. On the other hand, the ability of annexin II2-p11(2) to stimulate GFAP assembly under conditions where this latter is normally hampered (e.g., at alkaline pH values) might depend on annexin II2-p11(2)-induced changes in the structure of GFAP rod domain, possibly as a consequence of charge modifications. By contrast, the inability of annexin II2-p11(2) to bind to desmin would depend on desmin resistance to charge modifications.

Interaction of smooth muscle calponin and desmin

Interaction of smooth-muscle calponin and desmin was analyzed by means of ultracentrifugation, fluorescent spectroscopy and affinity chromatography. At low and intermediate ionic strength (30-50 mM NaCl) calponin is cosedimented with desmin with an apparent dissociation constant 3-15 microM and stoichiometry of 1 calponin/4-6 desmin. Calmodulin decreases the quantity of calponin bound to desmin. Increase of ionic strength up to 150 mM weakens calponin-desmin interaction, but even at this ionic strength part of calponin remains bound to desmin. Calponin increases the rate and extent of fluorescence quenching induced by polymerization of 5-iodoacetamidofluorescein-labeled desmin. Affinity chromatography data indicate that desmin-binding sites are located in the N-terminal 22 kDa fragment of calponin. Since calponin interacts with desmin with an affinity comparable with that of, e.g., tropomyosin and myosin we suppose that calponin-desmin interaction may be important for cytoskeleton organization.

Specific recognition of coiled coils by infrared spectroscopy: analysis of the three structural domains of type III intermediate filament proteins

The central domain of cytoplasmic intermediate filament (IF) proteins from vertebrates contains some 310 residues and forms a double-stranded coiled coil (rod) with a length of about 46 nm. The flanking terminal domains show a high cell type specific variability both in sequence and in length. Using Fourier transform infrared (FTIR) spectroscopy we measured secondary structures of isolated domains of type III and IV IF proteins and of the soluble tetramers and the filaments formed by type III IF proteins. The amide I spectrum of the desmin rod is virtually identical to the spectra of other coiled-coil proteins such as tropomyosin and the myosin rod. All these double-stranded coiled coils reveal spectra distinctly different from classical alpha-helical spectra. The spectrum of coiled coils is a triplet of approximately equally strong bands. One band occurs at normal alpha-helix position, while the other two are found at lower wavenumbers. Theoretical aspects of these findings are discussed in the accompanying paper by W. C. Reisdorf and S. Krimm [(1996) Biochemistry 35, 1383-1386]. The amino-terminal head domain of desmin has a multicomponent spectrum with major fractions of beta-sheet. The carboxy-terminal tail domains of desmin and the neurofilament proteins L and H, the latter in the phosphorylated and in the dephosphorylated forms, have very similar FTIR spectra, indicating mostly random structure. The spectrum of desmin type III protofilaments is very similar to the sum of the spectra of the three isolated domains. Polymerization into filaments seems to induce a small change in secondary structure.

Neurofilament protein heterotetramers as assembly intermediates

Evidence is presented for the existence of a soluble heterotetramer containing the low and middle molecular weight neurofilament (NF) proteins, NF-L and NF-M, and one containing the low and high molecular weight proteins, NF-L and NF-H, and for their role in filament assembly. When a mixture of either pair of proteins was renatured in 2 M urea, 20 mM Tris, pH 7.2, a new band representing a complex was observed in native gel electrophoresis. No new band was observed with a mixture of NF-M and NF-H. Two-dimensional gel electrophoresis showed that treatment of the complexes with SDS caused them to dissociate into their constituent polypeptide chains. Native neurofilaments dissociated in 2 M urea into a mixture of LM and LH complexes. Titration of NF-L with NF-M indicated that complex formation was complete at an approximately equimolar ratio of the two proteins. The LM complex had a sedimentation coefficient, s20,w, of 4.4 S, consistent with a tetrameric structure. Dialysis of a solution of the LM complex against 50 mM 4-morpholineethanesulfonic acid, 0.17 M NaCl, pH 6.25, led to the formation of 10-nm filaments in good yield. These results suggest that NF protein heterooligomers are intermediates in NF assembly and disassembly.

Desmin degradation and Ca(2+)-dependent proteolysis during myoblast fusion

It has already been reported that, in vitro, intermediate filaments such as desmin and vimentin are very susceptible to proteolysis by calpains (Ca(2+)-activated cysteine proteinases). On the other hand, desmin and m-calpain are both present at the onset of myoblast fusion and throughout this phenomenon. Based on these observations, the aim of this study was to demonstrate, with cultured rat myoblasts, that the amount of desmin decreased significantly as multinucleated myotubes were formed. Using immunoblot analysis, it has been shown that the desmin concentration decreased 41% as myoblasts fuse. Moreover, under conditions which stimulate myoblast fusion, desmin concentration was reduced by 21% compared to the control culture. Under our experimental conditions, which lead to a reduced desmin level, the amount of m-calpain was increased about three-fold. These results suggested that m-calpain could be involved in myoblast fusion via desmin cleavage. This hypothesis was confirmed by the results obtained after calpeptin treatment. In the presence of this cell-penetrating inhibitor of calpains, desmin seems not to be degraded. Taking into account the observations obtained after different hydrolysis assays and as compared to those observed on cultured cells, it seems conceivable that m-calpain would be able to initiate desmin cleavage leading to the formation of proteolytic fragments which should be immediately degraded.

Rearrangement of intermediate filament network of BHK-21 cells infected with vaccinia virus

Association between vaccinia virus (VV) structures and intermediate filaments in specific areas of the cytoplasm of infected cells (virus "factories") suggests that VV infection interferes with the cellular architecture by modifying the intermediate filament network. To analyse this question, we examined the array of intermediate filaments of BHK-21 cells infected with VV by laser scanning confocal microscopy using an anti-vimentin mouse monoclonal antibody. We observed a marked reorganization of intermediate filaments around the nucleus of infected cells. Bidimensional analysis of 32PO4-labeled intermediate filament proteins revealed that the acidic isoform of vimentin and two isoforms of desmin have increased phosphorylation levels in infected cells. Our results suggest that the reorganization of intermediate filaments observed during VV infection could be promoted by an increase in the phosphorylation level of the intermediate filament proteins, vimentin and desmin.

Desmin pathology in neuromuscular diseases

Desmin is an intermediate filament protein that in striated muscle is normally located at Z-bands, beneath the sarcolemma, and prominently at neuromuscular junctions. It is abundant during myogenesis and in regenerating fibers, but decreases in amount with maturation; in regenerating and denervated muscle fibers it is co-expressed with vimentin. Aggregates of desmin occur as nonspecific cytoplasmic bodies or cytoplasmic spheroid complexes, similar to the aggregates of keratin filaments in Mallory bodies or the neurofilament aggregates in Lewy bodies. In all three instances, alpha-B crystallin may be associated with desmin. There are now increasing numbers of neuromuscular disorders in which abnormal amounts of desmin, some abnormally phosphorylated, feature prominently in muscle fibres. Several of these diseases, including spheroid body myopathy, granulo-filamentous body myopathy and the dystrophinopathies, are familial. Ultrastructural and immunohistochemical studies of desmin have considerably broadened our understanding of the pathology of the cytoskeleton in muscle fibers and in certain hereditary neuromuscular diseases.

Chemical crosslinking with disuccinimidyl tartrate defines the relative positions of the two antiparallel coiled coils of the desmin protofilament unit

Filaments formed by desmin, the myogenic intermediate filament protein, were crosslinked with the lysine specific crosslinker DST (disuccinimidyl tartrate; 0.64 nm span) and three DST crosslinked peptides were characterized. Two correspond to crosslinks previously obtained with the longer crosslinker EGS (ethylene glycol bis(succinimidylsuccinate), 1.61 nm span) which defined the antiparallel on-stagger relationship of neighbouring coiled coils. The two DST crosslinks now provide the relative positions of the coiled coils within a limit of about 9 alpha-helical residues. The third DST crosslink most likely connecting two helices of a single coiled coil gives a direct measure of the distance spanned in DST crosslinks.

A synthetic peptide representing the consensus sequence motif at the carboxy-terminal end of the rod domain inhibits intermediate filament assembly and disassembles preformed filaments

All intermediate filament (IF) proteins share a highly conserved sequence motif at the COOH-terminal end of their rod domains. We have studied the influence of a 20-residue peptide, representing the consensus motif on filament formation and stability. Addition of the peptide at a 10-20-fold molar excess over keratins K8 plus K18 had a severe effect on subsequent IF assembly. Filaments displayed a rough surface and variable diameters with a substantial amount present in unravelled form. At higher peptide concentration (50-100-fold molar excess), IF formation was completely inhibited and instead only loose aggregates of "globular" particles were formed. The peptide also influenced performed keratin IF in a dose-dependent manner. While a three-fold molar excess was sufficient to cause partial fragmentation of IF, a 50-fold molar excess caused complete disassembly within 5 min. Loosely associated protofibrils, short needlelike IF fragments, and aggregates of globular particles were detected. The motif peptide also caused the disassembly of filaments formed by desmin, a type III IF protein. Peptide concentrations and incubation times required for complete disassembly were somewhat higher than for the filaments containing K8 plus K18. A 50-fold molar excess was sufficient to cause complete disassembly within 1 h. Peptides unrelated in sequence to the motif did not interfere with filament formation or stability even when present for more than 12 h at a 100-fold molar excess. The results suggest that the motif sequence normally binds to a specific acceptor site for which the motif peptide can successfully compete. Taken together with current models of IF structure the results indicate that normal binding of the motif sequence to its acceptor must play an essential role in IF formation, possibly by directing the proper alignment of neighboring tetramers or protofilaments. Finally we show that in vitro formed IF are much more sensitive and dynamic strutures than previously thought.

Autoimmune syndrome after neonatal induction of tolerance to alloantigens: analysis of the specificity and of the cellular and genetic origin of autoantibodies

BALB/c mice neonatally injected with 10(8) semiallogeneic (C57BL/6 x BALB/c)F1 spleen cells become tolerant to the H-2b alloantigens, but also develop a wide range of autoimmune manifestations characteristic of systemic lupus erythematosus (SLE). Indeed, in these mice, the presence of a hypergammaglobulinaemia, autoantibodies--including anti-ssDNA, anti-platelet, thymocytotoxic and rheumatoid factor antibodies--circulating immune complexes, cryoglobulins as well as renal glomerular deposition of immunoglobulins have been observed. In this study, we have shown that the allogenic effect and B cell chimaerism which characterize these F1 cell-injected mice is associated with the expression of a large spectrum of autoantibodies, including anti-ssDNA and anti-cytoskeleton antibodies, and that these autoantibodies are not multispecific. We took advantage of the fact that, in this model, autoantibodies are exclusively produced by F1 donor B cells to inject newborn BALB/c mice with F1 Xid spleen cells lacking the CD5+ B cell subset. Injection of 2 x 10(8) F1 Xid spleen cells triggers the production of anti-ssDNA as well as anti-BrMRBC antibodies, and these mice developed tissue lesions. Finally, analysis of the VH gene family expressed by monoclonal autoantibodies derived from F1 cell-injected mice showed that they used the 2 largest families J558 and 7183. These results suggest that the allogenic effect and B cell chimerism which characterize the neonatal induction of tolerance to MHC alloantigens is associated with the selective triggering of autoreactive B cells producing monospecific IgG autoantibodies. They also imply that upon stimulation by persisting alloreactive CD4+ T cells, either CD5- B cells are able to produce autoantibodies or autoantibody-producing CD5+ B cells can differentiate from Xid spleen cells.

Site-specific antibodies block kinase A phosphorylation of desmin in vitro and inhibit incorporation of myoblasts into myotubes

Desmin and vimentin are two type III intermediate filament (IF) proteins, which can be phosphorylated in vitro by cAMP-dependent kinase (kinase A) and protein kinase C, and the in vitro phosphorylation of these proteins appears to favor the disassembled state. The sites of phosphorylation for desmin and vimentin have been mapped to their amino-terminal headpiece domains; in chicken smooth muscle desmin the most kinase A-reactive residues are ser-29 and ser-35. In this study we have examined the phosphorylation of desmin by the catalytic subunit of kinase A by using anti-peptide antibodies directed against residues 26-36. The antibodies, which we call anti-D26, recognize both native and denatured desmin and can discriminate between intact desmin and those derivatives that do not possess residues 26-36. Pre-incubation of desmin with affinity purified anti-D26 blocks total kinase A catalyzed incorporation of 32P into desmin by 75-80%. When antibody-treated IFs are subjected to phosphorylation, no filament break-down is observed after 3 hours. Thus anti-D26 antibodies block phosphorylation of IF in vitro. We have also explored the role of desmin phosphorylation in skeletal muscle cell differentiation using these antibodies. Quail embryo cells, induced to differentiate along the myogenic pathway by infection with avian SKV retroviruses expressing the ski oncogene, were microinjected with affinity purified anti-D26 at the mononucleated, myoblast stage. By 24 h post-injection, the vast majority of uninjected cells had fused into multinucleated myotubes, but all microinjected cells were arrested in the process of incorporating into myotubes and remained mononucleated. This observation suggests that kinase A phosphorylation-induced dynamic behavior of the desmin/vimentin IF cytoskeleton may be one of the many cytoskeletal restructuring events that must take place during myoblast fusion.

Properties of the desmin tail domain: studies using synthetic peptides and antipeptide antibodies

Intermediate filament (IF) proteins have a common structural motif consisting of an alpha-helical rod domain flanked by non-alpha-helical amino-terminal head and carboxy-terminal tail domains. Coiled-coil interaction between neighboring rod domains is though to generate the backbone of the 10-nm filament. There must also be other interactions between subunits to bring them into alignment and to effect elongation of the filament, but these are poorly understood. To examine the involvement of the tail domain in filament structure and stabilization, we have studied the interaction between a synthetic peptide corresponding to residues 442-450 of avian desmin, and authentic desmin protein. The potential importance of this region lies in its hydrophilic nature and its high degree of homology among the Type III IF proteins and cytokeratins 8 and 18. The peptide, D442-450, binds to a 27-residue region between lys-436 and leu-463, the carboxy terminus. The presence of the peptide during assembly causes the filaments to appear much more loosely packed than normal desmin IF. We have also generated polyclonal antibodies against this peptide and attempted to localize this portion of the tailpiece along desmin IFs by immunological procedures. By immunoblotting, we found that anti-D442-450 antibodies recognize desmin and only those proteolytic fragments that contain the tailpiece. In contrast, the antibodies do not label any structure in adult gizzard smooth muscle and skeletal muscle myofibrils in immunofluorescence experiments during which conventional antidesmin antibodies do. At the ultrastructural level, anti-D442-450 antibodies label free desmin tetramers but not desmin IFs. These results show that, as part of an assembled IF, the epitope of anti-D442-450 is inaccessible to the antibodies, and suggest that either the tailpiece of an IF protein may not be entirely peripheral to the filament backbone, or the interaction between end domains during assembly masks this particular region of the IF molecule.

Immunocytochemistry of intermediate filaments in cultured arterial smooth muscle cells: differences in desmin and vimentin expression related to cell of origin and/or plating time

The objective of this study was to determine whether intermediate filament expression, including desmin and vimentin, in cultured smooth muscle cells (SMCs) is related to cyto-differentiation or proliferation. Using antibodies to desmin and vimentin, we studied by immunoperoxidase technique the distribution of these proteins in subcultured SMCs derived from porcine aorta and coronary artery. In addition, the proliferative potentiality of the cells was estimated by the incorporation of [3H]thymidine into DNA. The frequency of desmin-positive cells in coronary arterial SMCs of 3 and 6 population doubling levels was significantly higher as compared to findings with the aortic SMCs and depended on the plating time. No difference was evident at the 12 population doubling level. In contrast, vimentin was present in the majority of both aortic and coronary arterial SMCs. With regard to the localization of vimentin, two cell types were observed, one had reaction products to vimentin in both perinuclear and cell-peripheral areas (type-I cell), the other only in the cell-peripheral region (type-II cell). The relative proportion of the type-I and -II cells varied with the period of culture. Most of the SMCs showed the type-I cell on the first day and the number of type-II cells was increased on the sixth day. Quiescent SMCs in serum-free media had the same percentage of desmin-positive cells and frequency distribution of type-I and -II cells as did the proliferating SMCs incubated in media containing 5% serum. These results suggest that intermediate filament expression, including desmin and vimentin in cultured SMCs, is related to cell origin and/or plating time, but not to the proliferating activity, per se.

Myosin isoforms and cell heterogeneity in vascular smooth muscle. I. Developing and adult bovine aorta

Monoclonal anti-smooth muscle (SM-E7, SM-F11, and BF-48) and anti-nonmuscle (NM-A9 and NM-G2) myosin antibodies, Western blotting, and immunocytochemical procedures were used to study myosin isoform composition and distribution in the smooth muscle (SM) cells of bovine aorta differentiating in vivo and in vitro. Two myosin heavy chain (MHC) isoforms were identified by SM-E7 in adult aorta: SM-MHC-1 (Mr = 205 kDa) and SM-MHC-2 (Mr = 200 kDa), respectively. When tested with the SM-F11 antibody, SM-MHC-2 isoform showed distinct antigenic properties compared to SM-MHC-1. Two bands of 205 and 200 kDa were also present in the aortic SM tissue from 3-month-old fetus and were equally recognized by the BF-48 antibody. The 200-kDa SM myosin isoform was labeled by SM-F11 but not by SM-E7, thus indicating the existence of a fetal-specific SM-MHC-2 isoform. At the cellular level, both developing and adult bovine aortic tissues showed the existence of distinct patterns of myosin isoform expression. Three or even more aortic cell populations are differently distributed in areas which appear as (1) a network of interconnecting sheet-like or compact tissue (early fetus) and (2) enriched of collagenous-elastic or muscular tissue (adult animal). In addition, the SM-MHC-2 isoform of the fetal type appears to be uniquely distributed in cultured SM cells grown in vitro from adult bovine aortic explants. Our data indicate that in bovine aorta (1) MHC isoform expression is developmentally regulated and (2) the distribution of myosin isoforms is heterogenous both among and within aortic cells. These findings may be related to the distinct physiological properties displayed by SM during vascular myogenesis.

Changes in the composition of cytoskeletal and cytocontractile proteins of rat aortic smooth muscle cells during aging

Cytoskeletal proteins are used as differentiation markers of vascular smooth muscle cells (SMC). To study possible changes in SMC phenotype during aging, cytoskeletal and cytocontractile proteins were quantified in the aortic intima-medias of 4-, 12-, 30-, and 36-month-old rats by one- and two-dimensional gel electrophoresis. The percentages of myosin and desmin in total protein decreased with age, while those of actin and vimentin remained unchanged. Immunohistochemical comparison of the aortas from 4- and 30-month-old rats showed that the reduction of desmin reflected a selective disappearance of desmin in some cells. There was an age-related increase in the proportion of beta-actin at the expense of the alpha-isoform. Our results suggest an age-dependent modulation of the phenotype of vascular SMC towards the synthetic state, which is opposite to that observed during developmental differentiation.

An immunohistochemical study of canine tissues with vimentin, desmin, glial fibrillary acidic protein, and neurofilament antisera

In a wide range of canine tissues the immunoreactivity with commercially available antisera against intermediate filament antigens viz. vimentin, desmin, glial fibrillary acidic protein (GFAP) and neurofilament proteins, was studied. In addition, the results of formalin and Carnoy fixation were compared. Carnoy fixation appeared to result in optimal reactivity for all antisera. Epithelial cells did not react with any of the antisera, with exception of ovarian surface epithelium, which showed staining with the vimentin and desmin antisera. The vimentin antiserum induced staining of several cell types viz. fibroblasts, endothelial cells, chondrocytes, Schwann cells, ependymal cells, astrocytes, Leydig cells, synovial cells, podocytes and some parietal cells of Bowman's capsule. Sertoli cells showed a faint staining reaction. Muscle cells in various tissues reacted with the desmin antiserum. In the kidney a varying number of parietal cells appeared to react as did a restricted number of epithelial cells of proximal tubules and loops of Henle. GFAP reactivity was confined to glial cells, predominantly fibrous astrocytes, Schwann cells and axons. Additionally, some neuronal cell bodies in peripheral ganglia showed staining of varying intensity. Neurofilament staining was restricted to axons and some neurons. The immunoreactivity of canine tissues with these antisera is compared to findings in other species. The results confirm a broad interspecies cross-reactivity of these antisera. They can be used in studying the nature of canine tissues.

Immunofluorescent localization of desmin and vimentin in developing cardiac muscle of Syrian hamster

The distributions of desmin and vimentin were examined in frozen sections of cardiac muscle from embryonic, newborn, and adult Syrian hamster by using immunofluorescent methods. Frozen sections of newborn and adult skeletal muscle were used for comparison. Cardiac myocytes from day 9 in utero embryos already show a clear association of desmin with the sarcomeric myofibrils. In newborn hearts, desmin is localized in the myofibrillar Z-line areas as well as in the peripheral cytoplasm of the cell. Three days after birth, desmin is associated with the intercalated discs. Thus, in adult cardiac muscle, desmin is present in both Z-bands and intercalated discs. Skeletal muscle of newborn and adult hamster also contains desmin associated with the Z-lines of myofibrils. Vimentin is associated with the myofibrils of day 9 in utero cardiac muscle cells. The protein remains associated with the myofibrillar Z-lines in the newborns and adults. No detectable staining for vimentin was observed in newborn or adult hamster skeletal muscle. The existence of vimentin as well as desmin in differentiated cardiac muscle may be a consequence of the somewhat more epithelial-like nature of cardiac cells as compared to skeletal muscle syncitia.

Desmin-containing intermediate filaments: structural analysis using monoclonal antibodies

1. Monoclonal antibodies have been raised against N- and C-terminal desmin sequence regions. 2. The antibodies decorated desmin intermediate filaments in a helical fashion with four antibody molecules per helix turn. 3. The filaments could be decorated with both types of antibody consecutively. 4. These results support a model of intermediate filament assembly in which the tetrameric protofilaments are aligned in a staggered fashion with partial overlapping of the central rod domain regions of the desmin sequence.

A new monoclonal antibody, UFT-4, reacting with rat Kupffer cells. Immunohistochemical and immunoelectron microscopical analysis with reestimation of the reticuloendothelial system

A new monoclonal antibody, designated UFT-4, reacting with rat Kupffer cells, was produced using sinusoidal liver-cell fraction as an immunogen. Immunohistochemically, UFT-4 reacted with Kupffer cells, interdigitating cells (IDCs), sinus endothelial cells of the spleen, a proportion of reticulum cells, smooth muscle fibers, choroid plexus epithelia and some macrophages. Immunoelectron microscopy disclosed two types of intracytoplasmic positivity: a striated type and a diffuse type. Both appeared to depend on variations in the quantity and distribution of the same antigen. On the other hand, UFT-4 gave a negative result for blood monocytes and most macrophages in lymphatic sinuses, lymphoid follicles, splenic red pulp and loose connective tissue. Cells positive for UFT-4 were mostly considered to belong to a narrow spectrum of the classical reticuloendothelial system, explaining the close relationship existing between some endothelia and some macrophages or reticulum cells. SDS-PAGE analysis of immunoprecipitates showed that the antigen reacting with UFT-4 was of molecule of 36-42 kDa under reducing conditions in the presence of 2-mercaptoethanol. The present results suggest that UFT-4 will be very useful for the study of Kupffer cells and for reconsidering their origin and destination.

Desmin heterogeneity in the main electric organ of Electrophorus electricus

Desmin, the muscle-specific intermediate filament protein was purified from the main electric organ of Electrophorus electricus. It is shown that pure desmin can be separated into 5 isoforms presenting different isoelectric points. These isoforms have similar molecular weight, react with an antibody directed against desmin and generate identical peptides after digestion with protease V8 from Staphylococcus aureus.

Storage of phosphorylated desmin in a familial myopathy

The quantity and the electrophoretic characteristics of desmin were analyzed in a familial skeletal muscle disorder, characterized by the intra-sarcoplasmic accumulation of an electron-dense granulo-filamentous material facing the Z-lines and reacting strongly with polyclonal anti-desmin antibodies. The analysis was performed on biopsies from the deltoid muscles of 4 patients, members of 2 families. In the 4 biopsies, an increase in the relative amount of desmin compared to that of actin or insoluble proteins (3 fold) and in the number of isovariants (6 instead of 3) was observed. The isovariants of desmin were similar to those described in Purkinje fibres of the heart as a phosphorylated form of the protein [(1987) Eur. J. Cell Biol. 44, 68-78]. Therefore, post-translational events could affect both the polymerization and the amount of desmin filaments in this autosomal dominant familial myopathy.

Identification of the conserved, conformation-dependent cytokeratin epitope recognized by monoclonal antibody (lu-5)

The epitope recognized by the murine monoclonal antibody (mAB lu-5) recently described as a formaldehyde-resistant, "pan-epithelial marker" of great value in tumour diagnosis is located on the surface of cytokeratin filaments. It has been preserved during vertebrate evolution from amphibia to man. As this epitope is not reactive after SDS-polyacrylamide gel electrophoresis (SDS-PAGE), the epitope-bearing protein has been identified by a dot-blot antibody binding assay, using purified proteins in which the epitope is reconstituted. We show that the epitope is present in most cytokeratin polypeptides of both the acidic (type I) and basic (type II) subfamily but does not occur in other cytoskeletal proteins. The location of this widespread epitope is discussed with respect to homologies of amino acid sequences of cytokeratins and their conformations.

Characterization of dimer subunits of intermediate filament proteins

The fundamental subunit of the various types of intermediate-sized filaments (IF) has been shown to be a tetramer that is thought to represent a double dimer, i.e. an array of two laterally packed coiled-coils of alpha-helices. The two-chain state of intact IF proteins had up to this point not been isolated and characterized as has been done for other fibrous alpha-helical coiled-coil proteins. Using buffers containing 3 M-guanidinium hydrochloride we prepared dimers by depolymerization of IF or by reconstitution from fully denatured molecules. Dimers of desmin (from chicken gizzard), vimentin (from bovine lens tissue and cultured human fibroblasts) and the neurofilament protein NF-L (from bovine brain) as well as in vitro formed homodimers of human and rat cytokeratins numbers 8 (A), 18 (D) and 19 ("40K"), are characterized by ultracentrifugation techniques (sedimentation velocity and equilibrium), electron microscopy and chemical cross-linking. The results show that IF proteins from discrete complexes of two polypeptide chains in parallel orientation and probably in coiled-coil configuration, which apparently have a high tendency to further associate into double dimers. Implications of these results for concepts of IF organization and IF protein assembly are discussed.

Cytoskeletal features of normal and atheromatous human arterial smooth muscle cells

The distribution of actin, vimentin, desmin, and tropomyosin was studied in the media of the human aorta and femoral and coronary arteries, as well as in atheromatous plaques from the same arteries, by means of immunofluorescence, densitometric analysis of sodium dodecylsulfate-polyacrylamide gel electrophoresis, and bidimensional gel electrophoresis. The proportions of desmin-containing cells varied in the media of different arteries; 4 per cent of the cells in the aorta, 11 per cent in the coronary artery, and 37 per cent in the femoral artery contained desmin. In fibrous atheromatous plaques, independently of the artery, desmin-containing cells were almost absent, but they reappeared in complicated lesions. The content of vimentin per smooth muscle cell increased in fibrous atheromatous plaques, whereas the content of actin and tropomyosin was less than in normal media. Moreover, the alpha-actin predominance observed in the media was transformed to beta-actin predominance in the atheromatous plaques. These cytoskeletal changes provide new, possibly useful, biochemical markers for the characterization of smooth muscle cells during early and advanced phases of atheroma formation.

Desmin-containing stellate cells in rat liver; distribution in normal animals and response to experimental acute liver injury

Using immunohistochemical methods, we have confirmed that the perisinusoidal cells in rat liver express the intermediate filament protein, desmin, and we have used this marker for identification of the cells on light microscopy. The study has been extended to quantify the response of perisinusoidal cells to acute liver injury induced by carbon tetrachloride. A significant increase in desmin-positive cells was observed in areas of necrosis as early as 48 h following the administration of a single bolus of carbon tetrachloride. This reached a peak at 72 h, with a five-fold increase in desmin-positive cells in areas of necrosis. These observations are consistent with the hypothesis that perisinusoidal cells are involved in the response to acute liver injury. Anti-desmin antibodies are of potential value in further characterizing the functional role of perisinusoidal cells in normal and diseased liver.

Are the terminal domains in intermediate filaments organized as octameric complexes? Reevaluation of a recent suggestion

Recently L. M. Milam and H. Erickson ((1985) J. Ultrastruc. Res. 90, 251-260) reported the isolation of a particle thought to be an octomeric complex of the terminal domains of desmin. This complex was isolated after trypsin treatment of intact filaments. As the existence of such a complex would place important restrictions on the possible packing modes of subunits within the filament we have repeated their procedure and additionally characterized the particle in question by detailed protein chemical data. We find that the particle is not derived from the terminal domains but instead comprises a portion of the carboxy-terminal half of the alpha-helical rod domain. We further show that the terminal domains are very rapidly digested into small peptides during the trypsin treatment of the filaments. No inferences on the structure of intermediate filament are therefore possible from the data in the original report.

Tenacious binding of lipids to vimentin during its isolation and purification from Ehrlich ascites tumor cells

Vimentin enriched in cytoskeletal frameworks by Triton X-100 extraction of Ehrlich ascites tumor cells and purified from a low ionic strength extract of the cell residues by (NH4)2SO4 precipitation and DEAE-Sepharose and ssDNA-cellulose chromatography in the presence of 6 M urea was highly contaminated with lipids. Thin-layer chromatography of a chloroform-methanol extract of the purified protein revealed, besides small amounts of phospholipids, the presence of large quantities of neutral lipids.

Pair formation and promiscuity of cytokeratins: formation in vitro of heterotypic complexes and intermediate-sized filaments by homologous and heterologous recombinations of purified polypeptides

Cytokeratins are expressed in different types of epithelial cells in certain combinations of polypeptides of the acidic (type I) and basic (type II) subfamilies, showing "expression pairs." We have examined in vitro the ability of purified and denatured cytokeratin polypeptides of human, bovine, and rat origin to form the characteristic heterotypic subunit complexes, as determined by various electrophoretic techniques and chemical cross-linking, and, subsequently, intermediate-sized filaments (IFs), as shown by electron microscopy. We have found that all of the diverse type I cytokeratin polypeptides examined can form complexes and IFs when allowed to react with equimolar amounts of any of the type II polypeptides. Examples of successful subunit complex and IF formation in vitro include combinations of polypeptides that have never been found to occur in the same cell type in vivo, such as between epidermal cytokeratins and those from simple epithelia, and also heterologous combinations between cytokeratins from different species. The reconstituted complexes and IFs show stability properties, as determined by gradual "melting" and reassociation, that are similar to those of comparable native combinations or characteristic for the specific new pair combination. The results show that cytokeratin complex and IF formation in vitro requires the pairing of one representative of each the type I and type II subfamilies into the heterotypic tetramer but that there is no structural incompatibility between any of the members of the two subfamilies. These findings suggest that the co-expression of specific pair combinations observed in vivo has other reasons than general structural requirements for IF formation and probably rather reflects the selection of certain regulatory programs of expression during cell differentiation. Moreover, the fact that certain cytokeratin polypeptide pairs that readily form complexes in vitro and coexist in the same cells in vivo nevertheless show preferential, if not exclusive, partner relationships in the living cell points to the importance of differences of stabilities among cytokeratin complexes and/or the existence of extracytokeratinous factors involved in the specific formation of certain cytokeratin pairs.

Intermediate filament forming ability of desmin derivatives lacking either the amino-terminal 67 or the carboxy-terminal 27 residues

Amino acid sequence data and results from limited proteolytic digestion have been used to define the three-domain structure of intermediate filament proteins. A centrally located highly alpha-helical domain of about 310 residues well-conserved in sequence principles and length is flanked by the highly variable sequences of the non-alpha-helical headpiece and tailpiece. A direct involvement in filament formation of one or both terminal domains was previously proposed for desmin since chymotryptic removal of head and tailpiece provided a derivative unable to form filaments. In order to evaluate directly the importance of these regions we have prepared desmin derivatives lacking either the amino-terminal 67 (T-desmin) or carboxy-terminal 27 residues (L-desmin). Whereas the latter derivative is fully polymerization-competent the fragment lacking only the basic and arginine-rich headpiece cannot form filaments on its own and remains in a protofilamentous stage. These structures of T-desmin are not incorporated into filaments when mixed with protofilaments of desmin. If, however, the two proteins are mixed in 7 M-urea subsequent dialysis provides morphologically normal filaments containing T-desmin. The results suggest that at least certain hybrid protofilaments containing less than four headpieces are accepted in the filament. The removal of the 27 carboxy-terminal residues in L-desmin, although not interfering with filament formation, leads to a change in surface since filaments show lateral aggregation at 170 mM but not at 50 mM salt. The results are discussed in relation to current models of intermediate filament structure.

Patterns of expression and organization of cytokeratin intermediate filaments

Cytokeratins are a large multigene family comprising two polypeptide types, i.e. acidic (type I) and basic (type II) ones, which are distinguished on the basis of immunological, peptide mapping, mRNA hybridization, and primary amino acid sequence data. The acidic (type I) cytokeratins can be subdivided into at least two different subtypes on the basis of their carboxy-terminal sequences. Considerable interspecies conservation of sequences exists, even extending to the 3'-non-coding mRNA regions. Different pairs of type I and II cytokeratins show different resistance to dissociation in urea. Sequence differences of the type I cytokeratins containing functional domains may be an explanation of the observed preference of co-expression with certain type II cytokeratins. The distribution of the different type I and II cytokeratins in normal epithelia and in carcinomas is differentiation related and can be used for cell typing and identification. The cell type-specific expression of cytokeratin polypeptides is recognized at both the protein and the mRNA level. The building block of cytokeratin IFs is a heterotypic tetramer, consisting of two type I and two type II polypeptides arranged in pairs of laterally aligned coiled coils. This principle of tetrameric organization is thought to be generally applicable to IFs.

Cytoskeletal features of rat aortic cells during development. An electron microscopic, immunohistochemical, and biochemical study

Actin, vimentin, desmin, and tropomyosin distribution in rat aortic endothelial and smooth muscle cells has been studied during development using fetal (18 to 20 days of gestation), and 5- and 14-day-, and 5-, and 12-week-old rats. Endothelial cells of newborn animals actively replicate and contain many actin stress fibers, whereas, in adult animals, replication is minimal and actin stress fibers are rare. The actin, vimentin, desmin, and tropomyosin content of smooth muscle cells increases gradually from fetal to adult animals. The number of desmin-containing cells also increases from 13% in fetal rats to 51% in adult rats. The beta-actin isoform is predominant in fetal and newborn animals, but gradually the alpha-isoform becomes quantitatively the most important, as seen by bidimensional polyacrylamide gels. Several analogies exist between the features of developing smooth muscle and what is known for developing striated muscle cells. The evolution of cytoskeletal features from fetal to adult animals is remarkably the opposite of what takes place in: (1) rat aortic smooth muscle cells proliferating after an endothelial injury, (2) human arterial smooth muscle cells present in atheromas, and (3) actively growing rat aortic smooth muscle cells in vitro. Thus, the assumption that pathological or cultured smooth muscle cells are "dedifferentiated" is supported by our biochemical observations.