Differential behavior of gizzard isoactins

Evolutionary conservation of physical and functional interactions between phospholipase D and actin

Phospholipase D (PLD) enzymes from bacteria to mammals exhibit a highly conserved core structure and catalytic mechanism, but whether protein-protein interactions exhibit similar commonality is unknown. Our objective was to determine whether the physical and functional interactions of mammalian PLDs with actin are evolutionarily conserved among bacterial and plant PLDs. Highly purified bacterial and plant PLDs cosedimented with mammalian skeletal muscle alpha-actin, indicating direct interaction with F-actin. The binding of bacterial PLD to G-actin exhibited two affinity states, with dissociation constants of 1.13 pM and 0.58 microM. The effects of actin on the activities of bacterial and plant PLDs were polymerization dependent; monomeric G-actin inhibited PLD activity, whereas polymerized F-actin augmented PLD activity. Actin modulation of bacterial and plant PLDs demonstrated kinetic characteristics, efficacies, and potencies similar to those of human PLD1. Thus, physical and functional interactions between PLD and actin in PLD family members from bacteria to mammals are highly conserved throughout evolution.

Regulation of phospholipase D activity by actin. Actin exerts bidirectional modulation of Mammalian phospholipase D activity in a polymerization-dependent, isoform-specific manner

Many critical cellular processes, including proliferation, vesicle trafficking, and secretion, are regulated by both phospholipase D (PLD) and the actin microfilament system. Stimulation of human PLD1 results in its association with the detergent-insoluble actin cytoskeleton, but the molecular mechanisms and functional consequences of PLD-actin interactions remain incompletely defined. Biochemical and pharmacologic modulation of actin polymerization resulted in complex bidirectional effects on PLD activity, both in vitro and in vivo. Highly purified G-actin inhibited basal and stimulated PLD activity, whereas F-actin produced the opposite effects. Actin-induced modulation of PLD activity was independent of the activating stimulus. The efficacy and potency of the effects of actin were isoform-specific but broadly conserved among actin family members. Human betagamma-actin was only 45% as potent and 40% as efficacious as rabbit skeletal muscle alpha-actin, whereas its inhibitory profile was similar to the single actin species from the yeast, Saccharomyces cerevisiae. Use of actin polymerization-specific reagents indicated that PLD1 binds both monomeric G-actin, as well as actin filaments. These data are consistent with a model in which the physical state of the actin cytoskeleton is a critical determinant of its regulation of PLD activity.

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.

The rosette complex in gerbil Deiters cells contains gamma actin

The relationship of selected cytoskeletal elements with the rosette complex of Deiters cells was examined immunocytochemically in the gerbil cochlea. By light microscopy, the staining pattern for actin in the apical portion of Deiters cells corresponded with the location of the rosette complex. At the ultrastructural level, the actin antibodies bound selectively at the periphery of the dense trabeculae in the center of the complex. Comparative staining with a battery of polyclonal and isoform-specific monoclonal antibodies revealed selective presence of the gamma muscle actin isoform in this location. The loose meshwork at the periphery of the rosette complex stained selectively with a monoclonal antibody to vimentin. beta-tubulin was not associated with the rosette complex but occurred in abundance in the microtubule-rich stalk. Actin and vimentin were not detected in the apical compartment of Deiters cells at the extreme base of the cochlea, thus confirming their association with the rosette complex which is not present in regions of the gerbil cochlea tuned to frequencies of 20 kHz or higher (Spicer and Schulte, 1993, 1994). The cytoarchitecture of the rosette complex and its preferential distribution along the place-frequency map promote speculation that Deiters cells may play a role in regulating ion homeostasis and/or micromechanical response properties of the organ of Corti.

Indirect association of ezrin with F-actin: isoform specificity and calcium sensitivity

Whereas it has been demonstrated that muscle and nonmuscle isoactins are segregated into distinct cytoplasmic domains, the mechanism regulating subcellular sorting is unknown (Herman, 1993a). To reveal whether isoform-specific actin-binding proteins function to coordinate these events, cell extracts derived from motile (Em) versus stationary (Es) cytoplasm were selectively and sequentially fractionated over filamentous isoactin affinity columns prior to elution with a KCl step gradient. A polypeptide of interest, which binds specifically to beta-actin filament columns, but not to muscle actin columns has been conclusively identified as the ERM family member, ezrin. We studied ezrin-beta interactions in vitro by passing extracts (Em) over isoactin affinity matrices in the presence of Ca(2+)-containing versus Ca(2+)-free buffers, with or without cytochalasin D. Ezrin binds and can be released from beta-actin Sepharose-4B in the presence of Mg2+/EGTA and 100 mM NaCl (at 4 degrees C and room temperature), but not when affinity fractionation of Em is carried out in the presence of 0.2 mM CaCl2 or 2 microM cytochalasin D. N-acetyl-(leucyl)2-norleucinal and E64, two specific inhibitors of the calcium-activated protease, calpain I, protect ezrin binding to beta actin in the presence of calcium. Moreover, biochemical analysis of endothelial lysates reveals that a calpain I cleavage product of ezrin emerges when cell locomotion is stimulated in response to monolayer injury. Immunofluorescence analysis of leading lamellae reveals that anti-ezrin and anti-beta-actin IgGs can be simultaneously co-localized, extending the results of isoactin affinity fractionation of Em-derived extracts and suggesting that ezrin and beta-actin interact in vivo. To test the hypothesis that ezrin binds directly to beta-actin, we performed three sets of studies under a wide range of physiological conditions (pH 7.0-8.5) using purified pericyte ezrin and either alpha- or beta-actin. These included co-sedimentation, isoactin affinity fractionation, and co-immunoprecipitation. Results of these experiments reveal that purified ezrin does not directly bind to beta-actin filaments, either in solution or while isoactins are covalently cross-linked to Sepharose-4B. This is in contrast to our finding that ezrin and beta-actin could be co-immunoprecipitated or co-sedimented from Em-derived cell lysates. To explore whether calcium transients occur in cellular domains enriched in ezrin and beta-actin, we mapped cellular free calcium in endothelial monolayers crawling in response to injury.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Changes in the cytoskeleton of 3T3 fibroblasts induced by the phosphatase inhibitor, calyculin-A

Addition of the protein phosphatase inhibitor, calyculin-A, to 3T3 fibroblasts causes a marked change in cell morphology. Initially the cells become rounded, develop surface blebs and then detach from the substratum. In the detached cells an unusual ball-like structure is observed. This study focuses on the cytoskeleton during these calyculin-A-induced morphological changes. Stress fibres disappear as the cells begin to round and aggregates of actin are formed towards the apical surface of the cell. These aggregates condense, in the detached cells, to form the ball structure of approximately 3 microns diameter. Between the ball and the nucleus are cables of intermediate filaments that appear to be attached to the surface of the ball and to the nuclear lamina. Using a procedure designed for the isolation of nuclei the nucleus-ball complex can be obtained. Analysis of the nucleus-ball preparation by immunofluorescence and electron microscopy demonstrate that the ball contains actin and that intermediate filaments are located between the ball and the nucleus. In this preparation, the intermediate filaments also appear to attach to the surfaces of the ball and the nucleus. Electrophoretic analysis of the nucleus-ball preparation indicates that, in addition to actin, a major component of the ball is myosin. It is suggested that the formation of the ball is caused by an actin-myosin-based contractile process, initiated by the phosphorylation of myosin. The aggregation of the actomyosin draws together the intermediate filaments into the area between the ball and nucleus. This hypothesis requires that vimentin binds both to the nucleus and to some component of the ball.

The functional importance of multiple actin isoforms

Actin is a protein that plays an important role in cell structure, cell motility, and the generation of contractile force in both muscle and nonmuscle cells. In many organisms, multiple forms of actin, or isoactins, are found. These are products of different genes and have different, although very similar, amino acid sequences. Furthermore, these isoactins are expressed in a tissue specific fashion that is conserved across species, suggesting that their presence is functionally important and their behavior can be distinguished quantitatively from one another in vitro. In muscle cells, they are differentially distributed within the cell and some are specifically associated with structures such as costameres, mitochondria, and neuromuscular junctions. There is also good evidence for specific isoactin function in microvascular pericytes and in the intestinal brush border. However, the necessity of specific isoactins for various functions has not yet been conclusively demonstrated.

Nonuniform behavior of multiple isoactins in the same cell is a cell-dependent phenomenon

The functional significance of multiple isoactins in the same cell is still not understood. To address this question, we examined the response of smooth muscle and cardiac muscle alpha-isoactins to a serial extraction procedure applied to both muscle and nonmuscle cell types. We compared these extraction results with results obtained with the beta- and gamma-nonmuscle actin isoforms from the same cells. In differentiated BC3H1 nonfusing muscle cells (smooth muscle alpha-isoactin), in human rhabdomyosarcoma cells (cardiac alpha-isoactin), and in chick skeletal muscle cells (cardiac alpha-isoactin), different fractions were found selectively enriched in either the nonmuscle or the muscle-specific actin isoforms compared with their relative abundance in whole cell extracts. Conversely, when these same isoactins were examined either in undifferentiated BC3H1 cells or in mouse nonmuscle cells stably transfected with a cardiac alpha-isoactin gene, no enrichment of these isoforms above their relative abundance in whole cell extracts was observed. These results indicate that within the muscle or muscle-like cells examined, the different actin isoforms were either selectively utilized or localized. These results further show that isoactin-specific responses observed were apparently related to the cell type in which they were found and not to differences in inherent physical properties such as solubility of the different isoactins examined.

Actin-isoform pattern as a marker of normal or pathological smooth-muscle and fibroblastic tissues

The relative proportions of actin isoforms present in smooth-muscle (SM) and fibroblastic human and non-human tissue extracts were examined by densitometric evaluation of Coomassie-Blue-stained spots in two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) as well as by quantification of radiolabeled actin NH2-terminal peptide spots separated by two-dimensional paper electrophoresis. SM tissues contained alpha- and gamma-SM as well as beta- and gamma-cytoplasmic (CY) actins in different proportions in different organs. Species differences with respect to the ratios of the isoactins were also observed. Moreover, during pregnancy, both human and rat myometrium exhibited a changed actin-isoform pattern, there being an increased proportion of gamma-actin. Analysis of the NH2-terminal peptides showed that, in human myometrium, this was essentially due to an increase in the amount of the gamma-SM isoform. Fibroblastic tissues were found to contain only the beta- and gamma-CY isoforms, the ratio being approximately 2.6:1. Thus, the presence or absence of alpha-actin provides a reliable biochemical criterion for distinguishing between fibroblastic and SM cell populations and/or tissues. This distinction and the evaluation of changes in isoactin ratios may be useful in the study of differentiation as well as physiological and pathological phenomena, and for determining the origin of certain soft-tissue tumours.

A monoclonal antibody against alpha-smooth muscle actin: a new probe for smooth muscle differentiation

A monoclonal antibody (anti-alpha sm-1) recognizing exclusively alpha-smooth muscle actin was selected and characterized after immunization of BALB/c mice with the NH2-terminal synthetic decapeptide of alpha-smooth muscle actin coupled to keyhole limpet hemocyanin. Anti-alpha sm-1 helped in distinguishing smooth muscle cells from fibroblasts in mixed cultures such as rat dermal fibroblasts and chicken embryo fibroblasts. In the aortic media, it recognized a hitherto unknown population of cells negative for alpha-smooth muscle actin and for desmin. In 5-d-old rats, this population is about half of the medial cells and becomes only 8 +/- 5% in 6-wk-old animals. In cultures of rat aortic media SMCs, there is a progressive increase of this cell population together with a progressive decrease in the number of alpha-smooth muscle actin-containing stress fibers per cell. Double immunofluorescent studies carried out with anti-alpha sm-1 and anti-desmin antibodies in several organs revealed a heterogeneity of stromal cells. Desmin-negative, alpha-smooth muscle actin-positive cells were found in the rat intestinal muscularis mucosae and in the dermis around hair follicles. Moreover, desmin-positive, alpha-smooth muscle actin-negative cells were identified in the intestinal submucosa, rat testis interstitium, and uterine stroma. alpha-Smooth muscle actin was also found in myoepithelial cells of mammary and salivary glands, which are known to express cytokeratins. Finally, alpha-smooth muscle actin is present in stromal cells of mammary carcinomas, previously considered fibroblastic in nature. Thus, anti-alpha sm-1 antibody appears to be a powerful probe in the study of smooth muscle differentiation in normal and pathological conditions.

Immunolocalization of the gamma isoform of nonmuscle actin in cultured cells

In many vertebrate nonmuscle cells, the microfilament subunit protein, actin, exists as two isoforms, called beta and gamma, whose sequences differ only in their amino-terminal regions. We have prepared a peptide antibody specifically reactive with the amino-terminal sequence of gamma actin. This antibody reacted with nonmuscle actin as determined by Western blots of SDS gels, and reacted with the gamma, but not the beta, nonmuscle actin isoform as shown by Western blots of isoelectric focusing gels. In immunofluorescence experiments, the gamma peptide antibody stained microfilament bundles, ruffled edges, and the contractile ring of a variety of cultured cells, including mouse L cells, which have previously been reported to contain only the beta actin isoform (Sakiyama, S., S. Fujimura, and H. Sakiyama, 1981, J. Biol. Chem., 256:31-33). Double immunofluorescence experiments using the gamma peptide antibody and an antibody reactive with all actin isoforms revealed no differences in isoform localization. Thus, at the level of resolution of light microscopy, we have detected the gamma actin isoform in all microfilament-containing structures in cultured cells, and have observed no subcellular sorting of the nonmuscle actin isoforms.

Polymerization of beta-like actin from scallop adductor muscle

Scallop adductor muscle beta-like isoactin differs from rabbit skeletal muscle alpha-actin in the rate, extent and critical concentration of polymerization. The difference is temperature- and [KC1]-dependent. In the presence of DNase I scallop actin was shown to be depolymerized more rapidly than rabbit actin. It was suggested that the polymers formed by beta-actin are less stable than those formed by alpha-actin.

Microvascular pericytes contain muscle and nonmuscle actins

We have affinity-fractionated rabbit antiactin immunoglobulins (IgG) into classes that bind preferentially to either muscle or nonmuscle actins. The pools of muscle- and nonmuscle-specific actin antibodies were used in conjunction with fluorescence microscopy to characterize the actin in vascular pericytes, endothelial cells (EC), and smooth muscle cells (SMC) in vitro and in situ. Nonmuscle-specific antiactin IgG stained the stress fibers of cultured EC and pericytes but did not stain the stress fibers of cultured SMC, although the cortical cytoplasm associated with the plasma membrane of SMC did react with nonmuscle-specific antiactin. Whereas the muscle-specific antiactin IgG failed to stain EC stress fibers and only faintly stained their cortical cytoplasm, these antibodies reacted strongly with the fiber bundles of cultured SMC and pericytes. Similar results were obtained in situ. The muscle-specific antiactin reacted strongly with the vascular SMC of arteries and arterioles as well as with the perivascular cells (pericytes) associated with capillaries and post-capillary venules. The non-muscle-specific antiactin stained the endothelium and the pericytes but did not react with SMC. These findings indicate that pericytes in culture and in situ possess both muscle and nonmuscle isoactins and support the hypothesis that the pericyte may represent the capillary and venular correlate of the SMC.

Bovine aorta actin. Development of an improved purification procedure and comparison of polymerization properties with actins from other types of muscle

Crude actin extracts from acetone-dried powder of the muscle layer of bovine aorta contain an actin-modulating protein which promotes nucleation of actin monomers and decreases the average length of actin filaments in a Ca2+-dependent manner. This observation has allowed the development of an improved purification procedure for aorta actin which increases the yield 2- to 3-times. The actin obtained with this procedure consists of 77% alpha- and 23% gamma-isoelectric species. Pure aorta actin is indistinguishable from actins from skeletal, cardiac and chicken-gizzard smooth muscle in its polymerization rate, critical concentration, and reduced viscosity when polymerized with KCl at 25 degrees C. It differs from sarcomeric actins, but not from chicken-gizzard smooth muscle actin, in the temperature dependence of polymerization equilibria in KCl. This difference correlates with the amino acid replacements Val-17----Cys-17 and Thr-89----Ser-89, supporting a conclusion drawn from other studies that the N-terminal portion of actin polypeptide chain contains sites important for polymerization.

Effects of various amino acid replacements on the conformational stability of G-actin

Circular dichroic spectra of native, EDTA-treated and heat-denatured G-actin from chicken gizzard smooth muscle are virtually the same as those of rabbit skeletal muscle actin. The rates of changes produced by EDTA or heat in the secondary structure are, however, higher in the case of gizzard actin. Similar differences were found in the rates of inactivation as measured by loss of polymerizability during incubation with EDTA or Dowex 50. The results are explicable in terms of local differences in the conformation at specific site(s) important for maintaining the native state of actin monomer. Involvement of the ATP binding site was shown by measuring the equilibrium constant for the binding of ATP to the two actins. Difference in the conformation of some additional site(s) is indicated by a higher rate constant of inactivation of nucleotide-free actin observed for gizzard actin. No significant difference was found in the equilibrium constant for the binding of Ca2+ at the single high-affinity site in gizzard and skeletal muscle actin. Comparison of inactivation kinetics of actin from chicken gizzard, rabbit skeletal, bovine aorta, and bovine cardiac muscle suggests that the amino acid replacements Val-17----Cys-17 and/or Thr-89----Ser-89 have a destabilizing effect on the native conformation of G-actin. The results indicate that deletion of the acidic residue at position 1 of the amino acid sequence has no effect on the conformation of the ATP binding site and the high-affinity site for divalent cation as well.

Developmental change of protein constituents in chicken gizzards

Developmental change of protein constituents of chick gizzard smooth muscle was described by the fluorescent antibody technique and two-dimensional polyacrylamide gel electrophoresis. Myosin heavy chain, tropomyosin, and desmin were immunohistologically detected in 5-day-old gizzard primordia, but myoglobin was detected after 19 days of incubation. Two-dimensional polyacrylamide gel electrophoresis revealed that most structural proteins including beta- and gamma-actin are synthesized almost simultaneously in the primordium, and accumulate in three patterns by which the proteins examined are classified: (1) gradually increasing protein (gamma-actin, tropomyosin, desmin), (2) abruptly increasing protein at a certain stage (myosin, myoglobin), (3) decreasing or constantly kept protein (tubulin, beta-actin). Based on the quantitative analysis of protein constituents, the nature of regulatory system of protein synthesis in smooth muscle and the possible functional difference between beta- and gamma-actin are discussed.