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

Maize profilin isoforms are functionally distinct

Profilin is an actin monomer binding protein that, depending on the conditions, causes either polymerization or depolymerization of actin filaments. In plants, profilins are encoded by multigene families. In this study, an analysis of native and recombinant proteins from maize demonstrates the existence of two classes of functionally distinct profilin isoforms. Class II profilins, including native endosperm profilin and a new recombinant protein, ZmPRO5, have biochemical properties that differ from those of class I profilins. Class II profilins had higher affinity for poly-l-proline and sequestered more monomeric actin than did class I profilins. Conversely, a class I profilin inhibited hydrolysis of membrane phosphatidylinositol-4,5-bisphosphate by phospholipase C more strongly than did a class II profilin. These biochemical properties correlated with the ability of class II profilins to disrupt actin cytoplasmic architecture in live cells more rapidly than did class I profilins. The actin-sequestering activity of both maize profilin classes was found to be dependent on the concentration of free calcium. We propose a model in which profilin alters cellular concentrations of actin polymers in response to fluctuations in cytosolic calcium concentration. These results provide strong evidence that the maize profilin gene family consists of at least two classes, with distinct biochemical and live-cell properties, implying that the maize profilin isoforms perform distinct functions in the plant.

Neurotrophin regulation of beta-actin mRNA and protein localization within growth cones

Neurotrophins play an essential role in the regulation of actin-dependent changes in growth cone shape and motility. We have studied whether neurotrophin signaling can promote the localization of beta-actin mRNA and protein within growth cones. The regulated localization of specific mRNAs within neuronal processes and growth cones could provide a mechanism to modulate cytoskeletal composition and growth cone dynamics during neuronal development. We have previously shown that beta-actin mRNA is localized in granules that were distributed throughout processes and growth cones of cultured neurons. In this study, we demonstrate that the localization of beta-actin mRNA and protein to growth cones of forebrain neurons is stimulated by neurotrophin-3 (NT-3). A similar response was observed when neurons were exposed to forskolin or db-cAMP, suggesting an involvement of a cAMP signaling pathway. NT-3 treatment resulted in a rapid and transient stimulation of PKA activity that preceded the localization of beta-actin mRNA. Localization of beta-actin mRNA was blocked by prior treatment of cells with Rp-cAMP, an inhibitor of cAMP-dependent protein kinase A. Depolymerization of microtubules, but not microfilaments, inhibited the NT-3-induced localization of beta-actin mRNA. These results suggest that NT-3 activates a cAMP-dependent signaling mechanism to promote the microtubule-dependent localization of beta-actin mRNA within growth cones.

Structure, assembly, and dynamics of actin filaments in situ and in vitro

Actin, though highly conserved, exhibits a myriad of diverse functions, most of which ultimately depend on its intrinsic ability to rapidly assemble and disassemble filamentous structures. Many organisms synthesize multiple actin isoforms even within the same cell. Tissue-specific expression patterns and tight developmental regulation as well as a high conservation across species emphasize the functional importance of isoforms. The detailed knowledge of the structure, assembly, and dynamic behavior of actin provides important pieces in solving the puzzle of how the different isoforms can be so versatile despite their extremely high sequence identity.

Thin filament protein dynamics in fully differentiated adult cardiac myocytes: toward a model of sarcomere maintenance

Sarcomere maintenance, the continual process of replacement of contractile proteins of the myofilament lattice with newly synthesized proteins, in fully differentiated contractile cells is not well understood. Adenoviral-mediated gene transfer of epitope-tagged tropomyosin (Tm) and troponin I (TnI) into adult cardiac myocytes in vitro along with confocal microscopy was used to examine the incorporation of these newly synthesized proteins into myofilaments of a fully differentiated contractile cell. The expression of epitope-tagged TnI resulted in greater replacement of the endogenous TnI than the replacement of the endogenous Tm with the expressed epitope-tagged Tm suggesting that the rates of myofilament replacement are limited by the turnover of the myofilament bound protein. Interestingly, while TnI was first detected in cardiac sarcomeres along the entire length of the thin filament, the epitope-tagged Tm preferentially replaced Tm at the pointed end of the thin filament. These results support a model for sarcomeric maintenance in fully differentiated cardiac myocytes where (a) as myofilament proteins turnover within the cell they are rapidly exchanged with newly synthesized proteins, and (b) the nature of replacement of myofilament proteins (ordered or stochastic) is protein specific, primarily affected by the structural properties of the myofilament proteins, and may have important functional consequences.

Myofibrillogenesis in the developing chicken heart: assembly of Z-disk, M-line and the thick filaments

Myofibrillogenesis in situ was investigated by confocal microscopy of immunofluorescently labelled whole mount preparations of early embryonic chicken heart rudiments. The time-course of incorporation of several components into myofibrils was compared in triple-stained specimens, taken around the time when beating starts. All sarcomeric proteins investigated so far were already expressed before the first contractions and myofibril assembly happened within a few hours. No typical stress fibre-like structures or premyofibrils, structures observed in cultured cardiomyocytes, could be detected during myofibrillogenesis in the heart. Sarcomeric proteins like (α)-actinin, titin and actin were found in a defined localisation pattern even in cardiomyocytes that did not yet contain myofibrils, making up dense body-like structures. As soon as the heart started to beat, all myofibrillar proteins were already located at their exact position in the sarcomere. The maturation of the sarcomeres was characterised by a short delay in the establishment of the pattern for M-line epitopes of titin with respect to Z-disk epitopes and the incorporation of the M-line component myomesin, which preceded that of myosin binding protein-C. Thus dense body-like structures, made up of titin, (α)-actinin and actin filaments serve as the first organised complexes also during myofibrillogenesis in situ and titin functions as a ruler for sarcomere assembly as soon as its C termini have become localised. We suggest that assembly of thin and thick filament occurs independently during myofibrillogenesis in situ and that myomesin might be important for integrating thick filaments with the M-line end of titin.

Isoform sorting and the creation of intracellular compartments

The generation of isoforms via gene duplication and alternative splicing has been a valuable evolutionary tool for the creation of biological diversity. In addition to the formation of molecules with related but different functional characteristics, it is now apparent that isoforms can be segregated into different intracellular sites within the same cell. Sorting has been observed in a wide range of genes, including those encoding structural molecules, receptors, channels, enzymes, and signaling molecules. This results in the creation of intracellular compartments that (a) can be independently controlled and (b) have different functional properties. The sorting mechanisms are likely to operate at the level of both proteins and mRNAs. Isoform sorting may be an important consequence of the evolution of isoforms and is likely to have contributed to the diversity of functional properties within groups of isoforms.

Differential epitope tagging of actin in transformed Drosophila produces distinct effects on myofibril assembly and function of the indirect flight muscle

We have tested the impact of tags on the structure and function of indirect flight muscle (IFM)-specific Act88F actin by transforming mutant Drosophila melanogaster, which do not express endogenous actin in their IFMs, with tagged Act88F constructs. Epitope tagging is often the method of choice to monitor the fate of a protein when a specific antibody is not available. Studies addressing the functional significance of the closely related actin isoforms rely almost exclusively on tagged exogenous actin, because only few antibodies exist that can discriminate between isoforms. Thereby it is widely presumed that the tag does not significantly interfere with protein function. However, in most studies the tagged actin is expressed in a background of endogenous actin and, as a rule, represents only a minor fraction of the total actin. The Act88F gene encodes the only Drosophila actin isoform exclusively expressed in the highly ordered IFM. Null mutations in this gene do not affect viability, but phenotypic effects in transformants can be directly attributed to the transgene. Transgenic flies that express Act88F with either a 6x histidine tag or an 11-residue peptide derived from vesicular stomatitis virus G protein at the C terminus were flightless. Overall, the ultrastructure of the IFM resembled that of the Act88F null mutant, and only low amounts of C-terminally tagged actins were found. In contrast, expression of N-terminally tagged Act88F at amounts comparable with that of wild-type flies yielded fairly normal-looking myofibrils and partially reconstituted flight ability in the transformants. Our findings suggest that the N terminus of actin is less sensitive to modifications than the C terminus, because it can be tagged and still polymerize into functional thin filaments.

Subcutaneous tissue fibroblasts transfected with muscle and nonmuscle actins: A good in vitro model to study fibroblastic cell plasticity

Cultured fibroblasts develop several biochemical and morphological properties of smooth muscle cells, particularly the expression of alpha-smooth muscle actin, the actin isoform typical of vascular smooth muscle cells. They resemble modified fibroblasts or myofibroblasts observed in granulation tissue during wound repair and in fibrotic situations. We have analysed by immunolabeling the fate of exogenous epitope-tagged actin isoforms by transfection of the corresponding cDNAs into fibroblasts cultured from rat subcutaneous tissue. Tagged muscle actins were efficiently integrated into stress fibers and did not produce obvious changes in cell shape of transfected cells. Transfected nonmuscle actins in contrast changed the morphology and were not or poorly incorporated into stress fibers. These cultured subcutaneous fibroblasts behave similarly to smooth muscle cells when transfected with the same actin encoding cDNAs, indicating another common characteristic of these two cell types in sorting and targeting actin isoforms. Subcutaneous fibroblasts transfected with muscle and nonmuscle actin isoforms provide a good in vitro model to analyze the intracellular sorting of isoactins and to improve our knowledge of myofibroblast characterization and differentiation during tissue repair as well as to understand the relationships between modifications of actin cytoskeleton, adhesion and extracellular matrix proteins.

Regulation of actin dynamics through phosphorylation of cofilin by LIM-kinase

Cell division, cell motility and the formation and maintenance of specialized structures in differentiated cells depend directly on the regulated dynamics of the actin cytoskeleton. To understand the mechanisms of these basic cellular processes, the signalling pathways that link external signals to the regulation of the actin cytoskeleton need to be characterized. Here we identify a pathway for the regulation of cofilin, a ubiquitous actin-binding protein that is essential for effective depolymerization of actin filaments. LIM-kinase 1, also known as KIZ, is a protein kinase with two amino-terminal LIM motifs that induces stabilization of F-actin structures in transfected cells. Dominant-negative LIM-kinasel inhibits the accumulation of the F-actin. Phosphorylation experiments in vivo and in vitro provide evidence that cofilin is a physiological substrate of LIM-kinase 1. Phosphorylation by LIM-kinase 1 inactivates cofilin, leading to accumulation of actin filaments. Constitutively active Rac augmented cofilin phosphorylation and LIM-kinase 1 autophosphorylation whereas phorbol ester inhibited these processes. Our results define a mechanism for the regulation of cofilin and hence of actin dynamics in vivo. By modulating the stability of actin cytoskeletal structures, this pathway should play a central role in regulating cell motility and morphogenesis.

Actin dynamics in living mammalian cells

The actin cytoskeleton maintains the cellular architecture and mediates cell movements. To explore actin cytoskeletal dynamics, the enhanced green fluorescent protein (EGFP) was fused to human β-actin. The fusion protein was incorporated into actin fibers which became depolymerized upon cytochalasin B treatment. This functional EGFP-actin construct enabled observation of the actin cytoskeleton in living cells by time lapse fluorescence microscopy. Stable expression of the construct was obtained in mammalian cell lines of different tissue origins. In stationary cells, actin rich, ring-like structured ‘actin clouds’ were observed in addition to stress fibers. These ruffle-like structures were found to be involved in the reorganization of the actin cytoskeleton. In migratory cells, EGFP-actin was found in the advancing lamellipodium. Immobile actin spots developed in the lamellipodium and thin actin fibers formed parallel to the leading edge. Thus EGFP-actin expressed in living cells unveiled structures involved in the dynamics of the actin cytoskeleton.

Rat fibroblasts cultured from various organs exhibit differences in alpha-smooth muscle actin expression, cytoskeletal pattern, and adhesive structure organization

In vivo, alpha-smooth muscle actin (SMA) is expressed de novo and temporarily by fibroblastic cells during wound healing and correlates particularly with wound contraction. In culture, the presence of varying proportions of cells expressing and not expressing this actin isoform (alpha-SMA-positive and alpha-SMA-negative cells) is characteristic of fibroblastic populations from different tissues. It is possible that mechanisms controlling the expression of actin isoforms, and thus modulating cytoskeleton-related functions, play a major role in the organization of cell shape and motility. We have compared the cell shape as well as the cytoskeleton and focal contact organization in alpha-SMA-positive and alpha-SMA-negative rat fibroblasts from various organs (i.e., skeletal muscle, dermis, subcutaneous tissue, and lung). Within each category, i.e., alpha-SMA-positive or alpha-SMA-negative fibroblasts, no significant morphological differences were seen among populations derived from different tissues. In contrast, alpha-SMA-positive and alpha-SMA-negative fibroblasts were significantly different, independently of their origin: alpha-SMA-positive cells had larger average areas, higher numbers of narrow extensions at the edges, larger focal adhesions with the substratum, and a more important network of cellular fibronectin than alpha-SMA-negative cells. Thus, alpha-SMA-positive and alpha-SMA-negative variants naturally present in fibroblastic populations exhibit important phenotypic differences probably associated with distinct functional activities.

Sorting of beta-actin mRNA and protein to neurites and growth cones in culture

The transport of mRNAs into developing dendrites and axons may be a basic mechanism to localize cytoskeletal proteins to growth cones and influence microfilament organization. Using isoform-specific antibodies and probes for in situ hybridization, we observed distinct localization patterns for beta- and gamma-actin within cultured cerebrocortical neurons. beta-Actin protein was highly enriched within growth cones and filopodia, in contrast to gamma-actin protein, which was distributed uniformly throughout the cell. beta-Actin protein also was shown to be peripherally localized after transfection of beta-actin cDNA bearing an epitope tag. beta-Actin mRNAs were localized more frequently to neuronal processes and growth cones, unlike gamma-actin mRNAs, which were restricted to the cell body. The rapid localization of beta-actin mRNA, but not gamma-actin mRNA, into processes and growth cones could be induced by dibutyryl cAMP treatment. Using high-resolution in situ hybridization and image-processing methods, we showed that the distribution of beta-actin mRNA within growth cones was statistically nonrandom and demonstrated an association with microtubules. beta-Actin mRNAs were detected within minor neurites, axonal processes, and growth cones in the form of spatially distinct granules that colocalized with translational components. Ultrastructural analysis revealed polyribosomes within growth cones that colocalized with cytoskeletal filaments. The transport of beta-actin mRNA into developing neurites may be a sequence-specific mechanism to synthesize cytoskeletal proteins directly within processes and growth cones and would provide an additional means to deliver cytoskeletal proteins over long distances.

Isoform specificity in the relationship of actin to dendritic spines

Dendritic spines contain high concentrations of actin, but neither the isoforms involved nor the mechanism of accumulation is known. In situ hybridization with specific probes established that beta- and gamma-cytoplasmic actins are selectively expressed at high levels by spine-bearing neurons. Transfecting cultured hippocampal neurons with epitope-tagged actin isoforms showed that cytoplasmic beta- and gamma-cytoplasmic actins are correctly targeted to spines, whereas alpha-cardiac muscle actin, which is normally absent from neurons, formed aggregates in dendrites. The transfected actin cDNAs contained only coding domains, suggesting that spine targeting involves amino acid sequences in the proteins, an interpretation supported by experiments with chimeric cDNAs in which C-terminal actin sequences were found to be determinative in spine targeting. By contrast to actin, microtubule components, including tubulin and MAP2, were restricted to the dendritic shaft domain. The close association of cytoplasmic actins with spines together with their general involvement in cell surface motility further supports the idea that actin motility-based changes in spine shape may contribute to synaptic plasticity.

Sorting of beta-actin mRNA and protein to neurites and growth cones in culture

The transport of mRNAs into developing dendrites and axons may be a basic mechanism to localize cytoskeletal proteins to growth cones and influence microfilament organization. Using isoform-specific antibodies and probes for in situ hybridization, we observed distinct localization patterns for beta- and gamma-actin within cultured cerebrocortical neurons. beta-Actin protein was highly enriched within growth cones and filopodia, in contrast to gamma-actin protein, which was distributed uniformly throughout the cell. beta-Actin protein also was shown to be peripherally localized after transfection of beta-actin cDNA bearing an epitope tag. beta-Actin mRNAs were localized more frequently to neuronal processes and growth cones, unlike gamma-actin mRNAs, which were restricted to the cell body. The rapid localization of beta-actin mRNA, but not gamma-actin mRNA, into processes and growth cones could be induced by dibutyryl cAMP treatment. Using high-resolution in situ hybridization and image-processing methods, we showed that the distribution of beta-actin mRNA within growth cones was statistically nonrandom and demonstrated an association with microtubules. beta-Actin mRNAs were detected within minor neurites, axonal processes, and growth cones in the form of spatially distinct granules that colocalized with translational components. Ultrastructural analysis revealed polyribosomes within growth cones that colocalized with cytoskeletal filaments. The transport of beta-actin mRNA into developing neurites may be a sequence-specific mechanism to synthesize cytoskeletal proteins directly within processes and growth cones and would provide an additional means to deliver cytoskeletal proteins over long distances.

Isoform specificity in the relationship of actin to dendritic spines

Dendritic spines contain high concentrations of actin, but neither the isoforms involved nor the mechanism of accumulation is known. In situ hybridization with specific probes established that beta- and gamma-cytoplasmic actins are selectively expressed at high levels by spine-bearing neurons. Transfecting cultured hippocampal neurons with epitope-tagged actin isoforms showed that cytoplasmic beta- and gamma-cytoplasmic actins are correctly targeted to spines, whereas alpha-cardiac muscle actin, which is normally absent from neurons, formed aggregates in dendrites. The transfected actin cDNAs contained only coding domains, suggesting that spine targeting involves amino acid sequences in the proteins, an interpretation supported by experiments with chimeric cDNAs in which C-terminal actin sequences were found to be determinative in spine targeting. By contrast to actin, microtubule components, including tubulin and MAP2, were restricted to the dendritic shaft domain. The close association of cytoplasmic actins with spines together with their general involvement in cell surface motility further supports the idea that actin motility-based changes in spine shape may contribute to synaptic plasticity.

The motility-associated proteins GAP-43, MARCKS, and CAP-23 share unique targeting and surface activity-inducing properties

Local regulation of the cortical cytoskeleton controls cell surface dynamics. GAP-43 and MARCKS are two abundant cytosolic protein kinase C substrates that are anchored to the cell membrane via acyl groups and interact with the cortical cytoskeleton. Each of them has been implicated in several forms of motility involving the cell surface. Although their primary sequences do not reveal significant homologies, GAP-43, MARCKS, and the cortical cytoskeleton-associated protein CAP-23 (in the following, the three proteins will be abbreviated as GMC) share a number of characteristic biochemical and biophysical properties and an unusual amino acid composition. In this study we determined whether GMC may be related functionally. In double-labeling immunocytochemistry experiments GMC accumulated at unique surface-associated structures, where they codistributed. In transfected cells GMC induced the same range of characteristic changes in cell morphology and cell surface activities, including prominent blebs and filopodia. These activities correlated with local accumulation of transgene and had characteristic features of locally elevated actin dynamics, including loss of stress fiber structures, accumulation of beta-(cytosolic) actin at cell surface protrusions, and dynamic blebbing activity. Analysis of appropriate deletion and fusion constructs revealed that the surface accumulation pattern and cell surface activities were correlated and that minimal structural requirements included acylation-mediated targeting to the cell membrane and the presence of a predominantly GMC-type sequence composition. Based on these experiments and on the results of previous studies on GAP-43, MARCKS, and CAP-23, we propose that GMC may define a class of functionally related proteins whose local accumulation promotes actin dynamics and the formation of dynamic structures at the cell periphery. Superimposed on these general properties, differences in the regulation of membrane association and binding properties of effector domains would confer individual properties to each of these proteins.

Rescue of cardiac alpha-actin-deficient mice by enteric smooth muscle gamma-actin

The muscle actins in higher vertebrates display highly conserved amino acid sequences, yet they show distinct expression patterns. Thus, cardiac alpha-actin, skeletal alpha-actin, vascular smooth muscle alpha-actin, and enteric smooth muscle gamma-actin comprise the major actins in their respective tissues. To assess the functional and developmental significance of cardiac alpha-actin, the murine (129/SvJ) cardiac alpha-actin gene was disrupted by homologous recombination. The majority ( approximately 56%) of the mice lacking cardiac alpha-actin do not survive to term, and the remainder generally die within 2 weeks of birth. Increased expression of vascular smooth muscle and skeletal alpha-actins is observed in the hearts of newborn homozygous mutants and also heterozygotes but apparently is insufficient to maintain myofibrillar integrity in the homozygous mutants. Mice lacking cardiac alpha-actin can be rescued to adulthood by the ectopic expression of enteric smooth muscle gamma-actin using the cardiac alpha-myosin heavy chain promoter. However, the hearts of such rescued cardiac alpha-actin-deficient mice are extremely hypodynamic, considerably enlarged, and hypertrophied. Furthermore, the transgenically expressed enteric smooth muscle gamma-actin reduces cardiac contractility in wild-type and heterozygous mice. These results demonstrate that alterations in actin composition in the fetal and adult heart are associated with severe structural and functional perturbations.

Point mutations in human beta cardiac myosin heavy chain have differential effects on sarcomeric structure and assembly: an ATP binding site change disrupts both thick and thin filaments, whereas hypertrophic cardiomyopathy mutations display normal assembly

Hypertrophic cardiomyopathy is a human heart disease characterized by increased ventricular mass, focal areas of fibrosis, myocyte, and myofibrillar disorganization. This genetically dominant disease can be caused by mutations in any one of several contractile proteins, including beta cardiac myosin heavy chain (beta MHC). To determine whether point mutations in human beta MHC have direct effects on interfering with filament assembly and sarcomeric structure, full-length wild-type and mutant human beta MHC cDNAs were cloned and expressed in primary cultures of neonatal rat ventricular cardiomyocytes (NRC) under conditions that promote myofibrillogenesis. A lysine to arginine change at amino acid 184 in the consensus ATP binding sequence of human beta MHC resulted in abnormal subcellular localization and disrupted both thick and thin filament structure in transfected NRC. Diffuse beta MHC K184R protein appeared to colocalize with actin throughout the myocyte, suggesting a tight interaction of these two proteins. Human beta MHC with S472V mutation assembled normally into thick filaments and did not affect sarcomeric structure. Two mutant myosins previously described as causing human hypertrophic cardiomyopathy, R249Q and R403Q, were competent to assemble into thick filaments producing myofibrils with well defined I bands, A bands, and H zones. Coexpression and detection of wild-type beta MHC and either R249Q or R403Q proteins in the same myocyte showed these proteins are equally able to assemble into the sarcomere and provided no discernible differences in subcellular localization. Thus, human beta MHC R249Q and R403Q mutant proteins were readily incorporated into NRC sarcomeres and did not disrupt myofilament formation. This study indicates that the phenotype of myofibrillar disarray seen in HCM patients which harbor either of these two mutations may not be directly due to the failure of the mutant myosin heavy chain protein to assemble and form normal sarcomeres, but may rather be a secondary effect possibly resulting from the chronic stress of decreased beta MHC function.

Transfected muscle and non-muscle actins are differentially sorted by cultured smooth muscle and non-muscle cells

We have analyzed by immunolabeling the fate of exogenous epitope-tagged actin isoforms introduced into cultured smooth muscle and non-muscle (i.e. endothelial and epithelial) cells by transfecting the corresponding cDNAs in transient expression assays. Exogenous muscle actins did not produce obvious shape changes in transfected cells. In smooth muscle cells, transfected striated and smooth muscle actins were preferentially recruited into stress fibers. In non-muscle cells, exogenous striated muscle actins were rarely incorporated into stress fibers but remained scattered within the cytoplasm and frequently appeared organized in long crystal-like inclusions. Transfected smooth muscle actins were incorporated into stress fibers of epithelial cells but not of endothelial cells. Exogenous non-muscle actins induced alterations of cell architecture and shape. All cell types transfected by non-muscle actin cDNAs showed an irregular shape and a poorly developed network of stress fibers. beta- and gamma-cytoplasmic actins transfected into muscle and non-muscle cells were dispersed throughout the cytoplasm, often accumulated at the cell periphery and rarely incorporated into stress fibers. These results show that isoactins are differently sorted: not only muscle and non-muscle actins are differentially distributed within the cell but also, according to the cell type, striated and smooth muscle actins can be discriminated for. Our observations support the assumption of isoactin functional diversity.

beta-Actin messenger RNA localization and protein synthesis augment cell motility

In chicken embryo fibroblasts (CEFs), beta-actin mRNA localizes near an actin-rich region of cytoplasm specialized for motility, the lamellipodia. This localization is mediated by isoform-specific 3'-untranslated sequences (zipcodes) and can be inhibited by antizipcode oligodeoxynucleotides (ODNs) (Kislauskis, E.H., X.-C. Zhu, and R.H. Singer. 1994. J. Cell Biol. 127: 441-451). This inhibition of beta-actin mRNA localization resulted in the disruption of fibroblast polarity and, presumably, cell motility. To investigate the role of beta-actin mRNA in motility, we correlated time-lapse images of moving CEFs with the distribution of beta-actin mRNA in these cells. CEFs with localized beta-actin mRNA moved significantly further over the same time period than did CEFs with nonlocalized mRNA. Antizipcode ODN treatment reduced this cell translocation while control ODN treatments showed no effect. The temporal relationship of beta-actin mRNA localization to cell translocation was investigated using serum addition to serum-deprived cultures. beta-actin mRNA was not localized in serum-deprived cells but became localized within minutes after serum addition (Latham, V.M., E.H. Kislauskis, R.H. Singer, and A.F. Ross. 1994. J. Cell Biol. 126:1211-1219). Cell translocation increased over the next 90 min, and actin synthesis likewise increased. Puromycin reduced this cell translocation and blocked this induction in cytosolic actin content. The serum induction of cell movement was also inhibited by antizipcode ODNs. These observations support the hypothesis that beta-actin mRNA localization and consequent protein synthesis augment cell motility.

Differential expression of a novel isoform of alpha-tropomyosin in cardiac and skeletal muscle of the Mexican axolotl (Ambystoma mexicanum)

Alternative mRNA splicing is a fundamental process in eukaryotes that contributes to tissue-specific and developmentally regulated patterns of tropomyosin (TM) gene expression. Northern blot analyses suggest the presence of multiple transcripts of tropomyosin in skeletal and cardiac muscle of adult Mexican axolotls. We have cloned and sequenced two tropomyosin cDNAs designated ATmC-1 and ATmC-2 from axolotl heart tissue and one TM cDNA from skeletal muscle, designated ATmS-1. Nucleotide sequence analyses suggest that ATmC-1 and ATmC-2 are the products of the same alpha-TM gene produced via alternate splicing, whereas ATmC-1 and ATmS-1 are the identical isoforms generated from the alpha-gene. RT-PCR analysis using isoform-specific primer pairs and detector oligonucleotides suggests that ATmC-2 is expressed predominantly in adult axolotl hearts. ATmC-2 is a novel isoform, which unlike ATmC-1 and other known striated muscle isoforms expresses exon 2a instead of exon 2b.

Primary structure of mouse actin-related protein 1 (Arp1) and its tissue expression

Different types of actin-related proteins which constitute an actin-superfamily together with conventional actin have recently been described (Mullins et al., 1996). Among them, Arp1 exhibits the highest homology with conventional actin. With the aim of clarifying the cellular function of Arp1 in mammalian cells, we cloned the cDNA encoding mouse alpha-Arp1, one of the variants of Arp1, from a mouse diaphragm cDNA library; two types of alpha-Arp1 cDNAs, which are probably generated by alternative RNA splicing from a single gene, were obtained and the entire sequences were determined. They differed only in the presence or absence of an insertion of 1.3 kb in the 3'-non-cooling region but shared a common open reading frame. The deduced amino acid sequence was identical with that of human alpha-Arp1. Northern blot analysis showed that the alpha-Arp1 mRNA corresponding to the longer cDNA is transcribed not only in various non-muscle tissues but also in muscle tissues, while the transcript corresponding to the shorter one becomes expressed only in skeletal muscle as development progresses. It is suggested that alpha-Arp1 may play some role in muscle, as judged by the significant level of its expression.

Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Centractin (Arp1), an actin-related protein, is a component of the dynactin complex. To investigate potential functions of the protein, we used transient transfections to overexpress centractin in mammalian cells. We observed that the overexpressed polypeptide formed filamentous structures that were significantly longer and more variable in length than those observed in the native dynactin complex. The centractin filaments were distinct from conventional actin in subunit composition and pharmacology as demonstrated by the absence of immunoreactivity of these filaments with an actin-specific antibody, by resistance to treatment with the drug cytochalasin D, and by the inability to bind phalloidin. We examined the transfected cells for evidence of specific associations of the novel centractin filaments with cellular organelles or cytoskeletal proteins. Using immunocytochemistry we observed the colocalization of Golgi marker proteins with the centractin polymers. Additional immunocytochemical analysis using antibodies to non-erythroid spectrin (fodrin) and Golgi-spectrin (beta I sigma *) revealed that spectrin colocalized with the centractin filaments in transfected cells. Biochemical assays demonstrated that spectrin was present in dynactin-enriched cellular fractions, was coimmunoprecipitated from rat brain cytosol using antibodies to dynactin subunits, and was coeluted with dynactin using affinity chromatography. Immunoprecipitations and affinity chromatography also revealed that actin is not a bona fide component of dynactin. Our results indicate that spectrin is associated with the dynactin complex. We suggest a model in which dynactin associates with the Golgi through an interaction between the centractin filament of the dynactin complex and a spectrin-linked cytoskeletal network.

Phalloidin binding and rheological differences among actin isoforms

Actin is a highly conserved protein in eukaryotes, yet different isoforms of this protein can be found within the same cell. To begin to explore whether isoactin sequence diversity leads to functional differences in actin filaments, we have examined the phalloidin binding kinetics and the bulk rheologic properties of purified actin isoforms from a variety of eukaryotic sources. We observe differences in the phalloidin association kinetics between muscle alpha- and cytoplasmic actins. Phalloidin dissociates from all mammalian actin isoforms tested at the same slow rate, while dissociation from yeast actin is 1 order of magnitude more rapid. The actin isoforms form viscoelastic gels to varying degrees with skeletal muscle alpha-actin gels being the most elastic, smooth muscle alpha- and gamma-actins being less elastic, and beta-actin not forming elastic structures under our experimental conditions. The sequence variation among isoforms is discussed in light of these biophysical and biochemical differences.

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.

The intracompartmental sorting of myosin alkali light chain isoproteins reflects the sequence of developmental expression as determined by double epitope-tagging competition

In order to compare within the same cell the various degrees of specificity of myosin alkali light chain (MLC) isoproteins sorting to sarcomeres, a competition assay was established using double epitope tagging. Various combinations of two different MLC isoform cDNAs tagged with either a vesicular stomatitis virus VSV-G (VSV) or a medium T (mT) protein epitope were co-expressed in cultured cardiomyocytes from adult and neonatal rat ventricles. Expressed isoproteins were detected by means of anti-VSV and anti-mT antibodies and their sorting patterns were analyzed by confocal microscopy. The sorting specificity of MLC isoforms to sarcomeric sites was shown to increase in the order MLC3nm, to ML1sa, to MLC1sb, to MLC1f and MLC3f following the sequence of developmental expression. Expressed fast skeletal muscle isoforms (MLC1f and MLC3f) were always localized at the A-bands of myofibrils, while nonmuscle type (MLC3nm) was distributed throughout the cytoplasm. The slow skeletal muscle type (MLC1sa) showed a weak sarcomeric pattern if it was co-expressed with MLC3nm, but it was distributed throughout the cytoplasm when expressed in combination with MLC1f, MLC3f or the slow skeletal/ventricular muscle isoform (MLC1sb). The MLC1sb was localized at the A-bands when it was co-expressed with MLC3nm or MLC1sa, while it was also distributed to the cytoplasm if co-expressed with MLC1f or MLC3f. Further, expression of chimeric cDNAs revealed that the N-terminal lobe of each isoprotein is responsible for the isoform-specific sorting pattern.

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.

Beta cap73: a novel beta actin-specific binding protein

Whereas actin-binding proteins (ABPs) regulate network formation during the cell cycle, it is not known whether ABPs also function to sequester or target isoactins to specific subcellular compartments. Recently, we have shown that ezrin indirectly associates with beta, but not alpha actin filaments in a calcium- and cytochalasin-sensitive manner [Shuster and Herman, 1995: J. Cell Biol. 128:837-848]. To identify the beta actin-specific binding protein that fosters ezrin-beta actin interactions, we developed an isoactin affinity fractionation and F-isoactin overlay/Western blotting technique. Results reveal that a 73 kd polypeptide that co-precipitates with ezrin and beta actin [Shuster and Herman, 1995: J. Cell Biol. 128:837-848] can also binds directly to filaments of beta, but not alpha actin by isoactin overlay. In an effort to establish whether p73 plays a role in regulating beta actin dynamics in cells, we produced monoclonal antibodies by immunizing BALB/c mice with p73-containing lamellar lysates or high salt elutions from beta actin affinity columns. Two monoclonal antibodies were cloned that react with p73 present in fractions released from beta actin Sepharose-4B or purified to homogeneity by DEAE chromatography. Anti-p73 Western blots reveal that there is a 16-fold difference in p73 binding to beta actin vs. alpha actin affinity columns when experiments are performed in physiological salts. To characterize p73-beta actin binding in vitro and establish whether p73 binds along the lengths or at the barbed end of the beta actin filament, we asked whether cytochalasin D (CD) could displace p73 pre-bound to beta actin-Sepharose 4B. Anti-p73 Western blotting reveals that nanomolar concentrations of CD are capable of selectively eluting p73 and ezrin from beta actin Sepharose 4B, indicating that p73 binds beta actin via the barbed end. Simultaneous double antibody localization studies using anti-beta actin IgG and anti-p73 IgM reveal that p73 and beta actin are co-localized in the forward aspects of motile cytoplasmic domains, in close proximity to the plasma membrane. Because of its isoform-specific interactions with the barbed end of beta actin filaments, we have named this molecule beta cap73. These results indicate that isoform-specific actin-binding proteins can be identified from cortical cytoplasm, and suggest that beta cap73 may not only act to spatially regulate the intracellular distribution of isoactins, but may also facilitate forward protrusion formation through the regulated release of free filament ends during cell motility.