At least six different actins are expressed in a higher mammal: an analysis based on the amino acid sequence of the amino-terminal tryptic peptide

Proteins associated with purified human cytomegalovirus particles

Virion-associated proteins isolated from purified human cytomegalovirus particles (strain AD169) were used as substrates for chemical sequence analysis. Extracellular virions, noninfectious enveloped particles, and dense bodies were purified by negative viscosity-positive density gradient centrifugation, and their component proteins were separated by denaturing polyacrylamide gel electrophoresis. The deduced amino acid sequence of individual protein bands was used to identify six corresponding viral genes whose products have not previously been identified as virion constituents: UL47, UL25, UL88, UL85, UL26, and UL48.5. In addition, a 45-kDa cellular protein was identified, and the protein fragments sequenced have a high degree of amino acid identity with actin. However, antiactin monoclonal and polyclonal antibodies did not react with a specific protein in the virus preparations, suggesting that this 45-kDa protein is an immunologically distinct isoform of actin. The newly identified viral and cellular proteins were resistant to protease treatment of purified virions, suggesting that they are unlikely to be contaminants of the viral preparations.

Introns and protein revolution--an analysis of the exon/intron organisation of actin genes

A catalogue of intron positions obtained from a large number of actin genes has been compiled with a view to understanding the possible origin of intervening sequences. Actins are ubiquitous proteins conserved in evolution and an analysis of their gene structures from various organisms has revealed that there may be at least 25 intron positions distributed at different positions in the coding regions. A comparison of intron positions from a wide range of organisms from that of yeast to human actins shows that introns could be more ancestral in origin. The conservation in the observed intron patterns within the different tissue types hints at a possible functional significance of introns in present day actin genes.

Differential assembly of cytoskeletal and sarcomeric actins in developing skeletal muscle cells in vitro

Monoclonal antibodies (McAb) to actin were prepared to analyze the assembly of actin isoforms in developing muscle cells in vitro. One of the antibodies (SkA-06) was specific for alpha-sarcomeric actin isoforms in skeletal and cardiac muscles, while the others recognized cytoskeletal (beta, gamma) actin isoforms in smooth muscle and non-muscle tissues as well as the sarcomeric (alpha) actins. Using SkA-06 and a polyclonal antibody (PcAb) specific for cytoskeletal actins, the subcellular localization of the actin isoforms was examined by immunocytochemical methods. While in developing young myotubes, cytoskeletal and sarcomeric actins were co-localized in nascent myofibrils or stress-fiber-like structures, sarcomeric actins predominated in striated myofibrils in more developed myotubes. When FITC-labeled cytoskeletal and sarcomeric actins were introduced into young myotubes by a microinjection method, the latter became detectable in striated structures sooner than the former but they were finally incorporated into striated myofibrils. These results suggest that alpha-actin(s) as well as beta- and gamma-actins can be incorporated into myofibrils, but alpha-actin(s) is assembled preferentially into myofibrils in developing muscle cells.

Involvement of gamma and beta actin isoforms in mouse neuroblastoma differentiation

Two actin isoforms, gamma and beta, are contained within neuroblastoma cells. However, the relative amount and distribution of both isoforms within the cells are differentially regulated during neurite extension. The proportion of gamma-actin isoform became about four times greater than that of beta actin during neuroblastoma cell differentiation. Additionally, whereas beta actin appears to be concentrated in the cell cortex, gamma actin is also present throughout the cell body. Upon differentiation, neuroblastoma cells reorganize their actin cytoskeleton and gamma actin is induced to polymerize whereas beta actin polymers are partially disassembled. Moreover, both actin isoforms are differentially distributed within differentiated cells. Thus, gamma actin polymers are located both in the soma and proximal regions of extended neurites, whereas beta actin is enriched in the terminal tip of the neurites. Our results strongly suggest that both actin isoforms are involved in a different way in neuroblastoma cell differentiation.

Localization of actin in basal cells of stria vascularis

The distribution of actin in the lateral wall of the cochlear duct was investigated in the gerbil, rat, mouse and hamster. A monoclonal antibody specific for muscle alpha and gamma actins, and a polyclonal antiserum reactive with smooth muscle gamma and non-muscle beta actins yielded strong immunostaining of basal cells in the stria vascularis and of smooth muscle cells in lateral wall blood vessels. Both cell types stained in all four genera. Diffuse cytosolic staining was observed along the full-length of the basal cell layer including the blunt cell processes which they extend toward strial marginal cells. Immunoreactive basal cells appeared continuous with morphologically similar cells investing vessels penetrating the stria from the spiral ligament. The basal cells failed to bind antibodies to smooth muscle alpha actin and sarcomeric actin. By electron microscopic immunocytochemistry, gold labeling for actin was observed on dense, fine fibrils in the cytoplasm of the basal cells. In paraffin sections adjacent to those stained for actin, antibody to vimentin stained intermediate and basal cells in the stria vascularis whereas antibody to isoform 1 of the facilitated glucose transporter protein family (GLUT1) labeled only the non-overlapping apical and basal plasmalemma of basal cells. Content of vimentin, GLUT1 and muscle gamma actin supports the derivation of basal cells from mesoderm. The presence of stress fibers containing muscle gamma actin points to a possible contractile activity of basal cells which conceivably could be related to transport of K+ to and within the intrastrial compartment or regulation of blood flow in the stria vascularis.

A function for filamentous alpha-smooth muscle actin: retardation of motility in fibroblasts

Actins are known to comprise six mammalian isoforms of which beta- and gamma-nonmuscle actins are present in all cells, whereas alpha-smooth muscle (alpha-sm) actin is normally restricted to cells of the smooth muscle lineages. alpha-Sm actin has been found also to be expressed transiently in certain nonmuscle cells, in particular fibroblasts, which are referred to as myofibroblasts. The functional significance of alpha-sm actin in fibroblasts is unknown. However, myofibroblasts appear to play a prominent role in stromal reaction in breast cancer, at the site of wound repair, and in fibrotic reactions. Here, we show that the presence of alpha-sm actin is a signal for retardation of migratory behavior in fibroblasts. Comparison in a migration assay of fibroblast cell strains with and without alpha-sm actin revealed migratory restraint in alpha-sm actin-positive fibroblasts. Electroporation of monoclonal antibody (mAb) 1A4, which recognizes specifically the NH2-terminal Ac-EEED sequence of alpha-sm actin, significantly increased the frequency of migrating cells over that obtained with an unrelated antibody or a mAb against beta-actin. Time-lapse video microscopy revealed migratory rates of 4.8 and 3.0 microns/h, respectively. To knock out the alpha-sm actin protein, several antisense phosphorothioate oligodeoxynucleotide (ODNs) were tested. One of these, 3'UTI, which is complementary to a highly evolutionary conserved 3' untranslated (3'UT) sequence of alpha-sm actin mRNA, was found to block alpha-sm actin synthesis completely without affecting the synthesis of any other proteins as analyzed by two-dimensional gel electrophoresis. Targeting by antisense 3'UTI significantly increased motility compared with the corresponding sense ODN. alpha-Sm actin inhibition also led to the formation of less prominent focal adhesions as revealed by immunofluorescence staining against vinculin, talin, and beta1-integrin. We propose that an important function of filamentous alpha-sm actin is to immobilize the cells.

Placental villous stroma as a model system for myofibroblast differentiation

Different subtypes of myofibroblasts have been described according to their cytoskeletal protein patterns. It is quite likely that these different subtypes represent distinct steps of differentiation. We propose the human placental stem villi as a particularly suitable model to study this differentiation process. During the course of pregnancy, different types of placental villi develop by differentiation of the mesenchymal stroma surrounding the fetal blood vessels. In order to characterise the differentiation of placental stromal cells in the human placenta, the expression patterns of the cytoskeletal proteins vimentin, desmin, alpha- and gamma-smooth muscle actin, pan-actin, smooth muscle myosin, and the monoclonal antibody GB 42, a marker of myofibroblasts, were investigated on placental tissue of different gestational age (7th-40th week of gestation). Proliferation patterns were assessed with the proliferation markers MIB 1 and PCNA. Additionally, dipeptidyl peptidase IV distribution was studied in term placenta and the ultrastructure of placental stromal cells was assessed by electron microscopy. Different subpopulations of extravascular stromal cells were distinguished according to typical co-expression patterns of cytoskeletal proteins. Around the fetal stem vessels in term placental villi they were arranged as concentric layers with increasing stage of differentiation. A variable layer of extravascular stromal cells lying beneath the trophoblast expressed vimentin (V) or vimentin and desmin (VD). They were mitotically active. The next layer co-expressed vimentin, desmin, and alpha-smooth muscle actin (VDA). More centrally towards the fetal vessels, extravascular stromal cells co-expressed vimentin, desmin, alpha- and gamma-smooth muscle actin, and GB 42 (VDAG). Cells close to the fetal vessels additionally co-expressed smooth muscle myosin (VDAGM). Ultrastructurally, V cells resembled typical mesenchymal cells. VD cells corresponded to fibroblasts, while VDA and VDAG cells developed features of myofibroblasts. Cells of the VDAGM-type revealed a smooth muscle cell-related ultrastructure. In earlier stages of pregnancy, stromal cell types with less complex expression patterns prevailed. The media smooth muscle cells of the fetal vessels showed a mixture of different co-expression patterns. These cells were separated from extravascular stromal cells by a layer of collagen fibres. The results obtained indicate a clearly defined spatial differentiation gradient with increasing cytoskeletal complexity in human placental stromal cells from the superficial trophoblast towards the blood vessels in the centre of the stem villi. The spatial distribution of the various stages of differentiation suggests that human placental villi could be a useful model for the study of the differentiation of myofibroblasts.

Actin-related proteins in Anabaena spp. and Escherichia coli

Actin has been described in all eukaryotic cells as the major microfilament cytoskeletal protein. Although prokaryotic cells do not have a cytoskeleton, proteins related to the latter have been found in different prokaryotic species. We have found prokaryotic actin-related proteins in the enterobacterium Escherichia coli and in the cyanobacteria Anabaena cylindrica and Anabaena variabilis. They were identified by the following criteria: (1) by cross-reaction with a fluorescent conjugated anti-actin (rat-brain) mAb by Western blot analysis (in total cellular extracts); (2) specific binding of acetone powder and soluble cellular extracts to DNase I; and (3) specific binding of cells and total cellular extracts to phalloidin. In E coli, specific binding of phalloidin labelled with rhodamine to cells was detected by spectrofluorometry. In total cellular extracts, three bands of 60, 43 and 35 kDa were weakly recognized by the mAb by Western blot analysis; this recognition increased when phalloidin was added to the extracts. Furthermore, three polypeptides of kDa were isolated by binding to DNase I, showing pI values of 6.7, 6.65 and 6.6, less acidic than all reported actin pI values. In A. cylindrica and A. variabilis, specific binding of phalloidin labelled with rhodamine to cells was also detected by spectrofluorometry. In total and soluble cellular extracts, the mAb recognized two bands of 45 and 40 kDa by Western blot analysis, but only the first was purified by binding to DNase I, and it showed three isoforms of pI values 6.8, 6.5 and 6.4. These results suggest the presence, in prokaryotes, of proteins with similar biochemical characteristics to eukaryotic actin.

Immunocytochemical localization of caldesmon and calponin in chicken gizzard smooth muscle

The distribution of caldesmon and calponin in chicken gizzard smooth muscle was investigated with immunofluorescence and immunogold electron microscopy. Immunofluorescence microscopy showed that in verapamil treated (relaxed) muscles the distributions of caldesmon and myosin appeared to be uniform throughout the cytoplasm, but clearly more textured than that of actin filaments as revealed by the distribution of tropomyosin. In shortened muscles both caldesmon and myosin became segregated, in contrast to the distribution of actin, which remained uniform. The distribution of calponin was even more textured, with no similarity to those of caldesmon or myosin. Instead, considerable overlap was observed between calponin and the cytoskeletal protein desmin and, to a lesser extent, beta-actin. By immunogold electron microscopy caldesmon appeared mostly near and around myosin filaments in both relaxed and shortened muscle. Calponin, on the other hand, was found primarily at the periphery of cytoskeletal structures in the same general region as desmin, and very often adjacent to beta-actin, which is mainly in the core. These observations indicated that caldesmon and calponin are associated with different subsets of actin filaments, caldesmon with contractile actin, while calponin with cytoskeletal actin. Thus the in situ localization of caldesmon is consistent with its proposed regulatory function. Calponin, on the other hand, is unlikely to directly regulate actomyosin interactions in these cells; instead, it may function as a bridging protein between the actin and the intermediate filament networks.

Biochemical characterization of ezrin-actin interaction

The highly related actin isoforms are thought to have different functions. We recently demonstrated a polarized distribution of actin isoforms in gastric parietal cells and association of gastric ezrin with the cytoplasmic beta-actin isoform (Yao, X., Chaponnier, C., Gabbiani, G., and Forte, J. G. (1995) Mol. Biol. Cell. 6, 541-557). Here we used ultrastructural immunocytochemistry to verify that beta-actin is located within canalicular microvilli and the apical cortex of parietal cells, similar to the localization reported for ezrin. Furthermore, we tested whether ezrin binds preferentially to cytoplasmic beta-actin compared with the skeletal muscle alpha-actin isoform. Purified cytoplasmic beta-actin (from erythrocytes) and skeletal alpha-actin were assembled with gastric ezrin. Co-sedimentation experiments showed that gastric ezrin selectively co-pelleted with the beta-actin isoform and only very poorly with alpha-actin. Binding of erythrocytic beta-actin to ezrin is saturable with a molar ratio of approximately 1:10 (ezrin:actin) and a dissociation constant approximately 4.6 x 10(-8) M. In addition, ezrin promoted pyrene-labeled actin assembly, with predominant effects on filament elongation and a distinct preference for beta-actin compared with alpha-actin. Given these isoform-selective associations, we speculate that actin isoforms might segregate into different functional domains and exert specificity by interacting with isoform-orientated binding proteins.

Comparison of the cell cycle regulated synthesis and phosphorylation of stress proteins, actin isoforms and a novel actin-like protein following drug administration in cultured rat lymphocytes

Administration of phytohemagglutinin initiated cycling of rat lymphocytes in vitro, and following treatment with this drug and other drugs in combination, lymphocytes were pulse labeled with [3H] leucine of [32P] phosphate. The nuclei were isolated from lymphocytes and collected from partitions of the cell cycle, and the proteins analyzed from fluorographs following gel electrophoresis for protein biomarkers after drug exposure. Stress proteins (sps) were dependent on a specific drug or drugs in combination (i.e., interleukin-2, bleomycin) for their synthesis that occurred only during the G1-phase of the cell cycle. An "actin-like" protein (A4) with electrophoretic mobilities similar to the actin complex, was synthesized in S and G2 phases and phosphorylated in all phases of the cell cycle only following the administration of drugs in combination. A4 exhibited a binding affinity for sp 24 that was cell cycle regulated (i.e., A4 from S phase did not bind with sp 24, but A4 from G2 phase did bind with the sp. Protein A4 appeared similar in some structural aspects to the nonmuscular actin isoform family but differed in epitope, suggesting a unique relationship and represented a stable protein, perhaps a product from the mutation of an actin gene. The dependence of certain sps and protein A4 for their induction by drugs in combination may serve as biomarkers of chemical interaction and toxicity.

Titin expression as an early indication of heart and skeletal muscle differentiation in vitro. Developmental re-organisation in relation to cytoskeletal constituents

Established myogenic cell lines of different species and tissue origin have been used to study expression and organisation of muscle-specific proteins during differentiation. Furthermore, primary cultures of rat myocard cells were used to examine these same processes during dedifferentiation. In particular, we were interested in the general mechanism that underlies the changes in the supramolecular organisation of titin during in vitro myogenesis. It became obvious that in the differentiating muscle cell cultures the redistribution of desmin, actin and myosin in a typical, differentiation state dependent fashion, always showed a certain delay when compared to titin. The sequence of changes in the assembly of cytoskeletal and sarcomeric structures observed during differentiation of the cell lines was reversed during the process of dedifferentiation in cultured rat myocard cells. These results all indicate that titin is an early marker of myogenic differentiation, both in vivo and in vitro, and the typical reorganisation of this giant molecule is independent of species or muscle cell type.

Pleomorphic soft tissue myogenic sarcomas of adulthood. A reappraisal in the mid-1990s

325 diverse sarcomas, 39 rhabdomyosarcomas (RMS), including all histologic variants, and 135 leiomyosarcomas (LMS) were identified. Within these two groups, 18 (46%) of the RMS and 14 (10%) of the LMS represented pleomorphic variants. These neoplasms were studied by morphology (histology and ultrastructure) and by immunohistochemical methods employing antibodies to intermediate filaments (vimentin and desmin) and actin isoforms [alpha-smooth (sm) and alpha-sarcomeric (sr) actins]. Twenty-four pleomorphic malignant fibrous histiocytomas (MFH) and eight pleomorphic liposarcomas (LS) were examined in a similar fashion. By light microscopy, the pleomorphic RMS, LMS, and MFH were indistinguishable, as each was dominated by pleomorphic cells disposed in a haphazard growth pattern; moreover, many featured fascicular, storiform, and sclerotic zones. The distinction between these neoplasms became apparent only following immunohistochemistry and/or ultrastructural study. All pleomorphic RMS disclosed rudimentary sarcomeres and exhibited the following cytoskeletal profile: vimentin (+) (18 of 18), desmin (+) (14 of 18), alpha-sr actin (+) (18 of 18) and alpha-sm actin (+) (five of 18). All the pleomorphic LMS featured smooth-muscle differentiation of variable degrees in the form of cytoplasmic bundles of microfilaments and associated dense bodies; their cytoskeletal profile was vimentin (+) (14 of 14), desmin (+) (seven of 14), alpha-sr actin (+) (none of 14), and alpha-sm actin (+) (eight of 14). The latter was demonstrated in all moderately differentiated, but absent or only focally expressed in poorly differentiated variants. All pleomorphic MFH and LS were devoid of myogenic (skeletal or smooth) ultrastructural features and expressed vimentin solely. This combined morphological and immunohistochemical study illustrates the following: First, these pleomorphic sarcomas are often indistinguishable by histologic growth pattern alone; thus, an accurate diagnosis requires study with all of these techniques. Second, pleomorphic myogenic sarcomas are restricted to adults and are not uncommon neoplasms among pleomorphic sarcomas: RMS (28%), LMS (21%), MFH (38%), and LS (13%). Third, the study defines desmin-negative and alpha-sm actin-positive pleomorphic RMS, and desmin-negative and alpha-sm-actin-negative pleomorphic LMS.

Dominant negative effect of cytoplasmic actin isoproteins on cardiomyocyte cytoarchitecture and function

The intracompartmental sorting and functional consequences of ectopic expression of the six vertebrate actin isoforms was investigated in different types of cultured cells. In transfected fibroblasts all isoactin species associated with the endogenous microfilament cytoskeleton, even though cytoplasmic actins also showed partial localization to peripheral submembranous sites. Functional and structural studies were performed in neonatal and adult rat cardiomyocytes. All the muscle isoactin constructs sorted preferentially to sarcomeric sites and, to a lesser extent, also to stress-fiber-like structures. The expression of muscle actins did not interfere with cell contractility, and did not disturb the localization of endogenous sarcomeric proteins. In sharp contrast, ectopic expression of the two cytoplasmic actin isoforms resulted in rapid cessation of cellular contractions and induced severe morphological alterations characterized by an exceptional outgrowth of filopodia and cell flattening. Quantitative analysis in neonatal cardiomyocytes indicated that the levels of accumulation of the different isoactins are very similar and cannot be responsible for the observed isoproteins-specific effects. Structural analysis revealed a remodeling of the cytoarchitecture including a specific alteration of sarcomeric organization; proteins constituting the sarcomeric thin filaments relocated to nonmyofibrillar sites while thick filaments and titin remained unaffected. Experiments with chimeric proteins strongly suggest that isoform specific residues in the carboxy-terminal portion of the cytoplasmic actins are responsible for the dominant negative effects on function and morphology.

The specific NH2-terminal sequence Ac-EEED of alpha-smooth muscle actin plays a role in polymerization in vitro and in vivo

The blocking effect of the NH2-terminal decapeptide of alpha-smooth muscle (SM) actin AcEEED-STALVC on the binding of the specific monoclonal antibody anti-alpha SM-1 (Skalli, O., P. Ropraz, A. Trzeviak, G. Benzonana, D. Gillessen, and G. Gabbiani. 1986. J. Cell Biol. 103:2787-2796) was compared with that of synthetic peptides modified by changing the acetyl group or by substituting an amino acid in positions 1 to 5. Using immunofluorescence and immunoblotting techniques, anti-alpha SM-1 binding was abolished by the native peptide and by peptides with a substitution in position 5, indicating that AcEEED is the epitope for anti-alpha SM-1. Incubation of anti-alpha SM-1 (or of its Fab fragment) with arterial SM actin increased polymerization in physiological salt conditions; the antibody binding did not hinder the incorporation of the actin antibody complex into the filaments. This action was not exerted on skeletal muscle actin. After microinjection of the alpha-SM actin NH2-terminal decapeptide or of the epitopic peptide into cultured aortic smooth muscle cells, double immunofluorescence for alpha-SM actin and total actin showed a selective disappearance of alpha-SM actin staining, detectable at approximately 30 min. When a control peptide (e.g. alpha-skeletal [SK] actin NH2-terminal peptide) was microinjected, this was not seen. This effect is compatible with the possibility that the epitopic peptide traps a protein involved in alpha-SM actin polymerization during the dynamic filament turnover in stress fibers. Whatever the mechanism, this is the first evidence that the NH2 terminus of an actin isoform plays a role in the regulation of polymerization in vitro and in vivo.

The monoclonal antibody GB 42--a useful marker for the differentiation of myofibroblasts

The expression patterns of a variety of cytoskeletal antigens were studied in normal human tissues (placenta, umbilical cord, myometrium, colon, mammary gland, testis, skeletal muscle, myocardium) as well as in abnormal human tissues (palmar fibromatosis, fibrocystic disease of the mammary gland, mammary carcinoma). The immunohistochemical binding patterns of the monoclonal antibody GB 42 were compared to those of commercial antibodies directed against vimentin, desmin, smooth muscle myosin, pan actin, alpha-smooth muscle actin and gamma-smooth muscle actin. Methods applied comprised immunohistochemistry on cryostat sections and paraffin sections. Immunogold immunocytochemistry was performed on Lowicryl sections. The patterns of GB 42-binding were confirmed biochemically by SDS-PAGE and Western-blotting, and quantitative amino acid analysis. Our data suggest that the monoclonal antibody GB 42 recognizes an actin isoform which is identical to, or closely related to, gamma-smooth muscle actin. Unlike the commercially available antibody against gamma-smooth muscle actin, GB 42 does not cross-react with alpha-skeletal or alpha-cardiac actins. The GB 42-antigen is expressed in smooth muscle cells, myoepithelial cells and in later stages of differentiation of myofibroblasts, in all the tissues investigated. Throughout the development of smooth muscle cells and myofibroblasts, the appearance of the GB 42-antigen occurs after the expression of vimentin, desmin and alpha-smooth muscle actin, but prior to the expression of smooth muscle myosin. GB 42 is a reliable marker for higher stages of differentiation of smooth muscle cells and myofibroblasts.

Polarized distribution of actin isoforms in gastric parietal cells

The actin genes encode several structurally similar, but perhaps functionally different, protein isoforms that mediate contractile function in muscle cells and determine the morphology and motility in nonmuscle cells. To reveal the isoform profile in the gastric monomeric actin pool, we purified actin from the cytosol of gastric epithelial cells by DNase I affinity chromatography followed by two-dimensional gel electrophoresis. Actin isoforms were identified by Western blotting with a monoclonal antibody against all actin isoforms and two isoform-specific antibodies against cytoplasmic beta-actin and gamma-actin. Densitometry revealed a ratio for beta-actin/gamma-actin that equaled 0.73 +/- 0.09 in the cytosol. To assess the distribution of actin isoforms in gastric glandular cells in relation to ezrin, a putative membrane-cytoskeleton linker, we carried out double immunofluorescence using actin-isoform-specific antibodies and ezrin antibody. Immunostaining confirmed that ezrin resides mainly in canaliculi and apical plasma membrane of parietal cells. Staining for the beta-actin isoform was intense along the entire gland lumen and within the canaliculi of parietal cells, thus predominantly near the apical membrane of all gastric epithelial cells, although lower levels of beta-actin were also identified near the basolateral membrane. The gamma-actin isoform was distributed heavily near the basolateral membrane of parietal cells, with much less intense staining of parietal cell canaliculi and no staining of apical membranes. Within parietal cells, the cellular localization of beta-actin, but not gamma-actin, isoform superimposed onto that of ezrin. In a search for a possible selective interaction between actin isoforms and ezrin, we carried out immunoprecipitation experiments on gastric membrane extracts in which substantial amounts of actin were co-eluted with ezrin from an anti-ezrin affinity column. The ratio of beta-actin/gamma-actin in the immunoprecipitate (beta/gamma = 2.14 +/- 0.32) was significantly greater than that found in the cytosolic fraction. In summary, we have shown that beta- and gamma-actin isoforms are differentially distributed in gastric parietal cells. Furthermore, our data suggest a preferential, but not exclusive, interaction between beta-actin and ezrin in gastric parietal cells. Finally, our results suggest that the beta- and gamma-actin-based cytoskeleton networks might function separately in response to the stimulation of acid secretion.

Correlation of alpha-smooth muscle actin expression and contraction in Dupuytren's disease fibroblasts

We studied 11 nodules from patients with Dupuytren's contracture to determine whether alpha-smooth muscle actin expression in Dupuytren's fibroblasts is related to the generation of contractile force. Tissue was placed into explant culture and fibroblast strains were obtained. The mean percent of cultured Dupuytren's fibroblasts expressing alpha-smooth muscle actin, as determined by immunofluorescence, was 14 +/- 8 and ranged from 1% to 26%. The ability of Dupuytren's fibroblasts to generate contractile force was determined by using a previously described collagen lattice contraction assay. We observed a significant positive correlation between the expression of alpha-smooth muscle actin and the generation of contractile force in cell strains of Dupuytren's fibroblasts. In addition, six fibroblast strains from palmar fascia of individuals undergoing carpal tunnel release were examined. In six strains of palmar fibroblasts the mean percent of cells expressing alpha-smooth muscle actin was 5 +/- 3 and ranged from 1% to 9%. Six Dupuytren's fibroblast strains, in which more than 15% of the cells expressed alpha-smooth muscle actin, were significantly more contractile than the palmar fibroblasts. These results suggest that Dupuytren's fibroblasts can acquire smooth muscle characteristics and that the acquisition of a smooth muscle-like phenotype correlates with increased contractility.

Polyacrylamide gel electrophoretic methods in the separation of structural muscle proteins

Polyacrylamide gel electrophoresis plays a major role in analyzing the function of muscle structural proteins. This review describes one- and two-dimensional gel electrophoretic methods for qualitative and quantitative investigation of the muscle proteins, with special emphasis on determination of protein phosphorylation. The electrophoretic studies established the subunit structures of the muscle proteins, characterized their multiple forms, revealed changes in subunit composition or shifts in isoform distribution of specific proteins during development, upon stimulation or denervation of the muscle. Protein phosphorylation during muscle contraction is preferentially studied by two-dimensional gel electrophoresis. The same method demonstrated protein alterations in human neuromuscular diseases.

A posttranslational modification of beta-actin contributes to the slow dissociation of the spectrin-protein 4.1-actin complex of irreversibly sickled cells

Irreversibly sickled cells (ISCs) remain sickled even under conditions where they are well oxygenated and hemoglobin is depolymerized. In our studies we demonstrate that triton extracted ISC core skeletons containing only spectrin, protein 4.1, and actin also retain their sickled shape; while reversibly sickled cell (RSC) skeletons remodel to a round or biconcave shape. We also demonstrate that these triton extracted ISC core skeletons dissociate more slowly upon incubation at 37 degrees C than do RSC or control (AA) core skeletons. This observation may supply the basis for the inability of the ISC core skeleton to remodel its shape. Using an in vitro ternary complex dissociation assay, we demonstrate that a modification in beta-actin is the major determinant of the slow dissociation of the spectrin-protein 4.1-actin complex isolated from the ISC core skeleton. We demonstrate that the difference between ISC and control beta-actin is the inaccessibility of two cysteine residues in ISC beta-actin to labeling by thiol reactive reagents; due to the formation of a disulfide bridge between cysteine284 and cysteine373 in ISC beta-actin, or alternatively another modification of cysteine284 and cysteine373 which is reversible with DTT and adds less than 100 D to the molecular weight of beta-actin.

Chromosomal mapping of the human smooth muscle actin gene (enteric type, ACTA3) to 2p13.1 and molecular nature of the hindIII polymorphism

The human gene for smooth muscle actin (enteric type, ACTA3) has been isolated, and three overlapping clones, lambda HACTSG-17, -2, and -112, were used as probes for fluorescence in situ hybridization of human chromosomes. The gene was localized to chromosome 2p13.1. To clarify the molecular nature of the HindIII RFLP present in the first intron of the gene, the 1105-bp EcoRI-BamHI fragment contained in lambda HACTSG-17 was sequenced. PCR with primers designed from the determined sequence yielded either the 463- or the 439-bp product or both, using human DNA as template. The 463-bp product was cleavable with HindIII, but the 439-bp product was not. Comparison of their nucleotide sequences revealed that they differ in the presence/absence of a 24-bp sequence harboring a HindIII restriction site. Therefore, analysis of PCR products by size has been shown to be sufficient to detect the RFLP. The allelic frequency on 156 chromosomes was determined by PCR to be 45 (439 bp, corresponding to the formerly designated A1 allele):55 (463 bp, A2 allele) in the Japanese population.

Cytoskeletal features in longitudinal and circular smooth muscles during development of the rat portal vein

Immunohistochemistry of alpha-smooth muscle actin and desmin, two markers of smooth muscle cell differentiation, and electron-microscopic observation of thick filaments of myosin were performed on the media of the developing rat hepatic portal vein to gain insights into the chronology of differentiation of its longitudinal and circular smooth muscles. In accordance with the ultrastructural distribution of thin filaments, staining of alpha-smooth muscle actin is lightly positive in the myoblasts at postnatal day 1 and then extends in probably all muscle cells of the developing vessel. Desmin, which appears later than alpha-smooth muscle actin in the two muscles, is distributed throughout the longitudinal layer at day 8, whereas the first arrangements of thick filaments are detectable in most longitudinal muscle cells; at this stage, desmin and thick filaments are absent from the poorly differentiated circular muscle cells. The longitudinal muscle cells differentiate in a strikingly synchronized way from day 8 onwards, conferring a homogeneous structure to the developing and mature longitudinal layer. Several desmin-positive cells and a heterogeneous distribution of thick filaments occur in the circular muscle at day 14; the subsequent extension of these filaments in this layer results in a persisting heterogeneous distribution in the young 7-week-old adult. Many features of the mature smooth muscle cells are established within the third week in the longitudinal muscle, approximately one week before those of the circular layer. These results are consistent with the function of the longitudinal muscle as a spontaneously contractile smooth muscle unit, and emphasize the need for its fast maturation to fulfil its major role in the control of portal blood flow.

Immunohistochemical distribution pattern of intermediate filament proteins and muscle actin in feline and human mammary carcinomas

Thirty-seven feline and 38 human spontaneous mammary gland carcinomas were studied immunohistochemically. Commercially available antibodies directed against high and low molecular weight keratins (RCK-102 and NCL-5D3), vimentin, desmin, glial fibrillary acidic protein (GFAP), neurofilament (NF) proteins and muscle actin (HHF35) were used in the avidin biotin peroxidase complex (ABC) technique on formalin-fixed paraffin wax-embedded tumour tissue samples. Healthy feline and human mammary gland tissue adjacent to the neoplasms was also examined. The distribution pattern of intermediate filament proteins and muscle actin was comparable in healthy mammary gland tissue of the two species: both RCK-102 and NCL-5D3 antibodies reacted with luminal epithelial cells of ducts and acini, but basal/myoepithelial cells were stained by RCK-102 exclusively. In addition, basal/myoepithelial cells expressed vimentin and muscle actin in both species, and GFAP was found in some feline basal/myoepithelial cells. No immunoreactivity to desmin and NF proteins was observed. Feline mammary gland carcinoma cells reacted with RCK-102 (89%), NCL-5D3 (62%), vimentin (76%) and GFAP (30%) antibodies, while human mammary gland carcinoma cells reacted with RCK-102 (95%), NCL-5D3 (100%) and vimentin (13%) antibodies. HHF35 immunoreactivity was observed in stromal cells only. These results indicate that mammary gland carcinomas of both species share a heterogeneous immunophenotype with respect to intermediate filament proteins, which adds to the list of known similarities between mammary gland carcinomas of both species.

Multiple phenotypically distinct smooth muscle cell populations exist in the adult and developing bovine pulmonary arterial media in vivo

Different smooth muscle cell (SMC) functions may require different cell phenotypes. Because the main pulmonary artery performs diverse functions, we hypothesized that it would contain heterogeneous SMC populations. If the hypothesis were confirmed, we wished to determine the developmental origin of the different populations. Using specific antibodies, we analyzed the expression of smooth muscle (SM) contractile and cytoskeletal proteins (alpha-SM-actin, SM myosin, calponin, desmin, and meta-vinculin) in the main pulmonary artery of fetal (60 to 270 days of gestation), neonatal, and adult animals. We demonstrated the existence of a complex, site-specific heterogeneity in the structure and cellular composition of the pulmonary arterial wall. We found that at least four cell/SMC phenotypes, based on immunobiochemical characteristics, cell morphology, and elastic lamellae arrangement pattern, were simultaneously expressed within the mature arterial media. Further, we were able to assess phenotypic alterations in each of the four identified cell populations during development. We found that each cell population within the arterial media expressed alpha-SM-actin at least at certain stages of development, thus demonstrating its smooth muscle identity. However, each cell population progressed along different developmental pathways, suggesting the existence of multiple and distinct cell lineages. A novel anti-metavinculin antibody described in this study reliably distinguished one SMC population from the others during all the developmental stages analyzed. We conclude that the pulmonary arterial media is indeed composed of multiple phenotypically distinct cell/SMC populations with unique lineages. We speculate that these distinct cell populations may serve different functions within the arterial media and may also respond in unique ways to pathophysiological stimuli.

Serum-induced signal transduction determines the peripheral location of beta-actin mRNA within the cell

Cell motility is dependent upon the reorganization of the cellular cytoskeleton. Actin filaments form the major component of the cytoskeleton and respond rapidly to serum growth factors. We have previously shown that myoblasts sort the two cytoskeletal beta- and gamma-actin isoform mRNAs to different intracellular regions and that only beta-actin mRNA was associated with peripheral regions of cell motility (Hill, M.A. and P. Gunning. 1993. J. Cell Biol. 122: 825-832). We now show by in situ hybridization that 3T3 fibroblasts similarly sort actin isoform mRNAs and that peripheral beta-actin mRNA is regulated by serum. In the absence of serum, we could not detect beta-actin mRNA at the periphery. Addition of serum rapidly redistributed beta-actin mRNA to the periphery. gamma-actin mRNA distribution was not altered by serum addition at any time. Both proteins, as identified by immunochemistry with isoform-specific antibodies, were found in similar cellular structures. Serum-stimulated cell motility is mediated through the GTPase signal transduction pathway. We find that an RNA-binding protein, p62, that is part of this pathway, displays a localization pattern similar to beta-actin mRNA. Our results suggest a new biological mechanism which integrates signal transduction with the supply of an architectural component required for membrane remodeling. We propose that active transport of beta-actin mRNA to regions of cell motility is one possible objective of these signal transduction pathways.

Clostridial ADP-ribosylating toxins: effects on ATP and GTP-binding proteins

The actin cytoskeleton appears to be as the cellular target of various clostridial ADP-ribosyltransferases which have been described during recent years. Clostridium botulinum C2 toxin, Clostridium perfringens iota toxin and Clostridium spiroforme toxin ADP-ribosylate actin monomers and inhibit actin polymerization. Clostridium botulinum exoenzyme C3 and Clostridium limosum exoenzyme ADP-ribosylate the low-molecular-mass GTP-binding proteins of the Rho family, which participate in the regulation of the actin cytoskeleton. ADP-ribosylation inactivates the regulatory Rho proteins and disturbs the organization of the actin cytoskeleton.

Intracellular localization and biochemical function of variant beta-actin, which inhibits metastasis of B16 melanoma

We analyzed the biochemical nature of beta m-actin protein found in mouse B16 melanoma. When we carried out immunostaining with the antibody specific to beta m-actin, filamentous immunofluorescence was observed in B16-F1, a low-metastatic cell line expressing beta m-actin, but not in highly metastatic B16-F10 that did not express beta m-actin. When a purified actin fraction containing beta m-actin was polymerized and immunoprecipitated with anti-beta m-actin antibody, the immunoprecipitate contained beta m-, beta- and gamma-actin. This indicated that the beta m-actin was incorporated into an actin filament together with beta- and gamma-actin in vitro, and this phenomenon was consistently suggested by cellular double immunostaining with anti-beta m-actin and common anti-actin antibody. When the actin fraction containing beta m-actin under a regular depolymerizing condition was subjected to immuno-adsorption assay using anti-beta m antibody and protein-A Sepharose, the immunoadsorbed aggregates contained beta m-, beta- and gamma-actin. This indicates that the actin fraction was not completely depolymerized and contained beta m-actin-containing oligomers, which were too small to be precipitated with anti-beta m-actin antibody alone. The incomplete depolymerization of the beta m-actin-containing fraction was also suggested by the much lower DNase 1 inhibition activity of the beta m-actin-containing fraction than that of beta- and gamma-actin fraction. Furthermore, a DNase 1 binding assay showed that cytoplasmic supernatant prepared from B16-F1 under a low-ionic condition contained less monomeric actin than the cytoplasmic preparation from B16-F10. These results suggested that beta m-actin protein in B16 melanoma probably inhibits the dynamic conversion between the monomeric and polymerized forms of actin, leading to a decrease in cell motility and consequently the suppression of invasiveness and metastasis.

Alpha-skeletal actin is associated with increased contractility in the mouse heart

BALB/c mice express abnormally high levels of alpha-skeletal actin in the heart, which may be related to a duplication in the promoter of the alpha-cardiac actin gene. To evaluate the effects of overexpression of the alpha-skeletal actin isoform on cardiac contractile function, we studied these mice using the isolated perfused work-performing murine heart model and measured actin isoform expression in the same hearts. We quantified myocardial contractility from the maximum rate of contraction (+dP/dt) and time to peak pressure and relaxation from -dP/dt and time to half relaxation of left intraventricular pressure. Dot blots of total RNA hybridized against oligonucleotide sequences specific for either alpha-skeletal or alpha-cardiac actin mRNA showed that increased levels of alpha-skeletal actin RNA correlated significantly with increased contractility of hearts from the BALB/c mice (r = .80, n = 15, P < .001). The present study demonstrates a significant functional correlation between alpha-actin isoform content and cardiac contractile function and also that alpha-skeletal actin may promote an increased contractile function in the heart compared with alpha-cardiac actin.

Dependence of collagen remodelling on alpha-smooth muscle actin expression by fibroblasts

To study the relation between expression of the putative myofibroblast marker alpha-smooth muscle actin and the remodelling of extracellular matrix, immunocytochemical, gel electrophoresis, and collagen gel contraction studies were performed on two human fibroblast subtypes. Double immunolabelling for total actins and alpha-smooth muscle (sm) actin as well as affinity labelling of filamentous and monomeric actins in gingival fibroblasts demonstrated that alpha-sm was colocalized in stress fibres and in regions with high levels of monomeric actin throughout the cytoplasm. alpha-sm comprised up to 14% of total cellular actin as assessed by 2D gel electrophoresis. Thirteen different gingival and seven different periodontal ligament fibroblast lines constitutively expressed on alpha-sm actin. These cells exhibited up to 60% inter-line variations of fluorescence due to alpha-sm actin and up to 70% and 45% inter-line variation in the rate of collagen gel contraction. Quantitative, single cell fluorimetry of alpha-sm actin immunoreactivity demonstrated a linear relation between gel contraction and alpha-sm actin (correlation coefficients of 0.71 for gingival and 0.61 for periodontal ligament cells), but there was no detectable relationship between total actin content and gel contraction. In contrast, flow cytometry demonstrated that 99% of the total gated cells from cell lines exhibiting rapid gel contraction showed alpha-sm actin staining above background fluorescence as compared to only 35% of cells with slow rates of gel contraction. Contracting collagen gels stained with FITC-phalloidin showed cells with well-developed stress fibres that were progressively more compact and elongated during the time of maximal gel contraction. To examine the dependence of gel contraction on assembly of monomeric actin into actin filaments, cells were electroporated in the presence of phalloidin or cytochalasin D. Collagen gels exhibited up to 100% inhibition of gel contraction that was dose-dependent. Gel contraction was inhibited 93% by electroinjection of cells with alpha-sm actin antibody prior to incubation, but the antibody did not inhibit actin assembly after attachment and spreading on substrates. These data indicate that gel contraction is dependent on alpha-sm actin expression and that alpha-sm actin is a functional marker for a fibroblast subtype that rapidly remodels the extracellular matrix.

Actin isoform compartments in chicken gizzard smooth muscle cells

Differentiated smooth muscle cells typically contain a mixture of muscle (alpha and gamma) and cytoplasmic (beta and gamma) actin isoforms. Of the cytoplasmic actins the beta-isoform is the more dominant, making up from 10% to 30% of the total actin complement. Employing an antibody raised against the N-terminal peptide specific to beta-actin, which labels only the beta-isoform on two-dimensional gel immunoblots, we have shown that this isoform has a restricted localisation in smooth muscle. Using double-label immunofluorescence and immunoelectron microscopy of ultrathin sections of chicken gizzard, beta-actin was localised in the dense bodies and in longitudinal channels linking consecutive dense bodies that were also occupied by desmin. It was additionally found in the membrane-associated dense plaques, but was excluded from the actomyosin-containing regions of the contractile apparatus. Taken together with earlier results these findings identify a cytoskeletal compartment containing intermediate filaments, cytoplasmic actin and the actin cross-linking protein filamin. Using an antibody specific only for muscle actin, labelling was found generally around the myosin filaments of the contractile apparatus, but was absent from the core of the dense bodies that contained beta-actin. Thus, if dense bodies act as dual-purpose anchorage sites, for the cytoskeletal actin and the contractile actin, the thin filaments of the contractile apparatus must be anchored at the periphery of the dense bodies. A model of the structural organisation of the cell is presented and the possible roles of the cytoskeleton are discussed.

Alterations in the expression of the beta-cytoplasmic and the gamma-smooth muscle actins in hypertrophied urinary bladder smooth muscle

The obstruction of the bladder outlet induces a marked increase in bladder mass, and this is accompanied by reduced contractility of bladder smooth muscle and alteration in the cellular architecture. In this study, we show that the composition of various isoforms of actin, a major component of the contractile apparatus and the cytoskeletal structure of smooth muscle, is altered in response to the obstruction-induced bladder hypertrophy. Northern blot analysis of the total RNA isolated from hypertrophied urinary bladder muscle, using a cDNA probe specific for smooth muscle gamma-actin, shows over 200% increase in the gamma-actin mRNA. However, the estimate of the amount of actin from the 2D gel reveals only a 16% increase in gamma-actin, since the 2D gel electrophoresis does not distinguish gamma-smooth muscle actin from gamma-cytoplasmic actin. The bladder smooth muscle alpha-actin and the smooth muscle alpha-actin mRNA are not altered in response to the hypertrophy. The obstructed bladder also reveals a decrease in the beta-cytoplasmic actin (37%) and a concomitant diminution in the beta-cytoplasmic actin mRNA (29%). Hence, the composition of the actin isoforms in bladder smooth muscle is altered in response to the obstruction-induced hypertrophy. This alteration of the actin isoforms is observed at both the protein and mRNA levels.

Expression of actin isoforms in the guinea pig organ of Corti: muscle isoforms are not detected

The specificity of antibodies to actin was assayed by use of immunoblots and histological sections of control tissues enriched for each of six different isoforms. On immunoblots, all antibodies stained at most one band of protein in most of the control materials, with a molecular weight of approximately 43 kDa. Their pattern of staining of muscle and nonmuscle tissues indicated their isoform specificity. On tissue sections, immunocytochemical staining demonstrated cellular and subcellular localization of the different isoforms. Once characterized with regard to specificity, these antibodies were used to probe actin in the guinea pig organ of Corti. None of the four muscle isoforms of actin were found in either immunoblots or tissue sections of the organ of Corti. Both beta- and gamma-cytoplasmic isoforms of actin were present in hair cells and supporting cells. This leaves open to investigation the role which cytoplasmic actins play in these cells of the organ of Corti.

The altered expression of alpha-smooth muscle actin in basal cell epithelioma and its surrounding stroma: with special reference to proliferating cell nuclear antigen expression and adenoid differentiation

Altered expression of alpha-smooth muscle actin (alpha-SMA) is known to indicate the morphological, tumorigenic and immunological changes occurring in tumour and stromal cells. The purpose of this study was to analyse the dynamics of alpha-SMA expression in human basal cell epithelioma (BCE) cells and their surrounding stromal cells, in the process of differentiation towards cutaneous appendages such as hair, sebaceous, apocrine and eccrine glands. Using anti-alpha-SMA specific monoclonal antibody (MAb), 17 of 36 BCEs (47%) were shown to express alpha-SMA, despite the usual absence of alpha-SMA in all eukaryotic cells except muscle cells. Solid, adenoid and sclerosing types of BCE expressed alpha-SMA more frequently, and in greater amount, than cystic, keratotic and superficial types. Furthermore, the expression of alpha-SMA in BCE cells significantly paralleled the expression of proliferating cell nuclear antigen (PCNA) in these cells. Thus, the altered expression of alpha-SMA may reflect the growing properties of BCE cells under the specific cellular regulations for differentiation. In addition, we have found anti-alpha-SMA MAb-positive fibroblasts with smooth muscle differentiation (myofibroblasts) in desmoplastic stroma surrounding BCE nests in 13 of 36 cases (36%). Coincidental expression of alpha-SMA in both BCE cells and stromal cells was found in nine of the 13 cases (69%), indicating the possibility of the induction of myofibroblastic stromal changes in surrounding tissues by cytokines secreted from BCE cells [e.g. basic fibroblast growth factor (bFGF)-like factor].

Beta-actin specific monoclonal antibody

Using a synthetic peptide mimicking the NH2-terminus of beta-actin we have raised a monoclonal antibody specific for this cytoplasmic actin isoform. Specificity of the antibody was demonstrated by its labelling of the actin polypeptide only in tissues containing the beta isoform, by its exclusive recognition of the synthetic beta-actin peptide amongst those mimicking all six vertebrate isoactins, and by its selective recognition of the beta-actin spot in two-dimensional electrophoresis gels of smooth muscle extracts. The antibody bound to actin filaments in both living and fixed fibroblasts where it labelled the stress fiber bundles and, more predominantly, the peripheral actin rich lamellipodia. The characteristics of the antibody indicate that it should serve as a useful tool for studying isoactin distribution and function.

GM-CSF-induced granulation tissue formation: relationships between macrophage and myofibroblast accumulation

We have studied the formation of granulation tissue around osmotic minipumps delivering granulocyte macrophage-colony stimulating factor (GM-CSF) chronologically in the rat using electron microscopy and immunohistochemistry at the light and electron microscopic levels, with specific antibodies against alpha-smooth muscle (SM) actin and rat macrophages. At 2 and 3 days after pump implantation, GM-CSF application produced an extensive inflammatory reaction characterized by edema and the accumulation of polymorphonuclear cells and macrophages. Gradually, polymorphonuclear cells decreased in number and macrophages became arranged in large clusters. The expression of alpha-SM actin in fibroblastic cells of the granulation tissue started from the 4th day after pump implantation and progressed up to the 7th day. Double immunofluorescence staining showed macrophage clusters in relation to alpha-SM actin-rich fibroblastic cells. Electron microscopic examination confirmed that the fibroblasts containing alpha-SM actin-positive stress fibers were found initially in close proximity to clustered macrophages. The delivery of platelet-derived growth factor (PDGF) and tumor necrosis factor-alpha (TNF-alpha) by the osmotic minipump induced an accumulation of macrophages, but in a much smaller number compared with those seen after GM-CSF application; these macrophages were never assembled in clusters and, furthermore, TNF-alpha and PDGF did not stimulate alpha-SM actin expression in fibroblastic cells. Our results suggest that after GM-CSF administration, the cluster-like accumulation of macrophages plays an important role in stimulating alpha-SM actin expression in myofibroblasts. Our results may be relevant to the understanding of the processes leading to granulation tissue formation in this and other experimental models.

Calponin and SM 22 as differentiation markers of smooth muscle: spatiotemporal distribution during avian embryonic development

Calponin and SM 22 are two proteins related in sequence that are particularly abundant in smooth muscle cells. Here, the distribution patterns of calponin and SM 22 were compared with that of other smooth muscle contractile and cytoskeletal components in the avian embryo using immunofluorescence microscopy and immunoblotting. Like myosin-light-chain kinase and heavy caldesmon, both calponin and SM 22 were more or less exclusively found in smooth muscle cells, during embryonic development and in the adult. Labelling of other cell types including striated muscle was not observed. In contrast, tropomyosin, smooth muscle alpha-actin, filamin and desmin could also be detected in many other cell types in addition to smooth muscles, at least during part of embryonic life. Calponin and SM 22 appeared almost synchronously during the differentiation of all smooth muscle cell populations, though with a slight time difference in the case of the aorta. The appearance of calponin, SM 22 and heavy caldesmon was generally delayed in relation to desmin, tropomyosin, smooth muscle alpha-actin, myosin-light-chain kinase and filamin and a marked increase in abundance of these proteins was observed in the late embryo and in the adult. From these observations we can conclude that both calponin and SM 22 belong to a group of late differentiation determinants in smooth muscle and may constitute convenient and reliable markers to follow the differentiation of most, if not all, smooth muscle cell populations.

Frequency and specificity of antiheart antibodies in patients with dilated cardiomyopathy detected using SDS-PAGE and western blotting

OBJECTIVES

This study was designed to investigate the organ and disease specificity of antiheart antibodies in patients with dilated cardiomyopathy.

BACKGROUND

Autoimmune disease is characterized by the presence of circulating autoantibodies, and autoimmune mechanisms may play a role in the pathogenesis of dilated cardiomyopathy.

METHODS

An SDS-PAGE (sodium dodecylsulfate polyacrylamide gel electrophoresis) procedure followed by Western blotting was used to screen serum samples for antiheart antibodies of two immunoglobulin classes, IgM and IgG, from 52 patients with dilated cardiomyopathy and 48 patients with ischemic heart disease as control subjects. Use of two-dimensional gel electrophoresis followed by Western blotting and protein sequencing enabled us to identify the protein bands against which antiheart antibodies were produced in both groups of patients.

RESULTS

Strong IgG antiheart antibodies against myocardial proteins, cross-reacting with skeletal muscle proteins, were detected in significantly more patients with dilated cardiomyopathy (n = 24 [46%]) than with ischemic heart disease (n = 8 [17%]) (p = 0.001). Patients with dilated cardiomyopathy showed a significantly greater frequency and reactivity of IgG antiheart antibodies against six myocardial proteins (molecular weight 30, 35, 40, 60, 85 and 200 kD) than did patients with ischemic heart disease. These were identified as myosin light chain 1, tropomyosin, actin, heat shock protein (HSP)-60, an unidentified protein and myosin heavy chain, respectively.

CONCLUSIONS

We detected strong IgG antiheart antibodies in significantly more patients with dilated cardiomyopathy than with ischemic heart disease. The most immunogenic band was that corresponding to HSP-60. Antibodies against HSP-60 were found in 85% and 42% of patients with dilated cardiomyopathy and ischemic heart disease, respectively, confirming our hypothesis of an immune involvement in dilated cardiomyopathy.

Isoform-specific 3'-untranslated sequences sort alpha-cardiac and beta-cytoplasmic actin messenger RNAs to different cytoplasmic compartments

We demonstrate that in differentiating myoblasts, the mRNAs encoding two actin isoforms, beta-cytoplasmic, and alpha-cardiac, can occupy different cytoplasmic compartments within the same cytoplasm. beta-actin mRNA is localized to the leading lamellae and alpha-actin mRNA is associated with a perinuclear compartment. This was revealed by co-hybridizing, in situ, fluorochrome-conjugated oligonucleotide probes specific for each isoform. To address the mechanism of isoform-specific mRNA localization, molecular chimeras were constructed by insertion of actin sequences between the Lac Z coding region and SV-40 3'UTR in a reporter plasmid. These constructs were transiently expressed in a mixed culture of embryonic fibroblasts, myoblasts and myotubes, beta-galactosidase activity within transfectants was revealed by a brief incubation with its substrate (X-gal). Since the blue-insoluble reaction product co-localized with the specific mRNAs expressed from each construct, it was used as a bioassay for mRNA localization. Transfectants were scored as either perinuclear, peripheral or nonlocalized with respect to the distribution of the blue product. The percentage of transfectants within those categories was quantitated as a function of the various constructs. This analysis revealed that for each actin mRNA its 3'UTR is necessary and sufficient to direct reporter transcripts to its appropriate compartment; beta-actin peripheral and alpha-actin perinuclear. In contrast, sequences from the 5'UTR through the coding region of either actin gene did not localize the blue product. Therefore, 3'UTR sequences play a key role in modulating the distribution of actin mRNAs in muscle cells. We propose that the mechanism of mRNA localization facilitates actin isoform sorting in the cytoplasm.

Cell signalling and motility in Entamoeba histolytica

Entamoeba histolytica crawls as a polarized cell following external stimuli, with the translocation of signals modifying extracellular matrix interactions and the amoeba cytoskeleton. Nancy Guillén here describes how the gliding of E. histolytica cells requires the activity of the actomyosin complex, and how actomyosin functions related to motility are necessary for pathogenesis and for amoebal escape from the host immune response.

Immunohistochemistry with keratin, vimentin, desmin, and alpha-smooth muscle actin monoclonal antibodies in canine mammary gland: normal mammary tissue

Normal canine mammary gland tissue was studied immunohistochemically with monoclonal antibodies (MoAbs) directed against various human keratin types, vimentin, desmin, and alpha-smooth muscle actin. Both ductal and alveolar luminal cells were immunoreactive with MoAbs recognizing respectively human keratins no. 7, 8, 18 and 19. In addition, some ductal luminal cells were labelled with a keratin 4 and a keratin 10 MoAb. Basal/myoepithelial cells were immunoreactive only with MoAbs directed against keratin 14, keratins 14 and 17, and alpha-smooth muscle actin. The vimentin MoAb merely labelled solitary loose intraluminal cells representing macro-phages or sloughed epithelial cells. These findings correspond largely to observations made in human breast tissue.

Beta and gamma actin mRNAs are differentially located within myoblasts

Actin is of fundamental importance to all eukaryotic cells. Of the six mammalian actins, beta (beta) and gamma (gamma) cytoplasmic are the isoforms found in all nonmuscle cells and differ by only four amino acids at the amino-terminal region. Both genes are regulated temporally and spatially, though no differences in protein function have been described. Using fluorescent double in situ hybridization we describe the simultaneous intracellular localization of both beta and gamma actin mRNA. This study shows that myoblasts differentially segregate the beta and gamma actin mRNAs. The distribution of gamma actin mRNA, only to perinuclear and nearby cytoplasm, suggests a distribution based on diffusion or restriction to nearby cytoplasm. The distribution of beta actin mRNA, perinuclear and at the cell periphery, implicates a peripheral localizing signal which is unique to the beta isoform. The peripheral beta actin mRNA corresponded to cellular morphologies, extending processes, and ruffling edges that reflect cell movement. Total actin and gamma actin protein steady-state distributions were identified by specific antibodies. gamma actin protein was found in both stress fibers and at the cell plasma membrane and does not correspond to its mRNA distribution. We suggest that localized protein synthesis rather than steady-state distribution functionally differentiates between the actin isoforms.

Molecular cloning and characterization of actin genes from Echinococcus granulosus

An Echinococcus granulosus genomic library has been screened with a mouse beta-actin cDNA probe. Two clones carrying DNA fragments of about 15 kb, possibly derived from the same genome region, have been isolated. This 15-kb genomic region includes 2 actin-related sequences (EgactI and EgactII) separated by about 4 kb. The nucleotide sequences of both genes were determined. The EgactI sequence presents no introns, but an intron of 591 bp was observed in the EgactII sequence. The genes potentially encode 375 and 376 amino-acid-long actins, respectively, with a homology of 85.3%. The deduced amino acid sequences from both genes were compared to the actin sequences from other organisms, showing similarities ranging from 63.5% to 90.6%. The nucleotide sequence of a partial actin cDNA clone has been determined. The deduced amino acids sequence showed a homology of 90.3% and 88.0% in relation to the EgactI and EgactII sequences respectively, suggesting the existence of at least one more actin gene in E. granulosus. This hypothesis is reinforced by the number of bands detected in the Southern blot analysis. Experiments based on the amplification of DNA segments using 3'-specific actin primers indicate that the EgactI gene is transcribed in protoscoleces.

Heterogeneous isoactin gene expression in the adult rat gastrointestinal tract

BACKGROUND

Normal gastrointestinal development is a complex process involving the precise integration of multiple cell types. To gain a better understanding of these processes, the present study examined isoactin gene expression in the adult rat gastrointestinal tract.

METHODS

Northern blot analysis was performed on specified segments of the adult rat esophagus, stomach, small intestine, cecum, colon, rectum, and anus using actin isoform-specific complementary DNAs for all six vertebrate isoactins.

RESULTS

Smooth muscle and cytoplasmic isoactins were heterogeneously coexpressed in a segment-specific manner throughout the gastrointestinal tract. In addition, striated muscle isoactin expression was also detected in segments of the adult rat esophagus, stomach, colon, cecum, rectum, and anus. Histological analysis indicated that the adult rat esophagus, stomach, and anus contained significant quantities of skeletal muscle, providing a source for the striated muscle isoactins detected in these gut segments. A similar source of striated muscle isoactin expression in the cecum, colon, and rectum was not identified. Both coordinate and independent regulation of isoactin gene expression was observed in the gastrointestinal tract, although distinct patterns of autoregulation were absent.

CONCLUSIONS

This study represents the first complete analysis of isoactin gene expression in the adult rat gastrointestinal tract and provides the basis for future studies designed to investigate the factors responsible for these processes.

Alpha-smooth-muscle actin expression in normal and fibrotic human livers

To determine the significance of the expression of alpha-smooth-muscle actin in the fibrotic human liver, normal and diseased livers were stained with anti-alpha-smooth-muscle-actin antibody by an immunoperoxidase method. Vitamin A-containing lipocytes were also identified by the modified Kupffer's gold chloride method. In the normal human liver, lipocytes as well as vascular smooth muscle cells expressed alpha-smooth-muscle actin. In alcoholic liver disease, there was an increase in the cells positive for alpha-smooth-muscle actin adjacent to the fibrotic areas, but the response of lipocytes to the gold chloride reaction diminished. In chronic hepatitis, the cells positive for alpha-smooth-muscle actin increased around the enlarged portal areas, and the response to the gold chloride reaction did not change appreciably. An increase in the cells positive for alpha-smooth-muscle actin was associated with the progression of hepatic fibrosis in the liver of patients with alcoholic liver disease and chronic hepatitis.

Molecular genetics of actin function

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