The complete amino acid sequence of actins from bovine aorta, bovine heart, bovine fast skeletal muscle, and rabbit slow skeletal muscle. A protein-chemical analysis of muscle actin differentiation

Water channel aquaporin-2 directly binds to actin

Water channel aquaporin-2 (AQP2) strictly regulates body water homeostasis in mammals. Trafficking of AQP2 to the apical membrane is critical to the reabsorption of water in renal collecting ducts. Controlled apical positioning of AQP2 suggests the interaction of AQP2 with other proteins. To isolate AQP2-binding proteins, immunoaffinity chromatography of extracts from rat kidney papilla was performed using a column covalently coupled with anti-AQP2 antibody. Using this method 42-kDa protein was purified and subsequently identified as beta- and gamma-isoforms of actin by two-dimensional gel analysis and matrix-assisted laser desorption-ionization time-of-flight mass spectrometry. AQP2 was indeed coimmunoprecipitated with actin from cell lysates of rat kidney papilla. In addition, surface plasmon resonance analyses showed that the C-terminal fragment of AQP2 strongly bound to actin and the K(D) value was 3.18x10(-8)M. In this experiment we have elucidated the direct binding of channel protein AQP2 to cytoskeletal protein actin, providing a novel mechanism for trafficking of not only AQP2 but also recycling channel proteins.

Characterization of a cardiac-specific enhancer, which directs {alpha}-cardiac actin gene transcription in the mouse adult heart

Expression of the mouse alpha-cardiac actin gene in skeletal and cardiac muscle is regulated by enhancers lying 5' to the proximal promoter. Here we report the characterization of a cardiac-specific enhancer located within -2.354/-1.36 kbp of the gene, which is active in cardiocytes but not in C2 skeletal muscle cells. In vivo it directs reporter gene expression to the adult heart, where the proximal promoter alone is inactive. An 85-bp region within the enhancer is highly conserved between human and mouse and contains a central AT-rich site, which is essential for enhancer activity. This site binds myocyte enhancer factor (MEF)2 factors, principally MEF2D and MEF2A in cardiocyte nuclear extracts. These results are discussed in the context of MEF2 activity and of the regulation of the alpha-cardiac actin locus.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Cellular disorganization and extensive apoptosis in the developing heart of mice that lack cardiac muscle alpha-actin: apparent cause of perinatal death

Mice that lack cardiac muscle alpha-actin die during the perinatal period. Approximately 56% of mice that are homozygous null (-/-) for a functional cardiac alpha-actin gene do not survive to term, and the remainder generally die within 2 wk of birth. We found that there were neither morphologic differences nor differences in the extent of apoptosis between the mutant and normal hearts on embryonic day (E) 12 and E14 of development. However, apoptosis was greater in the hearts of homozygous null mice on E17 and postnatal day 1 when compared with wild-type hearts. The antiapoptotic factor Bcl-x/(L) was localized in regions adjacent to where apoptosis was detected. The distribution patterns of the apoptosis triggering protein p53 were similar to those of apoptotic cells. The growth of the prenatal and postnatal hearts of the cardiac alpha-actin-deficient mice was retarded, and the cytoplasmic filaments were disorganized. Although apoptotic cells were observed in both the atria and ventricles in the hearts of the homozygous null animals, the frequency was greater in the ventricles than in the atria. Our results indicate that the functional and structural disturbances in the mice with a homozygous lack of cardiac alpha-actin seem to be due to disorganized development of acto-myosin filaments in the affected cardiomyocytes. Other actin isoforms cannot compensate for the lack of cardiac alpha-actin, and this seems to induce apoptosis in defective cardiac myocytes, which are not able to cope with the increased workload in the perinatal phase.

Myofibrillogenesis in the developing chicken heart: Role of actin isoforms and of the pointed end actin capping protein tropomodulin during thin filament assembly

Recently, important differences between myofibrillogenesis in cultured cardiomyocytes vs. the three-dimensional setting in situ could be determined. We investigated thin filament assembly in situ by confocal microscopy of whole-mount preparations of immunostained embryonic chicken hearts. Of interest, a distinct localisation of different actin isoforms was observed in immature thin filaments. Cardiac alpha-actin is restricted to filaments with a length comparable to mature thin filaments as soon as the first contractions occur, while vascular alpha-actin makes up filaments that extend toward the M-band. The pointed-end actin filament capping protein tropomodulin can be found initially in close association with the plasma membrane, but attains its mature localisation pattern at the ends of the thin filaments only comparatively late during myofibrillogenesis. Thus tropomodulin acts as a length stabilising element of actin filaments also in developing cardiomyocytes in situ, but plays an additional role together with membrane-associated actin filaments in the earliest steps of myofibril assembly.

Human cytoplasmic actin proteins are encoded by a multigene family

We characterized nine human actin genes that we isolated (Engel et al., Proc. Natl. Acad. Sci. U.S.A. 78:4674-4678, 1981) from a library of cloned human DNA. Measurements of the thermal stability of hybrids formed between each cloned actin gene and alpha-, beta-, and gamma-actin mRNA demonstrated that only one of the clones is most homologous to sarcomeric actin mRNA, whereas the remaining eight clones are most homologous to cytoplasmic actin mRNA. By the following criteria we show that these nine clones represent nine different actin gene loci rather than different alleles or different parts of a single gene: (i) the restriction enzyme maps of the coding regions are dissimilar; (ii) each clone contains sufficient coding region to encode all or most of an entire actin gene; and (iii) each clone contains sequences homologous to both the 5' and 3' ends of the coding region of a cloned chicken beta-actin cDNA. We conclude, therefore, that the human cytoplasmic actin proteins are encoded by a multigene family.

Human cytoplasmic actin proteins are encoded by a multigene family

We characterized nine human actin genes that we isolated (Engel et al., Proc. Natl. Acad. Sci. U.S.A. 78:4674-4678, 1981) from a library of cloned human DNA. Measurements of the thermal stability of hybrids formed between each cloned actin gene and alpha-, beta-, and gamma-actin mRNA demonstrated that only one of the clones is most homologous to sarcomeric actin mRNA, whereas the remaining eight clones are most homologous to cytoplasmic actin mRNA. By the following criteria we show that these nine clones represent nine different actin gene loci rather than different alleles or different parts of a single gene: (i) the restriction enzyme maps of the coding regions are dissimilar; (ii) each clone contains sufficient coding region to encode all or most of an entire actin gene; and (iii) each clone contains sequences homologous to both the 5' and 3' ends of the coding region of a cloned chicken beta-actin cDNA. We conclude, therefore, that the human cytoplasmic actin proteins are encoded by a multigene family.

Effects of tropomyosin internal deletion Delta23Tm on isometric tension and the cross-bridge kinetics in bovine myocardium

Tropomyosin (Tm) spans seven actin monomers and contains seven quasi-repeating, loosely similar regions, 1-7. Deletion of regions 2-3 decreases the in vitro sliding speed of synthetic filaments of actin-Tm-Troponin (Tn), and weakens Tm binding to the actin-myosin subfragment 1 (S1) complex (acto-S1). The thin filament was selectively removed from bovine myocardium by gelsolin, and the actin filament was reconstituted, followed by further reconstitution with Tm and Tn. In this reconstitution, full-length Tm (control) was compared with Tm internal deletion mutant Delta23Tm, which lacks residues 47-123 (regions 2-3). The effects of phosphate, MgATP, MgADP and Ca2+ were studied in Tm-reconstituted myocardium and Delta23Tm-reconstituted myocardium at pH 7.00 and 25 degrees C. In Delta23Tm, both isometric tension and stiffness were about 40 % of the control. The Hill factor with Delta23Tm, deduced from the pCa-tension plot, was 1.4 times that of the control, but the Ca2+ sensitivity was the same. Sinusoidal analysis indicated that the cross-bridge number in force-generating states was not decreased with Delta23Tm. We conclude that the thin filament cooperativity is increased with Delta23Tm, presumably because of the increased density of the Ca2+-binding sites. We further conclude that tension per cross-bridge is 40 % of control and stiffness per cross-bridge is 40 % of control in Delta23Tm. These results are consistent with the idea that Tm modifies the actin-myosin interface so as to increase the stereospecific interaction between moieties of actin and myosin. In Delta23Tm, the interface may not have a perfect stereospecific match so that the tension- and stiffness-generating capacity is greatly diminished.

Temperature and the expression of seven muscle-specific protein genes during embryogenesis in the Atlantic cod Gadus morhua L

Seven cDNA clones coding for different muscle-specific proteins (MSPs) were isolated from the fast muscle tissue of Atlantic cod Gadus morhua L. In situ hybridization using cRNA probes was used to characterize the temporal and spatial patterns of gene expression with respect to somite stage in embryos incubated at 4 degrees C, 7 degrees C and 10 degrees C. MyoD transcripts were first observed in the presomitic mesoderm prior to somite formation, and in the lateral compartment of the forming somites. MyoD expression was not observed in the adaxial cells that give rise to the slow muscle layer, and expression was undetectable by in situ hybridization in the lateral somitic mesoderm after the 35-somite stage, during development of the final approximately 15 somites. RT-PCR analysis, however, confirmed the presence of low levels of the transcript during these later stages. A phylogenetic comparison of the deduced aminoacid sequences of the full-length MyoD cDNA clone and those from other teleosts, and inference from the in situ expression pattern suggested homology with a second paralogue (MyoD2) recently isolated from the gilthead seabream Sparus aurata. Following MyoD expression, alpha-actin was the first structural gene to be switched on at the 16-somite stage, followed by myosin heavy chain, troponin T, troponin I and muscle creatine kinase. The final mRNA in the series to be expressed was troponin C. All genes were switched on prior to myofibril assembly. The troponin C sequence was unusual in that it showed the greatest sequence identity with the rainbow trout Oncorhynchus mykiss cardiac/slow form, but was expressed in the fast myotomal muscle and not in the heart. In addition, the third TnC calcium binding site showed a lower level of sequence conservation than the rest of the sequence. No differences were seen in the timing of appearance or rate of posterior progression (relative to somite stage) of any MSP transcripts between embryos raised at the different temperatures. It was concluded that myofibrillar genes are activated asynchronously in a distinct temporal order prior to myofibrillar assembly and that this process was highly canalized over the temperature range studied.

A genomewide survey of developmentally relevant genes in Ciona intestinalis. IX. Genes for muscle structural proteins

Ascidians are simple chordates that are related to, and may resemble, vertebrate ancestors. Comparison of ascidian and vertebrate genomes is expected to provide insight into the molecular genetic basis of chordate/vertebrate evolution. We annotated muscle structural (contractile protein) genes in the completely determined genome sequence of the ascidian Ciona intestinalis, and examined gene expression patterns through extensive EST analysis. Ascidian muscle protein isoform families are generally of similar, or lesser, complexity in comparison with the corresponding vertebrate isoform families, and are based on gene duplication histories and alternative splicing mechanisms that are largely or entirely distinct from those responsible for generating the vertebrate isoforms. Although each of the three ascidian muscle types - larval tail muscle, adult body-wall muscle and heart - expresses a distinct profile of contractile protein isoforms, none of these isoforms are strictly orthologous to the smooth-muscle-specific, fast or slow skeletal muscle-specific, or heart-specific isoforms of vertebrates. Many isoform families showed larval-versus-adult differential expression and in several cases numerous very similar genes were expressed specifically in larval muscle. This may reflect different functional requirements of the locomotor larval muscle as opposed to the non-locomotor muscles of the sessile adult, and/or the biosynthetic demands of extremely rapid larval development.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Expression of foreign genes in lamprey embryos: an approach to study evolutionary changes in gene regulation

Evolution in development can be viewed as a sequence of changes in gene regulation. To investigate the cross-species compatibility of 5' upstream regulatory regions, we introduced exogenous gene constructs derived from a gnathostome genome into fertilized eggs of the Japanese lamprey, Lampetra japonica, a sister group of the gnathostomes. Eggs were injected with gene constructs in which a sequence encoding the green fluorescent protein (GFP) had been located downstream of either a virus promoter or 5' regulatory regions of medaka actin genes. Reporter gene expression was recorded for more than a month starting two days after injection. Although the expression patterns were highly mosaic and differed among individuals, GFP was expressed predominantly in the striated muscles of lamprey embryos when driven by the 5' upstream regions of the medaka muscle actin genes. This implies that a pan-vertebrate muscle-specific gene regulatory mechanism may have evolved before the agnathan/gnathostome divergence. This gene-transfer technique potentially facilitates the visualization of cells in various differentiating tissues throughout development. The introduction of developmental genes of the lamprey or other animals into lamprey embryos is another potentially important application, one that could provide us with information on the evolutionary changes in functions of genes or gene cascades.

Mass spectral determination of skeletal/cardiac actin isoform ratios in cardiac muscle

Skeletal and cardiac muscle contains actin isoforms that vary by two juxtaposed amino acids and two amino acid substitutions (Met299Leu and Ser358Thr). This close sequence homology does not allow cardiac and skeletal actin isoforms to be resolved in traditional SDS-PAGE analysis as the molecular weights (Deltamass = 32 Da) are not significantly different and the pIs are identical (5.2). Although cardiac actin is the predominant form in cardiac muscle, there appears to be a specific skeletal/cardiac actin ratio in a normal heart that may vary in a compromised or diseased heart. In an effort to ascertain the validity of this hypothesis we developed a mass spectrometric technique to measure the ratio of skeletal to cardiac actin. The technique involves purification of muscle actin and subsequent liquid chromatography coupled with electrospray ionization Fourier transform ion cylcotron resonance (LC/FTICR-MS) mass spectrometry. A 7 Tesla FTICR mass spectrometer was utilized to compare skeletal/cardiac actin isoform ratios. Additionally, a new dual electrospray ionization source was employed to determine accurate masses of the alpha-skeletal and alpha-cardiac actins.

Immunocytochemistry of myoepithelial cells in the salivary glands

MECs are distributed on the basal aspect of the intercalated duct and acinus of human and rat salivary glands. However, they do not occur in the acinus of rat parotid glands, and sometimes occur in the striated duct of human salivary glands. MECs, as the name implies, have structural features of both epithelial and smooth muscle cells. They contract by autonomic nervous stimulation, and are thought to assist the secretion by compressing and/or reinforcing the underlying parenchyma. MECs can be best observed by immunocytochemistry. There are three types of immunocytochemical markers of MECs in salivary glands. The first type includes smooth muscle protein markers such as alpha-SMA, SMMHC, h-caldesmon and basic calponin, and these are expressed by MECs and the mesenchymal vasculature. The second type is expressed by MECs and the duct cells and includes keratins 14, 5 and 17, alpha 1 beta 1 integrin, and metallothionein. Vimentin is the third type and, in addition to MECs, is expressed by the mesenchymal cells and some duct cells. The same three types of markers are used for studying the developing gland. Development of MECs starts after the establishment of an extensively branched system of cellular cords each of which terminates as a spherical cell mass, a terminal bud. The pluripotent stem cell generates the acinar progenitor in the terminal bud and the ductal progenitor in the cellular cord. The acinar progenitor differentiates into MECs, acinar cells and intercalated duct cells, whereas the ductal progenitor differentiates into the striated and excretory duct cells. Both in the terminal bud and in the cellular cord, the immediate precursors of all types of the epithelial cells appear to express vimentin. The first identifiable MECs are seen at the periphery of the terminal bud or the immature acinus (the direct progeny of the terminal bud) as somewhat flattened cells with a single cilium projecting toward them. They express vimentin and later alpha-SMA and basic calponin. At the next developmental stage, MECs acquire cytoplasmic microfilaments and plasmalemmal caveolae but not as much as in the mature cell. They express SMMHC and, inconsistently, K14. This protein is consistently expressed in the mature cell. K14 is expressed by duct cells, and vimentin is expressed by both mesenchymal and epithelial cells. After development, the acinar progenitor and the ductal progenitor appear to reside in the acinus/intercalated duct and the larger ducts, respectively, and to contribute to the tissue homeostasis. Under unusual conditions such as massive parenchymal destruction, the acinar progenitor contributes to the maintenance of the larger ducts that result in the occurrence of striated ducts with MECs. The acinar progenitor is the origin of salivary gland tumors containing MECs. MECs in salivary gland tumors are best identified by immunocytochemistry for alpha-SMA. There are significant numbers of cells related to luminal tumor cells in the non-luminal tumor cells that have been believed to be neoplastic MECs.

Staurosporine-induced cleavage of alpha-smooth muscle actin during myofibroblast apoptosis

To examine the possibility that staurosporine is applicable for the treatment of abnormal scar formation such as hypertrophic scar and keloid, the cellular process during staurosporine-induced apoptosis was analyzed in myofibroblasts isolated from a rat granulation tissue pouch. Staurosporine induced myofibroblast apoptosis in a time- and dose-dependent manner with typical morphologic changes. Staurosporine (1 microM) activated caspase-3 up to 3.6-fold by cleaving pro-caspase-3 (32 kDa) to active forms (17, 19, and 20 kDa). Microfilaments mainly composed of alpha-smooth muscle actin, a contractile protein characterizing myofibroblasts, were degraded during staurosporine-induced apoptosis. The degradation of alpha-smooth muscle actin bundles was detected as early as 1 h after the treatment with staurosporine. Recombinant active caspase-3 and staurosporine-stimulated caspase-3 both cleaved purified alpha-smooth muscle actin in vitro. These results suggested that alpha-smooth muscle actin is directly degraded by caspase-3 in response to apoptotic stimuli in myofibroblasts. In addition, bleomycin (100 ng per ml) and cisplatin (1 mM) also induced myofibroblast apoptosis by activating caspase-3, suggesting that these agents have a potential therapeutic value for abnormal scar formation.

The role of MeH73 in actin polymerization and ATP hydrolysis

In actin from many species H73 is methylated, but the function of this rare post-translational modification is unknown. Although not within bonding distance, it is located close to the gamma-phosphate of the actin-bound ATP. In most crystal structures of actin, the delta1-nitrogen of the methylated H73 forms a hydrogen bond with the carbonyl of G158. This hydrogen bond spans the gap separating subdomains 2 and 4, thereby contributing to the forces that close the interdomain cleft around the ATP polyphosphate tail. A second hydrogen bond stabilizing interdomain closure exists between R183 and Y69. In the closed-to-open transition in beta-actin, both of these hydrogen bonds are broken as the phosphate tail is exposed to solvent. Here we describe the isolation and characterization of a mutant beta-actin (H73A) expressed in the yeast Saccharomyces cerevisiae. The properties of the mutant are compared to those of wild-type beta-actin, also expressed in yeast. Yeast does not have the methyl transferase necessary to methylate recombinant beta-actin. Thus, the polymerization properties of yeast-expressed wild-type beta-actin can be compared with normally methylated beta-actin isolated from calf thymus. Since earlier studies of the actin ATPase almost invariably employed rabbit skeletal alpha-actin, this isoform was included in these comparative studies on the polymerization, ATP hydrolysis, and phosphate release of actin. It was found that H73A-actin exchanged ATP at an increased rate, and was less stable than yeast-expressed wild-type actin, indicating that the mutation affects the spatial relationship between the two domains of actin which embrace the nucleotide. At physiological concentrations of Mg(2+), the kinetics of ATP hydrolysis of the mutant actin were unaffected, but polymer formation was delayed. The comparison of methylated and unmethylated beta-actin revealed that in the absence of a methyl group on H73, ATP hydrolysis and phosphate release occurred prior to, and seemingly independently of, filament formation. The comparison of beta and alpha-actin revealed differences in the timing and relative rates of ATP hydrolysis and P(i)-release.

Elementary steps of the cross-bridge cycle in bovine myocardium with and without regulatory proteins

The role of regulatory proteins in the elementary steps of the cross-bridge cycle in bovine myocardium was investigated. The thin filament was selectively removed by gelsolin and the actin filament was reconstituted without tropomyosin or troponin. Further reconstitution was achieved by adding tropomyosin and troponin. The effects of MgATP and phosphate (Pi) on the rate constants of exponential processes were studied in control, actin filament-reconstituted, and thin filament-reconstituted myocardium at pCa < or = 4.66, pH 7.00, 25 degrees C. In control myocardium, the MgATP association constant was 9.1 +/- 1.3 mM(-1), and the Pi association constant 0.14 +/- 0.04 mM(-1). The equilibrium constant of the cross-bridge detachment step was 2.6 +/- 0.4, and the equilibrium constant of the force generation step was 0.59 +/- 0.04. In actin filament-reconstituted myocardium without regulatory proteins, the MgATP association constant was approximately the same, and the Pi association constant increased to 2.8x. The equilibrium constant of cross-bridge detachment decreased to 0.2x, but the equilibrium constant of the force generation step increased to 4x. These kinetic constants regained control values after reconstitution of the thin filament. These results indicate that tension/cross-bridge in the presence of regulatory proteins is approximately 1.5-1.7x of that in the absence of regulatory proteins. These results further indicate that regulatory proteins promote detachment of cross-bridges.

Dynamics of actin and alpha-actinin in nascent myofibrils and stress fibers

Actin labeled with fluorescein isothiocyanate (FITC) and alpha-actinin labeled with rhodamine (rh) were co-injected into chick embryonic cardiac myocytes and fibroblasts. In cardiomyocytes, FITC-actin was distributed in nonstriated lines, linearly arranged punctate structures with short intervals, and cross-striated bands with regular sarcomeric intervals. rh-alpha-Actinin was seen to be distributed in the same pattern in the former two portions, and in the center of each striation in the latter portion. Photobleaching of structures incorporated with these fluorescent analogs revealed that the fluorescent recovery rate of actin decreased in the order of nonstriated > punctated > striated portions, while that of alpha-actinin was low and stable at all portions. During the transition phase from punctate to regular sarcomere structures of these proteins, short spaced alpha-actinin dots adjoined each other and aligned with Z bands of neighboring myofibrils. It appears that both the difference in exchangeability between actin and alpha-actinin molecules and the movement of alpha-actinin dots during this phase of myofibrillogenesis are related to sarcomere lengthening and I-Z-I brush formation; adjoining dots of low-exchangeable alpha-actinin may provide favorable situations for exchangeable actin molecules in filaments to elongate and/or rearrange. In fibroblasts, both FITC-actin and rh-alpha-actinin formed nonstriated lines. In these cells, exchangeabilities of both proteins were high and similar in rate. This seems to indicate that stress fibers are constantly exchanging their components for motile and other vital functions of these cells. The high exchangeabilities of both proteins in stress fibers showthat these fibers are clearly different from nonstriated, stress-fiber like structures of nascent myofibrils.

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.

Remodeling the cardiac sarcomere using transgenesis

An underpinning of basic physiology and clinical medicine is that specific protein complements underlie cell and organ function. In the heart, contractile protein changes correlating with functional alterations occur during both normal development and the development of numerous pathologies. What has been lacking for the majority of these observations is an extension of correlation to causative proof. More specifically, different congenital heart diseases are characterized by shifts in the motor proteins, and the genetic etiologies of a number of different dilated and hypertrophic cardiomyopathies have been established as residing at loci encoding the contractile proteins. To establish cause, or to understand development of the pathophysiology over an animal's life span, it is necessary to direct the heart to synthesize, in the absence of other pleiotropic changes, the candidate protein. Subsequently one can determine whether or how the protein's presence causes the effects either directly or indirectly. By affecting the heart's protein complement in a defined manner, the potential to establish the function of different proteins and protein isoforms exists. Transgenesis provides a means of stably modifying the mammalian genome. By directing expression of engineered proteins to the heart, cardiac contractile protein profiles can be effectively remodeled and the resultant animal used to study the consequences of a single, genetic manipulation at the molecular, biochemical, cytological, and physiological levels.

Changes in contractile protein gene expression with ageing and with captopril-induced regression of hypertrophy in the spontaneously hypertensive rats

BACKGROUND

Left ventricular hypertrophy (LVH) is present in young spontaneously hypertensive rats (SHR) compared with normotensive Wistar-Kyoto (WKY) rats and treatment of SHR with captopril leads to regression of LVH. Hypertrophy produces changes in gene expression for myofibrillar proteins with increased ratios of skeletal to cardiac actin and beta to alpha-myosin heavy chain (MHC).

OBJECTIVES

The objective of this study was to follow changes in transcript prevalence for these four proteins during ageing and with captopril treatment in SHR and WKY rats.

METHODS

Untreated SHR and WKY rats were studied at 100, 156, 350 and 450 days. Groups at 100 and 350 days were divided into a treatment group (given captopril) and untreated controls. Transcripts were measured using in situ hybridization.

RESULTS

Both cardiac and skeletal actin were increased in untreated SHR compared to WKY rats (P<0.01 and P<0.05, respectively). alpha-MHC was increased (P<0.01) whilst beta-MHC was normal in 100-day-old SHR (an age when LVH was present) compared with WKY rats. With ageing, alpha-MHC declined and beta-MHC increased giving the increased ratio of beta to alpha-MHC transcripts reported by other investigators. Treatment of SHR led to a significant decline in skeletal actin transcripts (P< 0.01) and reversed the rise in beta-MHC expression that occurred with ageing (P< 0.01).

CONCLUSIONS

LVH in SHR is associated with increased skeletal and cardiac actin transcripts. Despite unequivocal LVH in SHR at 100 days of age, alpha rather than beta-MHC transcripts were increased. Only with ageing did the classically reported increased ratio of beta to alpha-MHC transcripts become apparent Captopril treatment reduced skeletal actin transcripts and reversed the increase in beta-MHC that occurred with ageing.

Genes expressed in the amphioxus notochord revealed by EST analysis

The notochord cell of the cephalochordate amphioxus adult is unique due to the occurrence of myofilaments in the cytoplasm. The present EST (expressed sequence tag) analysis targeted mRNAs of the amphioxus notochord to determine genes that are expressed there. Notochord cells were isolated from Branchiostoma belcheri adults, from which a cDNA library was constructed. Analysis of a set of 257 ESTs (both 5' and 3' ends) showed that about 11% of the cDNAs are related to muscle genes, while 9% of them are genes for extracellular matrix proteins associated with formation of the notochordal sheath. The muscle-related genes included actin, tropomyosin, troponin I, myosin regulatory light chain, myosin light chain kinase, myosin heavy chain, calmodulin, calponin, calcium vector protein, creatine kinase, muscle LIM protein, and SH3-binding glutamate-rich protein, suggesting that vertebrate skeletal and smooth muscle-type genes are simultaneously expressed in the amphioxus notochord. Nucleotide sequences of cDNAs for actin, tropomyosin, troponin I, and a few others were completely determined to substantiate the conclusions. The chordate muscle-type actin is distinguishable from the cytoplasmic-type actin by the usage of amino acid residues at 20 diagnostic positions. Interestingly, analysis of the usage of amino acid residues at these positions showed that the "amphioxus notochord actin" is a unique intermediate between muscle-type and cytoplasmic-type actins. These results strongly suggest that the notochord of adult amphioxus is a mechanical swimming organ and its role is quite different from the role of the vertebrate embryonic notochord, which functions as a source of signals required for body plan formation.

The skeletal muscle alpha-actin gene of channel catfish (Ictalurus punctatus) and its association with piscine specific SINE elements

The alpha-actin gene of channel catfish (Ictalurus punctatus) was cloned and sequenced. The gene has a similar organization and exhibited a high level of sequence similarity to those from other vertebrate animals. The upstream region of the alpha-actin gene included a TATA box, a CAAT box, three E-boxes, and a CArG box. Nested deletion segments containing these transcriptional motifs were fused to the reporter gene chloramphenicol acetyl transferase (CAT). Transfection of the clones into C2C12 cells indicated that all these motifs are required for transcriptional activities. The channel catfish alpha-actin gene is associated with two distinct short interspersed repetitive elements (SINEs). The first SINE element showed high levels of sequence similarity to the zebrafish Mermaid element, while the second SINE element is not similar to the Mermaid element except for an 8bp sequence CCCCGTGC suggesting their evolutionary linkage. However, the second SINE element appeared to co-exist with the Mermaid element in most cases and therefore was designated as the Merman element. Approximately 9000 copies and 1200 copies of the Mermaid and Merman elements exist per haploid channel catfish genome, respectively. BLAST searches indicated that both the Mermaid and the Merman elements were frequently associated with gene sequences, mostly those of aquatic animals, suggesting their evolutionary origin in association with aquatic organisms and their function in shaping the evolution of genomes in aquatic animals.

Drosophila ACT88F indirect flight muscle-specific actin is not N-terminally acetylated: a mutation in N-terminal processing affects actin function

Many eukaryotic proteins are co and post-translationally modified at their N termini by removal of one or two amino acid residues and N(alpha)-acetylation. Actins show two different forms of N-terminal processing dependent on their N-terminal sequence. In class II actins, which include muscle actins, the common primary sequence of Met-Cys-Asp-actin is processed to acetyl-Asp-actin. The functional significance of this in vivo is unknown. We have studied the indirect flight muscle-specific actin, ACT88F, of Drosophila melanogaster. Our results show that ACT88F is N-terminally processed in vivo as a class II actin by removal of the first two amino acid residues (Met and Cys), but that uniquely the N terminus is not acetylated. In addition we show that ACT88F is methylated, probably at His73. Flies carrying the mod(-) mutation fail to complete post-translational processing of ACT88F. We propose that the mod gene product is normally responsible for removing N-acetyl-cysteine from actin. The biological significance of this process is demonstrated by observations that retention of the N-acetyl-cysteine in ACT88F affects the flight muscle function of mod(-) flies. This suggests that the extreme N terminus affects actomyosin interactions in vivo, a proposal we have examined by in vitro motility assays of ACT88F F-actin from mod(-) flies. The mod(-) actin only moves in the presence of methylcellulose, a viscosity-enhancing agent, where it moves at velocities slightly, but significantly, reduced compared to wild-type. These data confirm that N-acetyl-cysteine at the N terminus affects actomyosin interactions, probably by reducing formation of the initial actomyosin collision complex, a process known to involve the actin N terminus.

A subpopulation of cardiomyocytes expressing alpha-skeletal actin is identified by a specific polyclonal antibody

The NH(2)-terminal decapeptide of alpha-skeletal actin that contains a primary sequence specific for this isoform was used to raise a polyclonal antibody in rabbits. Using sequential affinity chromatography, we recovered from serum antibodies reacting exclusively with alpha-skeletal actin when tested by immunoblotting and immunofluorescence. Epitope mapping by means of competition assays with synthetic peptides indicated that the acetyl group and the first 9 amino acids are essential for specificity. The monospecific antibody was then used to investigate the distribution of alpha-skeletal actin in the myocardium of newborn and normal or hypertensive (with or without fibrotic areas) adult rats. Immunostaining of normal heart revealed that alpha-skeletal actin is diffusely distributed within practically all myocardial fibers of the newborn rat, whereas it is restricted to a small proportion of adult rat cardiomyocytes, which appear intensely stained. A correlation, albeit not complete, was found between the distribution of alpha-skeletal actin and beta-myosin heavy chain. During cardiac hypertrophy induced by aortic ligature between the renal arteries, the expressions of alpha-skeletal actin mRNA and protein were increased. The distribution of immunostaining had a focal pattern similar to that of normal adult rats, reactive fibers being more numerous and more intensely stained compared with normal myocardium. Positive fibers were particularly abundant at the periphery of fibrotic areas. Using this antibody, we have demonstrated for the first time the differential distribution of alpha-skeletal actin in heart tissues. Changes in the distribution of this isoform in hypertrophic heart provide new insight into the mechanisms by which the heart adapts to work overload. This antibody will prove useful in exploring the mechanisms of expression of alpha-skeletal actin and in defining its role in physiological and pathological situations.

Characterization of an E-box-dependent cis element in the smooth muscle alpha-actin promoter

Identification of the regulators of smooth muscle specific gene expression is critical for understanding smooth muscle cell (SMC) differentiation and the alterations in SMC phenotype seen in vascular diseases. Previous studies have identified that a 2-bp mutation in a conserved cis-acting element (TGTTTATC) in the promoter of the chicken smooth muscle (SM) alpha-actin gene abolished nuclear factor binding and decreased transcriptional activity of a 271-bp SM alpha-actin promoter fragment when transfected into rat aortic SMC. However, the promoter region containing this conserved sequence has negative cis regulatory activity when studied in homologous systems. The goal of the present studies was to further characterize the transcriptional activity of the rat SM alpha-actin promoter region between -224 and -236 that is conserved across mammals. DNAse I analysis and electrophoretic mobility shift assays demonstrated that SMC nuclear proteins bound an extended sequence (TGTTTATCCCCATAA). Transient transfection experiments of wild-type and mutant rat SM alpha-actin promoter-luciferase constructs into rat aortic SMC revealed that promoter activity was enhanced by mutations of specific nucleotides in the TGTTTATCCCCA region. Interestingly, the TGTTTATCCCCA element in the rat SM alpha-actin promoter is centered between 2 canonical E-boxes. Mutations of the flanking E-boxes abolished the enhancement in promoter activity seen with mutation of the TGTTTATCCCCA element alone. Thus studies provide evidence for a regulatory cassette in the rat SM alpha-actin promoter that regulates gene expression via combinatorial interactions between 2 E-boxes and a newly described TGTTTATCCCCA element.

Carp liver actin: isolation, polymerization and interaction with deoxyribonuclease I (DNase I)

The aim of this study was to isolate and to characterize actin from the carp liver cytosol and to examine its ability to polymerize and interact with bovine pancreatic DNase I. Carp liver actin was isolated by ion-exchange chromatography, followed by gel filtration and a polymerization/depolymerization cycle or by affinity chromatography using DNase I immobilized to agarose. The purified carp liver actin was a cytoplasmic beta-actin isoform as verified by immunoblotting using isotype specific antibodies. Its isoelectric point (pI) was slightly higher than the pI of rabbit skeletal muscle alpha-actin. Polymerization of purified carp liver actin by 2 mM MgCl(2) or CaCl(2) was only obtained after addition of phalloidin or in the presence of 1 M potassium phosphate. Carp liver actin interacted with DNase I leading to the formation of a stable complex with concomitant inhibition of the DNA degrading activity of DNase I and its ability to polymerize. The estimated binding constant (K(b)) of carp liver actin to DNase I was calculated to be 1.85x10(8) M(-1) which is about 5-fold lower than the affinity of rabbit skeletal muscle alpha-actin to DNase I.

Smooth muscle-specific actin levels in the urine of renal transplant recipients: correlation with cyclosporine or tacrolimus nephrotoxicity

Cyclosporine A (CSA) and tacrolimus (FK506) are powerful immunosuppressive agents that have proven useful for antirejection therapy in patients with solid organ transplants, including kidney. However, both drugs are nephrotoxic, each producing similar histological patterns of injury to renal tubules and preglomerular arterioles, and this toxicity is a major cause of renal allograft dysfunction. A renal transplant biopsy presently represents the most reliable means of diagnosing nephrotoxicity caused by CSA or tacrolimus and distinguishing it from acute rejection. Because CSA and tacrolimus nephrotoxicity often involve arteriolar smooth muscle, whereas vascular smooth muscle is rarely involved in acute rejection, we investigated if the appearance of a smooth muscle-specific isoform of alpha-actin (SMA) in the urine of renal transplant recipients about to undergo a biopsy for graft dysfunction correlated with biopsy evidence of CSA or tacrolimus toxicity. Eighty-nine urine samples from 61 patients, plus 6 samples from healthy control subjects, were analyzed in a blinded manner by enzyme-linked immunosorbent assay using a specific anti-SMA monoclonal antibody. For the patient samples, the results of these assays were then correlated with the biopsy findings. Those 40 cases in which the biopsy showed evidence of CSA or tacrolimus nephrotoxicity had a significantly (P < 0.01) greater SMA level in the corresponding urine samples (0.089 +/- 0.126 microgram/mL; mean +/- SD) than the 49 cases without toxicity (0.018 +/- 0.027 microgram/mL) or 6 control subjects (0.003 +/- 0.007 microgram/mL), although there was considerable overlap of SMA values among these groups. The greatest SMA levels were seen in patients with CSA or tacrolimus nephrotoxicity that was likely to be relatively acute, namely those with thrombotic microangiopathy and those without previous biopsy evidence of toxicity. SMA levels correlated significantly with the estimated severity of arteriolopathy on biopsy. In patients with tubular but not arteriolar lesions of CSA or tacrolimus toxicity, the mean SMA level was not significantly greater than that in patients without toxicity. Urine SMA levels in patients with a biopsy specimen showing acute rejection were not significantly different from those in patients without rejection, and there was no correlation between urine SMA level and severity of rejection. Whereas the degree of overlap of SMA levels in patients with and without nephrotoxicity was far too great to consider this assay as a potential alternative to renal transplant biopsy for the diagnosis of nephrotoxicity, the assay may have potential as a marker for active arteriolar injury in renal transplant recipients and other patients receiving CSA or tacrolimus therapy.

Role of intron 1 in smooth muscle alpha-actin transcriptional regulation in activated mesangial cells in vivo

BACKGROUND

The activation of glomerular mesangial cells is one of the early, important features of progressive glomerular disease. Smooth muscle alpha-actin (SMalphaA) is an excellent marker of activated mesangial cells. However, the mechanisms of SMalphaA regulation are only available from in vitro investigation.

METHODS

We examined in vivo promoter analysis of the SMalphaA gene-utilizing transgenic mice harboring different promoter regions of the SMalphaA gene fused to chloramphenicol acetyl transferase (CAT). CAT activities were tested in primary cultured mesangial cells and in glomerular legions of Habu venom glomerulonephritis.

RESULTS

The DNA sequence -891 to +3828, which contains exon 1, intron 1, and the first 14 bp of exon 2 in addition to the 5'-flanking sequence of the SMalphaA gene, induced high levels of transcription in activated mesangial cells in in vivo habu venom glomerulonephritis and in cultured mesangial cells derived from transgenic mice. The DNA region -891 to -124 was a positive element in mesangial cells derived from transgenic mice. Deletions (3316 or 137 bp) in intron 1 reduced transcription to undetectable levels. The 137 bp sequence is highly conserved among several species, containing one CArG box element, which is one of the key motifs for transcriptional activation of contractile-related proteins. In vitro transfection analysis failed to demonstrate these positive effects of intron 1 and region -891 to -124. Conclusions. In vivo promoter analysis of the SMalphaA gene provided new information about the transcriptional regulation of SMalphaA in activated mesangial cells. The DNA region -891 to -124 has a positive effect on SMalphaA transcription in cultured mesangial cells. The intron 1 region (+1088 to +1224) plays a pivotal role in SMalphaA transcription in activated mesangial cells in vivo. Further analysis of this conserved region in intron 1, including the CArG motif, will be of great value in understanding the molecular mechanisms of mesangial activation.

Genomic organization and evolution of actin genes in the amphioxus Branchiostoma belcheri and Branchiostoma floridae

We previously described the cDNA cloning and expression patterns of actin genes from amphioxus Branchiostoma floridae (Kusakabe, R., Kusakabe, T., Satoh, N., Holland, N.D., Holland, L.Z., 1997. Differential gene expression and intracellular mRNA localization of amphioxus actin isoforms throughout development: implications for conserved mechanisms of chordate development. Dev. Genes Evol. 207, 203-215). In the present paper, we report the characterization of cDNA clones for actin genes from a closely related species, Branchiostoma belcheri, and the exon-intron organization of B. floridae actin genes. Each of these two amphioxus species has two types of actin genes, muscle and cytoplasmic. The coding and non-coding regions of each type are well-conserved between the two species. A comparison of nucleotide sequences of muscle actin genes between the two species suggests that a gene conversion may have occurred between two B. floridae muscle actin genes BfMA1 and BfMA2. From the conserved positions of introns between actin genes of amphioxus and those of other deuterostomes, the evolution of deuterostome actin genes can be inferred. Thus, the presence of an intron at codon 328/329 in vertebrate muscle and cytoplasmic actin genes but not in any known actin gene in other deuterostomes suggests that a gene conversion may have occurred between muscle and cytoplasmic actin genes during the early evolution of the vertebrates after separation from other deuterostomes. A Southern blot analysis of genomic DNA revealed that the amphioxus genome contains multiple muscle and cytoplasmic actin genes. Some of these actin genes seem to have arisen from recent duplication and gene conversion. Our findings suggest that the multiple genes encoding muscle and cytoplasmic actin isoforms arose independently in each of the three chordate lineages, and gene duplications and gene conversions established the extant actin multigene family during the evolution of chordates.

Skeletal and smooth muscle alpha-actin mRNA in endomyocardial biopsy samples of dilated cardiomyopathy patients

Skeletal or smooth muscle alpha-actin isoform is reexpressed in hypertrophied or failing heart in experimental animals, however, it has been unknown whether there is a switching of these genes in the human heart. In this study, we assessed alpha-actin isoform mRNA in biopsied specimens from the left ventricle of patients with dilated cardiomyopathy (DCM). The biopsy was performed in 21 patients (DCM; 16, normal; 5). Skeletal and smooth muscle alpha-actin mRNA levels were assessed by quantitative reverse transcriptase-polymerase chain reaction using a synthetic RNA as internal control. The level of both alpha-actin mRNAs correlated well with the mean cardiomyocyte diameter as an index of cardiac hypertrophy. Further, there was a clear correlation between the expression of skeletal alpha-actin mRNA or smooth muscle alpha-actin mRNA and the ejection fraction. These results suggested that mRNAs for skeletal and smooth muscle alpha-actin isoforms are increased in the hypertrophic and/or failing heart. These mRNAs may be an useful marker for the pathological state of cardiomyopathy.

Differences in contractile protein content and isoforms in phasic and tonic smooth muscles

The basis of tonic vs. phasic contractile phenotypes of visceral smooth muscles is poorly understood. We used gel electrophoresis and quantitative scanning densitometry to measure the content and isoform composition of contractile proteins in opossum lower esophageal sphincter (LES), to represent tonic muscle, and circular muscle of the esophageal body (EB), to represent phasic smooth muscle. The amount of protein in these two types of muscles is similar: approximately 27 mg/g of frozen tissue. There is no difference in the relative proportion of myosin, actin, calponin, and tropomyosin in the two muscle types. However, the EB contains approximately 2.4-times more caldesmon than the LES. The relative ratios of alpha- to gamma-contractile isoforms of actin are 0.9 in the LES and 0.3 in EB. The ratio between acidic (LC17a) and basic (LC17b) isoforms of the 17-kDa essential light chain of myosin is 0.7:1 in the LES, compared with 2.7:1 in the EB. There is no significant difference in the ratios of smooth muscle myosin SM1 and SM2 isoforms in the two muscle types. The level of the myosin heavy chain isoform, which contains the seven-amino acid insert in the myosin head, is about threefold higher in the EB compared with LES. In conclusion, the esophageal phasic muscle in contrast to the tonic LES contains proportionally more caldesmon, LC17a, and seven-amino acid-inserted myosin and proportionally less alpha-actin. These differences may provide a basis for functional differences between tonic and phasic smooth muscles.

Lung smooth muscle differentiation

The vascular and visceral smooth muscle tissues of the lung perform a number of tasks that are critical to pulmonary function. Smooth muscle function often is compromised as a result of lung disease. Though a great deal is known about regulation of smooth muscle cell replication and cell and tissue contractility, much less is understood regarding the phenotype of the contractile protein machinery of lung smooth muscle cells. This review focuses on the expression of cytoskeletal and contractile proteins of lung vascular and airway smooth muscle cells during development, in the adult and during vascular and airway remodeling. Emphasis is placed on the expression of the heavy chain of smooth muscle myosin, as well as the regulation of its gene. Important areas for future research are discussed.

Expression of centrin isoforms in the mammalian retina

Centrin is a calcium-binding phosphoprotein of centrosomes, mitotic spindle poles, and flagellar basal apparatus. Indirect immunofluorescence studies in human and rat retinas reveal centrin localization in two distinct cellular structures: at centrosomes of nonciliated neuronal cells as well as in basal bodies, and in larger amounts in the highly modified cilium--the connecting cilium--of photoreceptor cells. Western blot analyses of mammalian retinal proteins show two closely migrating centrin bands at about 20 kDa, the previously described molecular weight of centrins. Using isoform specific primers in PCR, the expression of two related but distinct forms of centrin (centrin 1 and centrin 2), can be identified in the retina of human and rat as well as in the mammalian testis, tissues where cilia are present. However, only one isoform (centrin 2) is expressed in nondifferentiated, nonciliated retinal cells (retinoblastoma cells), as well as in rat liver, skeletal muscle, and cardiac muscle. These observations suggest centrin 2 message may be universally expressed while centrin 1 message may be restricted to retina and testis which contain cells that have differentiated cilia or flagella, or their modifications.

PDGF, TGF-beta, and heterotypic cell-cell interactions mediate endothelial cell-induced recruitment of 10T1/2 cells and their differentiation to a smooth muscle fate

We aimed to determine if and how endothelial cells (EC) recruit precursors of smooth muscle cells and pericytes and induce their differentiation during vessel formation. Multipotent embryonic 10T1/2 cells were used as presumptive mural cell precursors. In an under-agarose coculture, EC induced migration of 10T1/2 cells via platelet-derived growth factor BB. 10T1/2 cells in coculture with EC changed from polygonal to spindle-shaped, reminiscent of smooth muscle cells in culture. Immunohistochemical and Western blot analyses were used to examine the expression of smooth muscle (SM)-specific markers in 10T1/2 cells cultured in the absence and presence of EC. SM-myosin, SM22alpha, and calponin proteins were undetectable in 10T1/2 cells cultured alone; however, expression of all three SM-specific proteins was significantly induced in 10T1/2 cells cocultured with EC. Treatment of 10T1/2 cells with TGF-beta induced phenotypic changes and changes in SM markers similar to those seen in the cocultures. Neutralization of TGF-beta in the cocultures blocked expression of the SM markers and the shape change. To assess the ability of 10T1/2 cells to contribute to the developing vessel wall in vivo, prelabeled 10T1/2 cells were grown in a collagen matrix and implanted subcutaneously into mice. The fluorescently marked cells became incorporated into the medial layer of developing vessels where they expressed SM markers. These in vitro and in vivo observations shed light on the cell-cell interactions that occur during vessel development, as well as in pathologies in which developmental processes are recapitulated.

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.

Calpain inhibitors, but not caspase inhibitors, prevent actin proteolysis and DNA fragmentation during apoptosis

Apoptosis, or programmed cell death, involves a cascade of regulatory events leading to the activation of specific proteases. However, the key substrates for these proteases remain to be identified. We previously demonstrated that levels of five unidentified polypeptides were specifically increased in neurons from embryonic chicken ciliary ganglia undergoing apoptosis by trophic deprivation. Here we show by microsequencing of two of these polypeptides that they are fragments of actin. One of them represents cleavage of actin at the site of interaction with DNase I. The same actin fragments are also found at early stages of apoptosis in chicken and rat dorsal root ganglion neurons, chicken spinal motoneurons and rat thymocytes. Actin fragmentation may play a role in the apoptotic process, since calpain inhibitors I and II both inhibit neuronal death and suppress actin fragmentation. In contrast, caspase (ICE family) inhibitors, though effective in delaying neuronal death, do not prevent actin cleavage or DNA fragmentation. These results indicate a key role for calpain-like proteases in neuronal programmed cell death and suggest that actin fragmentation in the cell is correlated with subsequent DNA fragmentation.

Expression and subcellular distribution of filamin isotypes in endothelial cells and pericytes

Two principal forms of the actin binding protein, filamin, are expressed in mammalian cells: nonmuscle and muscle isotypes (FLN-1 and FLN-2). A protein that copurifies with an alpha-naphthyl acetate hydrolyzing esterase from human omentum microvessel endothelial cells (EC) is isolated by nondenaturing electrophoresis, sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and electroblotting. The purified protein is subjected to in situ trypsin cleavage, reversed-phase high performance liquid chromatography (HPLC) and automated Edman degradation. Six peptide fragments from the protein are identified to have 60-66% identity with nonmuscle filamin (ABP-280). Two of these peptides are 100% identical to a previously sequenced human muscle filamin fragment. Polyclonal antibody is produced using a 16-residue synthetic peptide corresponding to a structural beta-sheet region of muscle filamin. Compared with a variety of vascular cells evaluated, retinal pericytes express an abundance of both muscle and non-muscle filamin isotypes. Pericytes contain at least 10 times more muscle filamin than human umbilical vein EC and at least three times the amount expressed in human omentum microvessel and bovine pulmonary artery EC. Differential detergent fractionation indicates that both filamin isotypes are primarily localized in the cytosol and membrane/organelle fractions of pericytes. Another actin crosslinking protein, alpha-actinin, is primarily found in the cytosol and cytoskeletal fractions. The dynamic regulation of actin microfilament organization in pericytes may be controlled in part by the two filamin isotypes, which in turn may contribute to pericyte contractility.

Molecular remodeling of cardiac contractile function

A number of techniques are now available that allow the contractile apparatus of the heart to be altered in a defined manner. This review focuses on those approaches that result in germ-line transmission of the remodeling event(s). Thus the desired modifications can be propagated stably throughout multiple generations and result in the creation of stable, new animal models. Necessarily, such stable changes need to be performed at the level of the genome, and two distinct but complementary approaches have been developed: transgenesis and gene targeting. Each results in the stable modification of the mammalian genome. Via gene targeting or gene ablation of sequences encoding various components of the sarcomere, the contractile apparatus of the heart can be altered dramatically. Ablating a gene may lead to a loss in function, which can help establish a function of the candidate sequence. Gene targeting can also be used to effect changes in the sequences encoding a functional domain of the contractile protein or at a single-amino acid residue, resulting in the establishment of precise structure-function relationships. With the use of transgenesis, the contractile apparatus of the heart can also be significantly remodeled. These approaches are rapidly creating a group of animals in which altered contractile protein complements will lead to a fundamental understanding of the structure-function relationships that underlie the function of the heart at the molecular, biochemical, whole organ, and whole animal levels.

Classification of human placental stem villi: review of structural and functional aspects

The stem villi of the human placenta represent the central branches of the villous trees. They are characterized by a condensed fibrous stroma in which the fetal arteries and veins as well as the arterioles and venules are embedded. Functionally they are accepted as the mechanically supporting structures of the villous trees, and they are supposed to control fetal blood flow to the maternofetal exchange area, which is located in the peripheral villi. To obtain further insights into the functions of the stem villi, the recent literature has been reviewed, and some immunohistochemical, ultrastructural, and reconstruction studies have been added. These new studies were aimed at identifying immunohistochemically different subtypes of stem villi, their branching patterns, the distribution of macrophages, the stromal proliferation patterns, and the differentiation of extravascular stromal cells. Our findings demonstrate that the stem villi and their precursors, the immature intermediate villi, can selectively be identified by anti-gamma-smooth muscle (sm) actin staining. Furthermore, the existence of three different subtypes of stem villi is shown; these differ regarding the presence and distribution of gamma-sm actin-positive cells. These cells were immunohistochemically and ultrastructurally identified as smooth muscle cells and myofibroblasts. Increasingly complex coexpression patterns of cytoskeletal proteins reflect a clearly defined differentiation gradient of extravascular stromal cells, which covers the whole range of an undifferentiated germinative layer beneath the trophoblast to highly differentiated myofibroblasts surrounding the medias of the stem vessels. Possible functions of the extravascular contractile system include the regulation of villous turgor and the control of intervillous blood flow impedance.

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.

Enhanced proliferation and migration and altered cytoskeletal proteins in early passage smooth muscle cells from young and old rat aortic explants

Smooth muscle cell (SMC) proliferation, migration, and cytoskeletal protein expression were studied in cultured cells obtained from the aortic explants of young (6-month) and old (30-month) Fischer 344XNB rats. Second-passage SMC were cultured on coverslips, and cytoskeletal fibers were examined by immunofluorescence microscopy using antibodies specific for smooth muscle myosin, alpha-smooth muscle actin, vimentin, desmin, and tubulin. The cytoskeletal fiber density was quantified as fluorescence intensity by confocal microscopy. The proliferation of SMC was analyzed from the growth curve of cells grown in culture from 0 to 14 days, and a Boyden chamber assay was used to quantify the SMC migration rate. The diameter of fresh SMC digested enzymatically from old rat aortae was 52.4% larger than that of the cells from young animals (20.0 +/- 3 microm vs 13.1 +/- 2 microm, P < 0.05). In SMC cultured from old animals, the intensities of smooth muscle myosin, alpha-smooth muscle actin, and vimentin decreased by 59.6, 41.2, and 54.8%, respectively; desmin and tubulin increased by 46.1 and 65.1% (all P < 0.001). Compared to SMC isolated from young rat aortae, the number of SMC cultured (second passage) from the old rat aorta was increased by 48.4, 27.2, and 26.9%, respectively, at Days 3, 7, and 14 in culture (P < 0.05, P < 0.01, and P < 0.001). The migration rate of SMC cultured from old rats was 59.3% higher than that of the cells obtained from young rats. These data show that alterations of the SMC cytoskeleton occur concomitantly with changes in SMC proliferation and migration rate during aging, suggesting that the age-associated changes in cytoskeletal proteins may play a role in remodeling of the aortic wall during aging.

Chemico-structural degradation of Carboniferous lingulid shells

Shells of Lingula squamiformis from argillaceous sediments at three horizons within the Dinantian Series, exposed at three localities in Scotland, have been studied to determine chemico-structural changes resulting from fossilization. Biomineral structures are essentially the same as those of living Lingula anatina with apatitic granules aggregating into spherules, up to 60 nm in size, and larger spheroidal bodies as well as rods and rarer lath-like plates. These aggregates and the original organic constituents were secreted as stratiform successions in two distinct layers, as in Recent L. anatina . The outer, lithified part of the primary layer bears microstructural moulds of a totally degraded periostracum and was probably composed mainly of acidic glycosaminoglycans (GAGs); the inner part evidently contained a higher proportion of spherular apatite within the GAGs than in living species. The secondary layer consists of variably complete rhythmic sets of compact, rod and plate (virgose), and membranous laminae. Compact laminae are normally cleaved along degraded walls of GAGs whereas the original GAGs-filled spaces of virgose laminae are partly occupied by recrystallized apatitic sheets with kaolinite. The phosphatized membranous laminae probably contained more spherular apatite in life than present day Lingula . The shell is canaliculate with chambers and galleries well developed in the virgose laminae. There is a decrease in concentration of amino acids from the posterior to the anterior of the valves of living Lingula anatina and Glottidia pyramidata resulting from the proteinaceous coat of the apatitic spherules. A similar distribution of hydroxyproline occurs, indicative of collagen in the body platform of living lingulids. Nearly all organic constituents have been degraded in the Carboniferous valves but threads, about 50 nm thick, occasionally traverse spaces in virgose laminae and even form a network coated with spherular apatite, which resembles webs of collagens or actin found in living lingulids. Acidic and aliphatic amino acids were extracted from L. squamiformis valves from Calderwood and Kinghorn whereas the narrower range of amino acids from the heavily pyritized valves from Ardross confirmed differential degradation of organic material during the fossilization of penecontemporaneous samples. The fossilization of complete shells of L. squamiformis is not due exclusively to catastrophic burial as has been deemed necessary to preserve Recent Lingula intact. The living shells of Carboniferous species were more apatitic than those of Recent Lingula , especially in the anteriomedian sectors of the secondary layers.

An inhibitory effect of actin on casein kinase II activity in vitro

The inhibitory effect of actin on protein phosphorylation by three distinct protein kinases (CK-II, A-kinase and MAP-kinase) was examined in vitro. It was found that: (i) actin inhibits the activities of alpha-monomeric CK-II (CK-IIalpha) as well as oligomeric CK-II (alpha2beta2) in a dose-dependent manner, but has no effect on the activities of the two other kinases; and (ii) actin-induced inhibition of CK-II activity is due to the binding of actin to the alpha-subunit of CK-II and is non-competitive with its phosphate acceptors. In addition, it is demonstrated that actin binds directly to CK-II: both actin and CK-II are coprecipitated by anti-serum against Drosophila CK-IIbeta or by specific IgG against Ascaris suum muscle actin. The results presented here suggest that actin can suppress CK-II-mediated signal transduction.

A sialic acid-binding lectin from ovine placenta: purification, specificity and interaction with actin

A sialic-acid-specific lectin from ovine placental cotyledons was purified by affinity chromatography on bovine submaxillary mucin-agarose followed by gel filtration, and it showed a molecular weight of 65000 by sodium dodecylsulfate-polyacrylamide gel electrophoresis. This lectin has the capacity to interact with actin, since it binds to actin-F in a cosedimentation assay and it acts as a mediator in the binding of actin to the affinity column. The lectin agglutinated rabbit and rat erythrocytes, but not human A, B or O erythrocytes. Haemagglutination inhibition assays of different saccharides, glycoproteins and glycolipids indicate that this lectin has affinity for sialic acid, which is enhanced by its O-acetylation. The N-terminal sequence of the protein shows 92% identity with rabbit and porcine uterine calreticulin.

Tissue and developmental specific expression of murine smooth muscle gamma-actin fusion genes in transgenic mice

Smooth muscle gamma-actin (SMGA) is an excellent marker of smooth muscle differentiation because it is essentially restricted to smooth muscle. As a first step toward unraveling the mechanisms underlying smooth muscle development and differentiation, we have examined the tissue-specific and developmental expression patterns of six constructs carrying portions of the murine SMGA gene linked to chloramphenicol acetyltransferase (CAT) in stable lines of transgenic mice. Based on the transgenic studies most, if not all, of the regulatory elements necessary for proper spatial and temporal expression of SMGA are present within a 13.7 kb segment of the SMGA gene containing 4.9 kb of upstream sequence, exon 1, intron 1, and a portion of exon 2 up to the start codon for translation. A second construct (SMGA11.6CAT) that lacks the distal 2.1 kb of upstream sequence but is otherwise identical to SMGA13.7CAT shows a similar level of smooth muscle-specific CAT activity. However, SMGA9.3CAT fusion gene containing only 571 bp of 5' flanking sequence, but otherwise identical to SMGA13.7CAT, and SMGA6.0CAT containing only the 4.9 kb upstream sequence, exon 1, and a miniintron 1 show a more than a 100-fold reduction of CAT activity in most smooth muscle-rich tissues. Furthermore, removal of most or all of intron 1 from a transgene with 571 bp of upstream sequence (SMGA2.0 CAT and SMGA0.6CAT) results in a near-complete or complete loss of activity, respectively, in all tissues. Overall, the studies suggest that upstream elements between -2.7 kb and -571 bp and elements within intron 1 are required for high levels of SMGA gene expression in an appropriate temporal-spatial fashion.

Different effects of estrogen and progesterone on experimental atherosclerosis in female versus male rabbits. Quantification of cellular proliferation by bromodeoxyuridine

BACKGROUND

The aim of the present study was to compare the effect of estrogen and progesterone on the development of experimental atherosclerosis in female versus male rabbits to assess possible sex-specific differences.

METHODS AND RESULTS

A total of 32 female and 32 male New Zealand White rabbits were ovariectomized or castrated. In addition to a 0.5% cholesterol diet, the rabbits received estradiol alone (1 mg/kg body wt [BW] per week), progesterone alone (25 mg/kg BW per week), or combined estradiol-progesterone in these dosages during 12 weeks. Ovariectomized female and castrated male rabbits served as control groups without hormone treatment. Before excision of the vessels, bromodeoxyuridine labeling was performed to determine the extent of cellular proliferation in the atherosclerotic lesions. The aortic arch was analyzed immunohistologically and morphometrically. An inhibitory effect of estrogen on intimal plaque size was found in female rabbits compared with the ovariectomized control group (0.7 +/- 0.5 versus 3.7 +/- 2.5 mm2, P < .002; proliferating cells, 3.1 +/- 1.8% versus 8.5 +/- 2.6%, P < .002). In combination with progesterone, however, estrogen was not able to reduce intimal plaque size or cellular proliferation. In contrast, estradiol in castrated male rabbits was not associated with an inhibitory effect on cellular proliferation or intimal thickening compared with controls (estrogen treatment, 7.6 +/- 2.1% proliferating cells and 2.8 +/- 1.0 mm2 neointima; control group, 7.2 +/- 2.1% cellular proliferation and 2.9 +/- 1.2 mm2 intimal thickening).

CONCLUSIONS

Our data suggest that the atheroprotective effect of estrogen is probably due to a mechanism that is present in female rabbits only.