Isolation and characterization of six different chicken actin genes

Posttranscriptional and Posttranslational Regulation of Actin

Actin is one of the most abundant intracellular proteins, essential in every eukaryotic cell type. Actin plays key roles in tissue morphogenesis, cell adhesion, muscle contraction, and developmental reprogramming. Most actin studies have focused on its regulation at the protein level, either directly or through differential interactions with over a hundred intracellular binding partners. However, numerous studies emerging in recent years demonstrate specific types of nucleotide-level regulation that strongly affect non-muscle actins during cell migration and adhesion and are potentially applicable to other members of the actin family. This regulation involves zipcode-mediated actin mRNA targeting to the cell periphery, proposed to mediate local synthesis of actin at the cell leading edge, as well as the recently discovered N-terminal arginylation that specifically targets non-muscle β-actin via a nucleotide-dependent mechanism. Moreover, a study published this year suggests that actin's essential roles at the organismal level may be entirely nucleotide-dependent. This review summarizes the emerging data on actin's nucleotide-level regulation. Anat Rec, 301:1991-1998, 2018. © 2018 Wiley Periodicals, Inc.

Human Smooth Muscle α-Actin Promoter Drives Cre Recombinase Expression in the Cranial Suture in Addition to Smooth Muscle Cell

Tissue-specific gene deletion by the Cre-loxp system is a powerful tool to investigate the roles of specific genes. To determine the specificity and efficiency of the Cre-mediated recombination under the control of the human smooth muscle alpha-actin promoter, we mated SMalphaA-Cre mice and R26R reporter mice. Cre-mediated recombination was observed in visceral and vascular smooth muscle cells. Partial recombination was also found in heart and musculoskeletal connective tissues. Highly efficient recombination was found in cranial sutures. Hence, we propose that SMalphaA-Cre mice are good tool for conditionally deleting gene function in the cranial suture in addition to smooth muscle cells.

Smooth muscle α-actin is a direct target of PLZF: effects on the cytoskeleton and on susceptibility to oncogenic transformation

Changes in cell morphology and rearrangements of the actin cytoskeleton are common features accompanying cell transformation induced by various oncogenes. In this study, we show that promyelocytic leukemia zinc finger protein (PLZF) binds to the promoter of smooth muscle α-actin, reducing mRNA and protein levels encoded by this gene and resulting in a reorganization of the actin cytoskeleton. In cultures of chicken embryo fibroblasts (CEF), this effect on α-actin expression is correlated with a change in cellular phenotype from spindle shaped to polygonal and flattened. This morphological change is dependent on Ras function. The polygonal, flattened CEF show a high degree of resistance to the transforming activity of several oncoproteins. Our results support the conclusion that reorganization of the actin cytoskeleton plays an important role in tumor suppression by PLZF.

Non-electrophoretic differential detergent fractionation proteomics using frozen whole organs

Non-electrophoretic methods based on two-dimensional liquid chromatography followed directly by tandem mass spectrometry (2D-LC/MS(2)) have become the preferred method for high-throughput expression proteomics and are widely applied to fresh tissues. Pre-fractionation techniques are also used in combination with 2D-LC/MS(2) to both increase the proteome size and to assign cellular locations. Data from such experiments have become central to systems biology analyses. Here we apply a differential detergent (pre)fractionation (DDF) followed by 2D-LC/MS(2) to frozen archival tissues. Our results show that by using frozen archival tissues, we do not lose proteome coverage or the ability to assign proteins to cellular compartments. In addition, we were able to assign 'biological process' Gene Ontology (GO) annotations, which will facilitate systems biological modeling of our proteomics data.

Cysteine-rich LIM-only proteins CRP1 and CRP2 are potent smooth muscle differentiation cofactors

Cysteine-rich LIM-only proteins, CRP1 and CRP2, expressed during cardiovascular development act as bridging molecules that associate with serum response factor and GATA proteins. SRF-CRP-GATA complexes strongly activated smooth muscle gene targets. CRP2 was found in the nucleus during early stages of coronary smooth muscle differentiation from proepicardial cells. A dominant-negative CRP2 mutant blocked proepicardial cells from differentiating into smooth muscle cells. Together with SRF and GATA proteins, CRP1 and CRP2 converted pluripotent 10T1/2 fibroblasts into smooth muscle cells, while muscle LIM protein CRP3 inhibited the conversion. Thus, LIM-only proteins of the CRP family play important roles in organizing multiprotein complexes, both in the cytoplasm, where they participate in cytoskeletal remodeling, and in the nucleus, where they strongly facilitate smooth muscle differentiation.

Impaired vascular contractility and blood pressure homeostasis in the smooth muscle alpha-actin null mouse

The smooth muscle (SM) alpha-actin gene activated during the early stages of embryonic cardiovascular development is switched off in late stage heart tissue and replaced by cardiac and skeletal alpha-actins. SM alpha-actin also appears during vascular development, but becomes the single most abundant protein in adult vascular smooth muscle cells. Tissue-specific expression of SM alpha-actin is thought to be required for the principal force-generating capacity of the vascular smooth muscle cell. We wanted to determine whether SM alpha-actin gene expression actually relates to an actin isoform's function. Analysis of SM alpha-actin null mice indicated that SM alpha-actin is not required for the formation of the cardiovascular system. Also, SM alpha-actin null mice appeared to have no difficulty feeding or reproducing. Survival in the absence of SM alpha-actin may result from other actin isoforms partially substituting for this isoform. In fact, skeletal alpha-actin gene, an actin isoform not usually expressed in vascular smooth muscle, was activated in the aortas of these SM alpha-actin null mice. However, even with a modest increase in skeletal alpha-actin activity, highly compromised vascular contractility, tone, and blood flow were detected in SM alpha-actin-defective mice. This study supports the concept that SM alpha-actin has a central role in regulating vascular contractility and blood pressure homeostasis, but is not required for the formation of the cardiovascular system.

Antisense oligodeoxynucleotide complementary to smooth muscle alpha-actin inhibits endothelial-mesenchymal transformation during chick cardiogenesis

alpha-Smooth-muscle actin (SMA) is the major isoform of adult vascular tissues. During early development, SMA is expressed in various mesodermally derived tissues in a spatiotemporally restricted manner; however, its exact role remains unknown. We examined its role in the formation of chicken atrioventricular (AV) endocardial cushion tissue. This developmental process possesses the characteristics of endothelial-mesenchymal transformation and is partly TGF beta-dependent. Immunohistochemistry showed that SMA was (1) expressed homogeneously in the newly formed appendages of transforming endothelial/mesenchymal cells, and (2) distributed in a punctate manner in the lamellipodia/filopodia of invading mesenchymal cells. Antisense oligodeoxynucleotide (ODNs) specific for SMA reduced both SMA expression and mesenchymal formation in AV endothelial cells cultured with myocardium on a collagen gel lattice. Perturbation of SMA by antisense ODN also inhibited TGF beta-inducible migratory appendage formation in a cultured AV endothelial monolayer. However, it did not inhibit cell:cell separation or cellular hypertrophy. These results suggest that the expression of SMA is necessary for migratory appendage formation during the TGF beta-dependent initial phenotypic changes that occur in endothelial-mesenchymal transformation.

Actin mRNA isoforms are differentially sorted in normal osteoblasts and sorting is altered in osteoblasts from a skeletal mutation in the rat

Actin isoform sorting has been shown to occur in a variety of cell types in culture. To this list we add osteoblasts, in which we show by in situ hybridization that beta-actin is distributed primarily in cell processes and on one side of the nucleus and gamma-actin has a perinuclear distribution. Osteoblasts from the skeletal mutation toothless (tl), evaluated under identical conditions, fail to sort these actin isoforms differentially and exhibit diffuse labeling as their major manifestation. Northern analyses of actin mRNAs showed no differences between normal and mutant cultures. Shortened osteoblast life span and an inability to direct osteoclast-mediated bone resorption have recently been demonstrated in tl mutants. The present results suggest that a failure of osteoblasts to sort actin mRNAs may be related to one or both of these pathological manifestations in this mutation and represent, to our knowledge, the first correlation of an actin mRNA-sorting abnormality with a mammalian disease.

Expression of smooth muscle alpha-actin in mesenchymal cells during formation of avian endocardial cushion tissue: a role for transforming growth factor beta3

During early cardiac morphogenesis, outflow tract (OT) and atrio-ventricular (AV) endothelial cells differentiate into mesenchymal cells, which have characteristics of smooth muscle-like myofibroblasts, and which form endocardial cushion tissue, the primordia of valves, and septa in the adult heart. During this embryonic event, transforming growth factor beta3 (TGF beta3) is an essential element in the progression of endothelial-transformation into mesenchyme. TGF beta(s) are known to be a potent inducer for mesodermal differentiation and a promoter for differentiation of endothelial cells into smooth muscle-like cells. Using a monoclonal antibody against smooth muscle-specific alpha-actin (SMA), we examined the immunohistochemical staining of this form of actin in avian endocardial cushion tissue formation. To determine whether TGF beta3 initiates the expression of SMA, the pre-migratory AV endothelial monolayer was cultured with or without chicken recombinant TGF beta3 and the expression of SMA was examined immunochemically. Migrating mesenchymal cells expressed SMA beneath the cell surface membrane. These cells showed a reduction of endothelial specific marker antigen, QH1. Stationary endothelial cells did not express SMA. The deposition of SMA in the mesenchymal tissue persisted until the end of the fetal period. Pre-migratory endothelial cells cultured in complete medium (CM199) that contained TGF beta3 expressed SMA, whereas cells cultured in CM199 alone did not. At the onset of the endothelial-mesenchymal transformation, migrating mesenchymal cells express SMA and the expression of this form of actin is upregulated by TGF beta3. The induction of the expression of SMA by TGF beta3 is one of the initial events in the cytoskeletal reorganization in endothelial cells which separate from one another during the initial phenotypic change associated with the endothelial-mesenchymal transformation.

Down-regulation of actin genes precedes microfilament network disruption and actin cleavage during p53-mediated apoptosis

Inactivation of Simian Virus 40 large T antigen, in cells immortalized with conditional mutants, leads to activation of p53 and apoptosis. We used the mRNA differential display method to identify genes differentially expressed during this process. We found that steady-state levels of mRNA for cytoplasmic actins decreased early during apoptosis. We also showed that, although the steady-state level of the corresponding proteins is not profoundly affected, they are substrates for an interleukin 1-beta converting enzyme (ICE)-like protease activated during the process. However, only a very small fraction of actin is proteolysed during the early stages of apoptosis. The microfilament network is affected and non polymerized actin accumulates in apoptotic bodies after the decrease of mRNA levels, but before a significant amount of actin is cleaved. This suggests that down-regulation of actin genes may be involved in microfilament rearrangements during p53-mediated apoptosis.

Distribution of alpha-vascular smooth muscle actin in the smooth muscle cells of the gastrointestinal tract of the chicken

Immunoreactivity specific for alpha-vascular smooth muscle actin (ASMA) was examined in the enteric smooth muscle cells along the entire length of the gastrointestinal tract of the chicken. Specificity for gamma-smooth muscle actin (GSMA) and desmin was also examined. All smooth muscle layers, i.e. the muscularis mucosae, and the circular and longitudinal muscle layers, showed immunoreactivity specific for GSMA and desmin throughout the gastrointestinal tract whereas immunoreactivity for ASMA differed between regions and muscle layers. In the oesophagus and crop, immunoreactivity for ASMA was observed in the muscularis mucosae and the inner and outer muscle layers, together with staining for GSMA and desmin. In the proventriculus, immunoreactivity for ASMA was observed in all smooth muscle cells in the inner layer of the muscularis mucosae and the longitudinal muscle layer. In the outer layer of the muscularis mucosae, immunoreactivity for ASMA on smooth muscle cells was observed on the luminal side and decreased in the serosal direction. In the intermediate muscles, immunoreactivity for ASMA was observed in the luminal portion, the intensity of staining decreasing gradually in the serosal direction. In contrast to the intermediate muscles, the latter muscles were negative for ASMA. In the pyloric region, the outer part was weakly immunopositive, while the inner part was intensely positive. In the small and large intestines, the muscularis mucosae and the longitudinal muscle layer were positive for ASMA. The outer part of the circular muscle layer was immunonegative for ASMA whereas the inner part was positive. The complex structure and contractile functions of each organ and muscle layers may be related to the difference patterns of expression of ASMA molecules in the smooth muscle cells.

The nucleotide sequence, structure, and preliminary studies on the transcriptional regulation of the bovine alpha skeletal actin gene

The promoters of mammalian striated muscle actin gene contain binding sites for a number of transcription factors. Examples are the CArG boxes, which bind a protein identical to or related to serum response factor (SRF), E boxes, which bind myogenic determination factors such as MyoD and myogenin, and -CCGCCC- motifs, which bind the transcription factor Sp1. To date, the only mammalian sequences isolated and analyzed are from rodent and human. We have now isolated and sequenced the bovine gene encoding alpha skeletal actin, including almost 3 kb of 5'-flanking region. When compared to the human and rodent genes (the only ones previously cloned and for which 5'-flanking sequences to only approximately -750 are known), there was the expected conservation in the coding region. A comparison of the promoter regions indicated that the bovine gene has three CArG boxes in the 5'-flanking region in positions identical to those in other species. The bovine proximal promoter is unique from those of human and rodent in that it has only one E box in the vicinity of the TATA box, near -350, whereas the other mammals have three. Far upstream sequences reveal clusters of E boxes near -2,500 and -1,500. A minimal promoter element, to -297, which has no E boxes, is sufficient to activate transcription in myotubes derived from rat L6 and mouse C2C12 myoblasts.

Molecular analysis of bovine actin gene and pseudogene sequences: expression of nonmuscle and striated muscle isoforms in adult tissues

Most studies on the tissue distribution of actin isoform transcripts have been done in small mammals such as rat and mouse. We have begun a characterization of the actin gene family in a large mammal, the bovine. The alpha skeletal gene was isolated, and an isoform-specific probe to the 3' untranslated region of the transcript identified. This probe, in combination with isoform specific probes for alpha cardiac, beta nonmuscle, and gamma nonmuscle actins, was used to examine expression of nonmuscle and striated muscle actin gene transcription in different tissues. In contrast to other species so far examined, striated muscle isoforms were more strictly tissue specific, with virtually no alpha cardiac isoform transcripts detected in skeletal muscle and almost no alpha skeletal transcripts in cardiac tissue. The distribution of the beta and gamma nonmuscle actins was also unique in bovine compared to other species. A partial beta-actin pseudogene, and the chromosomal DNA flanking one end of it, were also cloned and sequenced. This chromosomal site was found to be homologous to a viral integration site previously identified in simian virus 40 (SV40)-transformed rat cells, suggesting that this region of the chromosome may be a preferred target for insertion events.

Heterochronic expression of an adult muscle actin gene during ascidian larval development

Adultation is a heterochronic mode of development in which adult tissues and organs differentiate precociously during the larval phase. We have investigated the expression of an adult muscle actin gene during adultation in the ascidian Molgula citrina. Ascidians contain multiple muscle actin genes which are expressed in the larva, the adult, or during both phases of the life cycle. In ascidian species with conventional larval development, the larval mesenchyme cells, which are believed to be progenitors of the adult mesoderm, remain undifferentiated and do not express the muscle actin genes. In M. citrina, the mesenchyme cells differentiate precociously during larval development, suggesting a role in adultation. An adult muscle actin gene from M. citrina was obtained by screening a mantle cDNA library with a probe containing the coding region of SpMA1, a Styela plicata adult muscle actin gene. The screen yielded a cDNA clone designated McMA1, which contained virtually the complete coding and 3' noncoding regions of a muscle actin gene. The deduced McMA1 and SpMA1 proteins exhibit 97% identity in amino acid sequence and may be encoded by homologous genes. The McMA1 gene is expressed in juveniles and adults, but not in larval tail muscle cells, suggesting that it is an adult muscle actin gene. In situ hybridization with a 3' non-coding region probe was used to determine whether the McMA1 gene is expressed during adultation in M. citrina. McMA1 mRNA was first detected exclusively in the mesenchyme cells during the late tailbud stage and continued to accumulate in these cells during their migration into the future body cavity and heart primordium in the hatched larva. The McMA1 transcripts persisted in mesenchyme cells after larval metamorphosis. It is concluded that an adult muscle actin gene shows a heterochronic shift of expression into the larval phase during adultation in M. citrina.

An analysis of actin isoforms expressed in hair-cell enriched fractions of the chick basilar papilla by the polymerase chain reaction technique

Actin mRNA was characterised in hair-cell enriched fractions of the chick basilar papilla, by means of the reverse-transcription polymerase chain reaction technique. Primers were directed against the 3' untranslated portions of the actin mRNAs. Evidence for beta-cytoplasmic and gamma-cytoplasmic actin mRNA was found; no evidence was found for alpha-skeletal, alpha-cardiac or type 5 cytoplasmic actin mRNAs. Since beta-actin is known to form bundles of filaments whereas gamma-actin does not, this suggests that the hair-cell stereocilia are composed of beta-actin.

Cytoplasmic beta-actin promoter produces germ cell and preimplantation embryonic transgene expression

The cytoplasmic beta-actin promoter, commonly used as strong promoter in many gene regulation studies, produces a pattern of male germ cell and preimplantation, embryonic gene expression in transgenic mice. In seven of ten expressing transgenic lines, a chicken beta-actin-lacZ fusion gene was expressed in adult testes. In addition, five of the ten lines demonstrated transgene expression in the preimplantation mouse embryo. This is the first example of transgene expression at the stages of both gamete and early embryo. Overall, the site or transgene integration appeared to influence transgene expression in adult tissues.

A cytoplasmic actin gene from the silkworm Bombyx mori is expressed in tissues of endodermal origin and previtellogenic germ cells of transgenic Drosophila

A cytoplasmic actin gene from Bombyx mori introduced into Drosophila melanogaster by P-element mediated transformation, is efficiently transcribed in larvae, pupae and adults of the host. The exogenous mRNA has the same size as the one observed in the Bombyx cells and the intron located within the coding region is properly excised, indicating a correct recognition of the exogenous sequences by the Drosophila transcriptional and splicing machineries. The expression of the Bombyx gene in Drosophila tissues was determined by transforming flies with a hybrid gene in which a large part of the Bombyx actin coding sequences was replaced by those of the bacterial lac Z gene. This chimaeric gene is specifically and highly expressed, from the embryo to the adult of the transgenic lines, in tissues of endodermal origin, the midgut and its derivatives, i.e. gastric caeca, the outer layer of the proventriculus, and in the Malpighian tubules. This gene is also expressed, at a lower level, in germ cells but restricted to the sixteen cell cysts during previtellogenesis. The expression of the Bombyx gene during development of transgenic flies was compared to that of the two Drosophila endogenous cytoplasmic actin genes and the results are discussed.

Molecular cloning and expression of the chicken smooth muscle gamma-actin mRNA

We have investigated the expression of chicken smooth muscle gamma-actin mRNA by isolation and characterization of cDNAs representing this actin isoform and utilizing the cDNA to probe RNA from adult and developing cells. Nucleotide sequence elucidated from an apparent full length smooth muscle gamma-actin cDNA revealed that it contained 94 bp of 5' non-translated sequence, an open reading frame of 1131 bp, and 97 bp of 3' non-translated sequence. Within the 376 amino acid sequence deduced from the chicken cDNA were diagnostic amino acids at the NH2- and COOH-terminal regions which provided unequivocal identification of the gamma-enteric smooth muscle actin isoform. In addition, the chicken gamma-enteric actin deduced from our cDNA clones was found to differ from the sequence reported in earlier protein studies [J. Vandekerckhove and K. Weber, FEBS Lett. 102:219, 1979] by containing a proline rather than a glutamine at position 359 of the protein, indicating that the avian gamma-enteric actin isoform is identical to its mammalian counterpart. Comparison of the 5' and 3' non-translated sequence determined from the chicken cDNA to that elucidated for rat, mouse, and human showed that there is not a high degree of cross-species sequence conservation outside of the coding regions among these mRNAs. Northern hybridization analyses demonstrated that the gamma-enteric actin mRNA is expressed in adult aorta and oviduct tissues but not in adult skeletal muscle, cardiac muscle, liver, brain, and spleen tissues. The gamma-enteric actin mRNA was first observed in measurable quantities in gizzard tissue from 4-5 day embryos and increased in content in developing smooth muscle cells through 16-17 embryonic days. Following this initial increase during embryonic development, the gamma-enteric actin mRNA exhibits a decline in content until approximately 7 days posthatching, after which there is an increase in content to maximal levels found in adult gizzard tissue. In general, the developmental appearance of the gamma-enteric mRNA parallels that observed for this protein in previous studies indicating that the developmental expression of smooth muscle gamma-actin is regulated, in part, by an increased content of mRNA in chicken visceral smooth muscle cells during myogenesis.

Actin filament structure probed with monoclonal antibodies

The interaction of two monoclonal antibodies (mAbs) with actin has been characterized to map the epitopes defined by these mAbs and to determine the accessibility of these sites in the actin filament (F-actin). Both mAbs react specifically with actin in radioimmunoassays and Western blot assays, and by immunoprecipitation. The location of the epitopes within the primary structure of actin has been determined using limited proteolysis of actin and Western blots, or using immunoprecipitation of truncated actin fragments synthesized in a cell free translation assay. Both mAbs bind to the C-terminal fragment of actin (residues 68-375) produced by chymotrypsin cleavage. One epitope is further localized to a 9.9 kD peptide corresponding to residues 5-93. Therefore, the epitope defined by this mAb (2G11.4) lies between residues Lys68 and Glu93 of actin. The location of the other epitope was determined by immunoprecipitation of actin fragments synthesized in vitro. Removal of residues 356-365 from the C-terminus of actin completely abolished the binding of mAb 4E3.adl. Therefore, this mAb defines an epitope that involves residues between Trp356 and Ala365. The accessibility of these epitopes in native F-actin was determined with solution binding assays and characterized by immunoelectron microscopy. Monoclonal antibody 4E3.adl binds strongly to filaments, resulting in bundling or decoration of F-actin depending on the valency of the mAb, and indicating that the epitope is readily accessible in F-actin. In contrast, mAb 2G11.4 disrupts F-actin structure, resulting in the formation of an amorphous immunoprecipitate. These results place constraints on models of actin filament structure.

Antisense RNA inhibition of HPRT synthesis

The abundant production of antisense hypoxanthine phosphoribosyltransferase (HPRT) RNA in NIH-3T3, COS, or HeLa cells leads to an inhibition of HPRT synthesis. HPRT enzyme levels in cells transfected with mouse HPRT antisense RNA expression vectors are reduced to less than 1% of parental cell activity, resulting in resistance to 6-thioguanine (6TG). The expression of antisense HPRT RNA leads to a marked reduction in the steady-state levels of endogenous HPRT mRNA. Furthermore, we demonstrate that intron-specific antisense RNA, complementary to sequences adjacent to splice donor or acceptor sites of the first intron of the mouse HPRT gene, are effective in depressing endogenous HPRT levels. These studies suggest that antisense RNA can inhibit gene expression in the nucleus, possibly by perturbing nuclear RNA processing.

Analysis of a CR1 (chicken repeat) sequence flanking the 5' end of the gene encoding alpha-skeletal actin

Genomic Southern blots of chicken DNA, probed with radiolabeled DNA fragments that flank the 5'-end of the asa gene encoding chicken alpha-skeletal actin, indicate the presence of repetitive nucleotide sequences. Sequence analysis of this region reveals a member of the CR1 family of middle-repetitive elements. A 186-bp restriction fragment carrying the 3'-end of this CR1 element binds factor(s) present in nuclear extracts, as assayed by band-shift electrophoresis. However, the CR1 repeat has limited influence on transcription from the alpha-skeletal actin promoter, as assessed by CAT assays of transfected chicken myoblasts and fibroblasts.

Nucleotide sequence of an actin-encoding gene from Hydra attenuata: structural characteristics and evolutionary implications

We have determined the complete nucleotide sequence of an actin-encoding gene from Hydra attenuata as well as partial sequences of cDNA clones from two additional actin-encoding genes. The gene from the genomic clone contains a single intron, and has promoter and polyadenylation signals similar to those found in other species. The hydra genome has a very A + T-rich base composition (71%). This is reflected in the codon usage of the actin-encoding genes, which is strongly biased towards codons having A or T in the third position. The hydra actin-encoding gene family consists of three or more transcribed genes, two of which are very closely related to each other and probably arose by a recent gene duplication. Hydra actin, like other invertebrate actins, is more similar to the non-muscle isotypes of vertebrates than to the vertebrate muscle actins. Hydra actin is more similar to animal actins than to those of plants or fungi, which is consistent with the view that all metazoans arose from a single protist ancestor.

Regulation of chicken alpha and beta actin genes and their hybrids inserted into myogenic mouse cells

We have investigated the regulation of intact non-muscle (beta) and muscle-specific (skeletal alpha) chicken actin genes and of hybrids of these two genes (alpha 5'-beta 3' and beta 5'-alpha 3') transferred into the mouse myogenic non-fusing cell line BC3H1. BC3H1 cells express members of the actin multigene family in a differentiation-dependent manner. When proliferating, the cells accumulate large amounts of non-muscle actin mRNA; when the cells are induced to differentiate, the amount of non-muscle actin mRNA decreases and the amount of muscle-specific actin mRNA increases. The transferred beta-actin gene is efficiently expressed in undifferentiated cells and appropriately down-regulated upon differentiation. In contrast, the transferred alpha-actin gene is inefficiently expressed and not consistently up-regulated. Results with the intact and hybrid genes, taken together, are consistent with the hypothesis that both 5' and 3' halves of these genes contain sequences important in regulating the efficiency and/or developmental timing of their expression in BC3H1 cells. By nuclear run-on experiments we found no evidence for gene-specific changes in the rate of transcription of the transferred actin genes during myogenesis. We conclude that the differentiation-dependent changes in expression of the intact beta-actin gene in BC3H1 cells must be regulated at the post-transcriptional level.

Temporal resolution and sequential expression of muscle-specific genes revealed by in situ hybridization

The expression of muscle-specific mRNAs was analyzed directly within individual cells by in situ hybridization to chicken skeletal myoblasts undergoing differentiation in vitro. The probes detected mRNAs for sarcomeric myosin heavy chain (MHC) or the skeletal, cardiac, and beta isoforms of actin. Precise information as to the expression of these genes in individual cells was obtained and correlated directly with analyses of cell morphology and interactions, cell cycle stage, and immunofluorescence detection of the corresponding proteins. Results demonstrate that mRNAs for the two major muscle-specific proteins, myosin and actin, are not synchronously activated at the time of cell fusion. The mRNA for alpha-cardiac actin (CAct), known to be the predominant embryonic actin isoform in muscle, is expressed prior to cell fusion and prior to the expression of any isoform of muscle MHC mRNA. MHC mRNA accumulates rapidly immediately after fusion, whereas skeletal actin mRNA is expressed only in larger myofibers. Single cells expressing CAct mRNA have a characteristic short bipolar morphology, are in terminal G1, and do not contain detectable levels of the corresponding protein. In a pattern of expression reciprocal to that of CAct mRNA, beta-actin mRNA diminishes to low or undetectable levels in myofibers and in cells of the morphotype which expresses CAct mRNA. Finally, the intracellular distribution of mRNAs for different actin isoforms was compared using nonisotopic detection of isoform-specific oligonucleotide probes. This work illustrates a generally valuable approach to the analysis of cell differentiation and gene expression which directly integrates molecular, morphological, biochemical, and cell cycle information on individual cells.

Expression of actin mRNAs in denervated chicken skeletal muscle

The expression of actin genes in chicken pectoralis muscle denervated 1 week after hatching was examined 1-8 weeks after the operation by RNA blot hybridization using a generic actin cDNA probe and DNA probes specific for alpha-skeletal and alpha-cardiac actin genes. Total and alpha-skeletal actin mRNAs/microgram total RNA decreased to about half of the levels found in contralateral control muscle, while the expression of alpha-cardiac actin mRNA was up-regulated. Consequently, alpha-cardiac actin mRNA formed about 15% of the total actin mRNA as compared to less than 1% found in control muscle. The expression of actin genes in the denervated muscle was similar to that in the late embryonic muscle. These results suggest that innervation is required to show the expression pattern of striated muscle actin genes found in mature muscle.

A third striated muscle actin gene is expressed during early development in the amphibian Xenopus laevis

During early embryonic development in the frog Xenopus laevis, several muscle-specific actin genes encoding distinct actin protein isoforms are activated in cells of the embryonic muscle. In addition to the cardiac (or alpha 1) and skeletal (or alpha 2) actin genes, a third muscle-specific actin gene is expressed in the same embryonic tissue. We have determined the complete nucleotide sequence of this third gene and examined its expression in embryonic and adult tissues. During embryogenesis, this femoral (alpha 3) actin gene is activated several hours later than its cardiac and skeletal counterparts and its transcripts are first detected after neurulation. The gene encodes a skeletal-type actin protein and is expressed exclusively in skeletal muscle in the adult frog. Two copies of this gene have been isolated from the tetraploid species Xenopus laevis, differing by only a few nucleotides in their protein-coding sequence. The related, diploid species, Xenopus tropicalis, possesses a single copy of the alpha 3 gene and its transcript is similarly conserved in nucleotide sequence. However, the X. tropicalis gene is expressed exclusively in embryonic stages of development. Comparison of the X. laevis and X. tropicalis alpha 3 gene promoters reveals extensive sequence homology, including several copies of a repeated motif that is common to other vertebrate striated-muscle actin gene promoters.

Expression of the three unlinked isocoding actin genes of Physarum polycephalum

The actin gene family in Physarum polycephalum contains four unlinked loci: ardA, ardB, ardC, and ardD. The ardA locus is complex and probably contains two genes which we designated ardA2-7 and ardA2-17. cDNA clones corresponding to the ardB and ardC loci were isolated. Nucleic acid sequencing showed that these two cDNAs coded for the only abundant form of Physarum actin, which is 96% homologous to human gamma-cytoplasmic actin. The ardA2-17 gene also codes for this same actin protein (Nader et al., Gene 48, 133-144, 1986). The coding regions of ardB and ardC differ by 15 nucleotides. A comparison of the ardB and ardC sequences with ardA2-17 showed 73 and 77 nucleotide substitutions, respectively, in the coding regions. The noncoding regions of these three sequences were not homologous to each other or to the noncoding regions of actin genes from other organisms. Southern genomic hybridizations indicated that the ardA2-7 and ardD genes have weak sequence similarities to the three isocoding actin genes and thus form a different subclass of the family. Northern hybridizations showed that the ardB and ardC transcripts varied in abundance but were present in all the developmental stages. No ardA2-17 transcripts were seen. The relative abundance of the ardB and ardC transcripts was measured in amoebae and plasmodia by S1 nuclease protection and dot hybridization assays. A ratio of approximately 3:1 for ardC versus ardB was found for both stages. P. polycephalum is the first organism shown to contain three unlinked isocoding actin genes, of which at least two are expressed.

Use of oligodeoxynucleotide probes for quantitative in situ hybridization to actin mRNA

We have employed an analytical approach for the development of an in situ hybridization methodology using synthetic oligodeoxynucleotide probes for actin messenger RNA detection in cultures of chicken fibroblasts and myoblasts. The methodology developed shows that oligonucleotides can complement the use of nick-translated probes in specific situations. Since they can be made to specific nucleic acid regions independent of restriction enzyme sites, they may be the most convenient approach for analysis of gene families among which sequences are highly conserved. However, it was found that oligonucleotides synthesized to different regions of a messenger RNA behave in situ with differing efficiencies, indicating that not all target sequences are equivalent. Therefore it was necessary to screen several oligonucleotide probes to a target molecule to find the optimal one. The convenience of using synthetic DNA probes makes it worthwhile to explore some of these characteristic properties so as to increase the sensitivity of this approach beyond its application to targets in high abundance.

Actin typing of rhabdomyosarcomas shows the presence of the fetal and adult forms of sarcomeric muscle actin

We analyzed actin expression in two human rhabdomyosarcomas as well as in three rhabdomyosarcomas induced in rats by the injection of nickel sulfide. All five tumors exhibited appreciable amounts of the sarcomeric alpha-actin types, in line with their myogenic differentiation. The level of these actins was particularly high in the rat tumors, which according to morphological criteria, all showed a higher degree of differentiation than the human tumors. Interestingly, in both human tumors and in two of the three rat tumors, the level of the cardiac alpha-actin type was significantly higher than that of adult skeletal muscle alpha-actin. Taken together with the results of recent reports indicating that the cardiac alpha-actin type is a marker of embryonic and fetal skeletal muscle, our findings indicate that rhabdomyosarcomas express the embryonic sarcomeric actin isoform.