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

Beta-Actin Is Involved in Modulating Erythropoiesis during Development by Fine-Tuning Gata2 Expression Levels

The functions of actin family members during development are poorly understood. To investigate the role of beta-actin in mammalian development, a beta-actin knockout mouse model was used. Homozygous beta-actin knockout mice are lethal at embryonic day (E)10.5. At E10.25 beta-actin knockout embryos are growth retarded and display a pale yolk sac and embryo proper that is suggestive of altered erythropoiesis. Here we report that lack of beta-actin resulted in a block of primitive and definitive hematopoietic development. Reduced levels of Gata2, were associated to this phenotype. Consistently, ChIP analysis revealed multiple binding sites for beta-actin in the Gata2 promoter. Gata2 mRNA levels were almost completely rescued by expression of an erythroid lineage restricted ROSA26-promotor based GATA2 transgene. As a result, erythroid differentiation was restored and the knockout embryos showed significant improvement in yolk sac and embryo vascularization. These results provide new molecular insights for a novel function of beta-actin in erythropoiesis by modulating the expression levels of Gata2 in vivo.

Severe forms of Baraitser-Winter syndrome are caused by ACTB mutations rather than ACTG1 mutations

ACTB and ACTG1 mutations have recently been reported to cause Baraitser-Winter syndrome (BRWS) - a rare condition characterized by ptosis, colobomata, neuronal migration disorder, distinct facial anomalies and intellectual disability. One of the patients carrying an ACTB mutation was previously diagnosed with Fryns-Aftimos syndrome (FAS), which is a rare and severe, multiple congenital anomaly (MCA) syndrome whose symptoms partially overlap with that of BRWS. However, several patients with Fryns-Aftimos were considered not to fit into the ACTB and ACTG1 spectrum because of their severe impairment and additional malformations. We report on three patients who had been diagnosed with FAS. All three patients carry a mutation in the ACTB gene. On the basis of the ACTB mutations and analysis of the clinical findings, we reclassify the diagnosis of these patients as severe BRWS. We suggest that mutations in ACTB cause a distinctly more severe phenotype than ACTG1 mutations, despite the structural similarity of beta- and gamma-actins and their overlapping expression pattern. We expand the spectrum of BRWS and confirm that FAS is not a separate entity but an early and severe manifestation of BRWS.

Dynamic actin gene family evolution in primates

Actin is one of the most highly conserved proteins and plays crucial roles in many vital cellular functions. In most eukaryotes, it is encoded by a multigene family. Although the actin gene family has been studied a lot, few investigators focus on the comparison of actin gene family in relative species. Here, the purpose of our study is to systematically investigate characteristics and evolutionary pattern of actin gene family in primates. We identified 233 actin genes in human, chimpanzee, gorilla, orangutan, gibbon, rhesus monkey, and marmoset genomes. Phylogenetic analysis showed that actin genes in the seven species could be divided into two major types of clades: orthologous group versus complex group. Codon usages and gene expression patterns of actin gene copies were highly consistent among the groups because of basic functions needed by the organisms, but much diverged within species due to functional diversification. Besides, many great potential pseudogenes were found with incomplete open reading frames due to frameshifts or early stop codons. These results implied that actin gene family in primates went through “birth and death” model of evolution process. Under this model, actin genes experienced strong negative selection and increased the functional complexity by reproducing themselves.

Actin isoforms in neuronal development and function

The actin cytoskeleton contributes directly or indirectly to nearly every aspect of neuronal development and function. This diversity of functions is often attributed to actin regulatory proteins, although how the composition of the actin cytoskeleton itself may influence its function is often overlooked. In neurons, the actin cytoskeleton is composed of two distinct isoforms, β- and γ-actin. Functions for β-actin have been investigated in axon guidance, synaptogenesis, and disease. Insight from loss-of-function in vivo studies has also revealed novel roles for β-actin in select brain structures and behaviors. Conversely, very little is known regarding functions of γ-actin in neurons. The dysregulation or mutation of both β- and γ-actin has been implicated in multiple human neurological disorders, however, demonstrating the critical importance of these still poorly understood proteins. This chapter highlights what is currently known regarding potential distinct functions for β- and γ-actin in neurons as well as the significant areas that remain unexplored.

Skeletal muscle α-actin diseases (actinopathies): pathology and mechanisms

Mutations in the skeletal muscle α-actin gene (ACTA1) cause a range of congenital myopathies characterised by muscle weakness and specific skeletal muscle structural lesions. Actin accumulations, nemaline and intranuclear bodies, fibre-type disproportion, cores, caps, dystrophic features and zebra bodies have all been seen in biopsies from patients with ACTA1 disease, with patients frequently presenting with multiple pathologies. Therefore increasingly it is considered that these entities may represent a continuum of structural abnormalities arising due to ACTA1 mutations. Recently an ACTA1 mutation has also been associated with a hypertonic clinical presentation with nemaline bodies. Whilst multiple genes are known to cause many of the pathologies associated with ACTA1 mutations, to date actin aggregates, intranuclear rods and zebra bodies have solely been attributed to ACTA1 mutations. Approximately 200 different ACTA1 mutations have been identified, with 90 % resulting in dominant disease and 10 % resulting in recessive disease. Despite extensive research into normal actin function and the functional consequences of ACTA1 mutations in cell culture, animal models and patient tissue, the mechanisms underlying muscle weakness and the formation of structural lesions remains largely unknown. Whilst precise mechanisms are being grappled with, headway is being made in terms of developing therapeutics for ACTA1 disease, with gene therapy (specifically reducing the proportion of mutant skeletal muscle α-actin protein) and pharmacological agents showing promising results in animal models and patient muscle. The use of small molecules to sensitise the contractile apparatus to Ca(2+) is a promising therapeutic for patients with various neuromuscular disorders, including ACTA1 disease.

Functional characterization of the human α-cardiac actin mutations Y166C and M305L involved in hypertrophic cardiomyopathy

Inherited cardiomyopathies are caused by point mutations in sarcomeric gene products, including α-cardiac muscle actin (ACTC1). We examined the biochemical and cell biological properties of the α-cardiac actin mutations Y166C and M305L identified in hypertrophic cardiomyopathy (HCM). Untagged wild-type (WT) cardiac actin, and the Y166C and M305L mutants were expressed by the baculovirus/Sf9-cell system and affinity purified by immobilized gelsolin G4-6. Their correct folding was verified by a number of assays. The mutant actins also displayed a disturbed intrinsic ATPase activity and an altered polymerization behavior in the presence of tropomyosin, gelsolin, and Arp2/3 complex. Both mutants stimulated the cardiac β-myosin ATPase to only 50 % of WT cardiac F-actin. Copolymers of WT and increasing amounts of the mutant actins led to a reduced stimulation of the myosin ATPase. Transfection of established cell lines revealed incorporation of EGFP- and hemagglutinin (HA)-tagged WT and both mutant actins into cytoplasmic stress fibers. Adenoviral vectors of HA-tagged WT and Y166C actin were successfully used to infect adult and neonatal rat cardiomyocytes (NRCs). The expressed HA-tagged actins were incorporated into the minus-ends of NRC thin filaments, demonstrating the ability to form hybrid thin filaments with endogenous actin. In NRCs, the Y166C mutant led after 72 h to a shortening of the sarcomere length when compared to NRCs infected with WT actin. Thus our data demonstrate that a mutant actin can be integrated into cardiomyocyte thin filaments and by its reduced mode of myosin interaction might be the basis for the initiation of HCM.

Nonredundant roles of cytoplasmic β- and γ-actin isoforms in regulation of epithelial apical junctions

The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific siRNAs and cell-permeable inhibitory peptides. Unique roles of cytoplasmic actin isoforms in regulating structure and remodeling of adherens and tight junctions are revealed.

Association with the actin cytoskeleton is critical for normal architecture and dynamics of epithelial tight junctions (TJs) and adherens junctions (AJs). Epithelial cells express β-cytoplasmic (β-CYA) and γ-cytoplasmic (γ-CYA) actins, which have different cellular localization and functions. This study elucidates the roles of cytoplasmic actins in regulating structure and remodeling of AJs and TJs in model intestinal epithelia. Immunofluorescence labeling and latrunculin B treatment reveal affiliation of dynamic β-CYA filaments with newly assembled and mature AJs, whereas an apical γ-CYA pool is composed of stable perijunctional bundles and rapidly turning-over nonjunctional filaments. The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific small interfering RNAs and cell-permeable inhibitory peptides. These experiments demonstrate unique roles of β-CYA and γ-CYA in regulating the steady-state integrity of AJs and TJs, respectively. Furthermore, β-CYA is selectively involved in establishment of apicobasal cell polarity. Both actin isoforms are essential for normal barrier function of epithelial monolayers, rapid AJ/TJ reassembly, and formation of three-dimensional cysts. Cytoplasmic actin isoforms play unique roles in regulating structure and permeability of epithelial junctions.

Downregulation of β-actin gene and human antigen R in human keratoconus

PURPOSE

The purpose of this study was to determine the expression levels and regulation of β-actin in the stroma of keratoconus (KC) and normal corneas.

METHODS

A total of 15 different human corneas from both KC and normal individuals were used for this study. Additionally, 3 Fuch's dystrophic corneas were also used. The β-actin gene expression was analyzed at the transcriptional and translational levels in the epithelium and stroma of the KC and normal corneas. The human antigen R (HuR) gene expression was analyzed by real-time PCR in the stroma of five KC and five normal corneas. The keratocytes from three normal and three KC corneas were cultured in the presence of serum, and the expression levels of β-actin and human antigen R (HuR) were analyzed by using confocal imaging in both normal and KC fibroblasts.

RESULTS

The expression of the β-actin gene was downregulated in the stroma of the six KC corneas but not in the stroma of six normal and Fuchs' dystrophic corneas. Immunofluorescence detection of β-actin showed that it was absent in the KC fibroblast. The real-time PCR analysis of the HuR gene showed a relative 4.7-fold lower expression in KC corneas relative to the normal corneas, which was further confirmed by the immunofluorescence detection of HuR in fibroblasts of KC corneas.

CONCLUSIONS

Although ubiquitous β-actins are essential for cell survival during early embryogenesis, the effects on various stages of development are not well understood. Our results show that β-actin is downregulated in the corneal stroma of patients with KC, which may be related to reduced levels of a stabilizing factor (HuR) for β-actin mRNA. We propose that loss of β-actin in the corneal stroma might be a triggering factor in the development of KC.

Small heat shock proteins and the cytoskeleton: an essential interplay for cell integrity?

The cytoskeleton is a highly complex network of three major intracellular filaments, microfilaments (MFs), microtubules (MTs) and intermediate filaments (IFs). This network plays a key role in the control of cell shape, division, functions and interactions in animal organs and tissues. Dysregulation of the network can contribute to numerous human diseases. Although small HSPs (sHSPs) and in particular HSP27 (HSPB1) or αB-crystallin (HSPB5) display a wide range of cellular properties, they are mostly known for their ability to protect cells under stress conditions. Mutations in some sHSPs have been found to affect their ability to interact with cytoskeleton proteins, leading to IF aggregation phenotypes that mimick diseases related to disorders in IF proteins (i.e. desmin, vimentin and neuro-filaments). The aim of this review is to discuss new findings that point towards the possible involvement of IFs in the cytoprotective functions of sHSPs, both in physiological and pathological settings, including the likelihood that sHSPs such as HSPB1 may play a role during epithelial-to-mesenchymal transition (EMT) during fibrosis or cancer progression. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Smooth muscle actin expression in primary bone tumours

Alpha isoform of smooth muscle actin (SMA) expression has been reported in giant cell tumour of bone (GCTB) and other benign and malignant bone tumours, but the pattern of SMA expression and the precise nature of SMA-expressing cells in these lesions is uncertain. We determined by immunohistochemistry the expression of SMA and other muscle and vascular markers in normal bone, GCTB and a wide range of primary benign and malignant bone tumours. Cultured stromal cells of GCTB, chondroblastoma (CB), and aneurysmal bone cyst (ABC) were also analysed for SMA expression. SMA was only noted in blood vessels in normal bone. SMA was expressed by mononuclear stromal cells (MSC) cultured from GCTB, ABC and CB. SMA was strongly and diffusely expressed by MSC in non-ossifying fibroma, fibrous dysplasia, and "brown tumour" of hyperparathyroidism. SMA expression was also noted in GCTB, ABC, CB, chondromyxoid fibroma, malignant fibrous histiocytoma of bone and osteosarcoma. Little or no SMA was noted in Langerhans cell histiocytosis, simple bone cyst, Ewing's sarcoma, osteoblastoma, osteoid osteoma, enchondroma, osteochondroma, chondrosarcoma, myeloma, lymphoma, chordoma and adamantinoma. Our findings show that there is differential SMA expression in primary bone tumours and that identifying the presence or absence of SMA is useful in the differential diagnosis of these lesions. The nature of SMA-expressing cells in bone tumours is uncertain but they are negative for desmin and caldesmon and could represent either myofibroblasts or perivascular cells, such as pericytes.

Cells lacking β-actin are genetically reprogrammed and maintain conditional migratory capacity

Vertebrate nonmuscle cells express two actin isoforms: cytoplasmic β- and γ-actin. Because of the presence and localized translation of β-actin at the leading edge, this isoform is generally accepted to specifically generate protrusive forces for cell migration. Recent evidence also implicates β-actin in gene regulation. Cell migration without β-actin has remained unstudied until recently and it is unclear whether other actin isoforms can compensate for this cytoplasmic function and/or for its nuclear role. Primary mouse embryonic fibroblasts lacking β-actin display compensatory expression of other actin isoforms. Consistent with this preservation of polymerization capacity, β-actin knockout cells have unchanged lamellipodial protrusion rates despite a severe migration defect. To solve this paradox we applied quantitative proteomics revealing a broad genetic reprogramming of β-actin knockout cells. This also explains why reintroducing β-actin in knockout cells does not restore the affected cell migration. Pathway analysis suggested increased Rho-ROCK signaling, consistent with observed phenotypic changes. We therefore developed and tested a model explaining the phenotypes in β-actin knockout cells based on increased Rho-ROCK signaling and increased TGFβ production resulting in increased adhesion and contractility in the knockout cells. Inhibiting ROCK or myosin restores migration of β-actin knockout cells indicating that other actins compensate for β-actin in this process. Consequently, isoactins act redundantly in providing propulsive forces for cell migration, but β-actin has a unique nuclear function, regulating expression on transcriptional and post-translational levels, thereby preventing myogenic differentiation.

De novo mutations in the actin genes ACTB and ACTG1 cause Baraitser-Winter syndrome

Brain malformations are individually rare but collectively common causes of developmental disabilities^1–3. Many forms occur sporadically and have reduced reproductive fitness, pointing towards a causative role for de novo mutations^4,5. Here we report our studies of Baraitser-Winter syndrome, a well-defined syndrome characterized by distinct craniofacial features, ocular colobomata and a neuronal migration defect^6,7. By using whole-exome sequencing in three proband-parent trios, we identified de novo missense changes in the cytoplasmic actin genes ACTB and ACTG1 in one and two probands, respectively. Sequencing of both genes in fifteen additional patients revealed disease-causing mutations in all probands, including two recurrent de novo mutations (ACTB p.Arg196His and ACTG1 p.Ser155Phe). Our results confirm that trio-based exome sequencing is a powerful approach to discover the genes causing sporadic developmental disorders, emphasize the overlapping roles of cytoplasmic actins in development, and suggest that Baraitser-Winter syndrome is the predominant phenotype associated with mutations of these two genes.

Identification of troponin I and actin, alpha cardiac muscle 1 as potential biomarkers for hearts of electrically stimulated chickens

Methods

In this study, proteomics methods have been used to study the effects of different currents and voltages used to stun chickens. Protein profiles of chicken hearts were constructed to detect differences in protein expression and modification. The different voltages studied were 10 V, 40 V and 70 V, while the currents examined were 0.25 A, 0.5 A, and 0.75 A. The profiles obtained from these stunning conditions were compared to the non-stunned (0 A, 0 V) sample.

Results

Proteomics analyses using 2D Platinum ImageMaster 6.0 and Matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry (MALDI-TOF) identified troponin I and alpha cardiac muscle actin 1 in the electrically stimulated heart samples. The overexpression of the proteins was further confirmed at the transcriptional level by Real Time PCR.

Conclusion

The results from MALDI-TOF and Real Time PCR agreed; therefore, this method for identifying biomarkers of electrically stimulated chicken hearts provides a novel approach for differentiation the hearts of increased electrically stimulated chickens from those of non-stunned chickens.

Angiogenin enhances cell migration by regulating stress fiber assembly and focal adhesion dynamics

Angiogenin (ANG) acts on both vascular endothelial cells and cancer cells, but the underlying mechanism remains elusive. In this study, we carried out a co-immunoprecipitation assay in HeLa cells and identified 14 potential ANG-interacting proteins. Among these proteins, β-actin, α-actinin 4, and non-muscle myosin heavy chain 9 are stress fiber components and involved in cytoskeleton organization and movement, which prompted us to investigate the mechanism of action of ANG in cell migration. Upon confirmation of the interactions between ANG and the three proteins, further studies revealed that ANG co-localized with β-actin and α-actinin 4 at the leading edge of migrating cells. Down-regulation of ANG resulted in fewer but thicker stress fibers with less dynamics, which was associated with the enlargements of focal adhesions. The focal adhesion kinase activity and cell migration capacity were significantly decreased in ANG-deficient cells. Taken together, our data demonstrated that the existence of ANG in the cytoplasm optimizes stress fiber assembly and focal adhesion formation to accommodate cell migration. The finding that ANG promoted cancer cell migration might provide new clues for tumor metastasis research.

Actin-towards a deeper understanding of the relationship between tissue context, cellular function and tumorigenesis

It is well-established that the actin cytoskeleton plays an important role in tumor development yet the contribution made by nuclear actin is ill-defined. In a recent study, nuclear actin was identified as a key mediator through which laminin type III (LN1) acts to control epithelial cell growth. In the breast, epithelial tumors are surrounded by an environment which lacks LN1. These findings point to actin as a potential mediator of tumor development. Here our current understanding of the roles of cytoplasmic and nuclear actin in normal and tumor cell growth is reviewed, relating these functions to cell phenotype in a tissue context.

Airways in smooth muscle α-actin null mice experience a compensatory mechanism that modulates their contractile response

We hypothesized that ablation of smooth muscle α-actin (SM α-A), a contractile-cytoskeletal protein expressed in airway smooth muscle (ASM) cells, abolishes ASM shortening capacity and decreases lung stiffness. In both SM α-A knockout and wild-type (WT) mice, airway resistance (Raw) determined by the forced oscillation technique rose in response to intravenous methacholine (Mch). However, the slope of Raw (cmH(2)O·ml(-1)·s) vs. log(2) Mch dose (μg·kg(-1)·min(-1)) was lower (P = 0.007) in mutant (0.54 ± 0.14) than in WT mice (1.23 ± 0.19). RT-PCR analysis performed on lung tissues confirmed that mutant mice lacked SM α-A mRNA and showed that these mice had robust expressions of both SM γ-A mRNA and skeletal muscle (SKM) α-A mRNA, which were not expressed in WT mice, and an enhanced SM22 mRNA expression relative to that in WT mice. Compared with corresponding spontaneously breathing mice, mechanical ventilation-induced lung mechanical strain increased the expression of SM α-A mRNA in WT lungs; in mutant mice, it augmented the expressions of SM γ-A mRNA and SM22 mRNA and did not alter that of SKM α-A mRNA. In mutant mice, the expression of SM γ-A mRNA in the lung during spontaneous breathing and its enhanced expression following mechanical ventilation are consistent with the likely possibility that in the absence of SM α-A, SM γ-A underwent polymerization and interacted with smooth muscle myosin to produce ASM shortening during cholinergic stimulation. Thus our data are consistent with ASM in mutant mice experiencing compensatory mechanisms that modulated its contractile muscle capacity.

Smooth muscle α actin is specifically required for the maintenance of lactation

Smooth muscle α-actin (Acta2) is one of six highly conserved mammalian actin isoforms that appear to exhibit functional redundancy. Nonetheless, we have postulated a specific functional role for the smooth muscle specific isoform. Here, we show that Acta2 deficient mice have a remarkable mammary phenotype such that dams lacking Acta2 are unable to nurse their offspring effectively. The phenotype was rescued in cross fostering experiments with wild type mice, excluding a developmental defect in Acta2 null pups. The mechanism for the underlying phenotype is due to myoepithelial dysfunction postpartum resulting in precocious involution. Further, we demonstrate a specific defect in myoepithelial cell contractility in Acta2 null mammary glands, despite normal expression of cytoplasmic actins. We conclude that Acta2 specifically mediates myoepithelial cell contraction during lactation and that this actin isoform therefore exhibits functional specificity.

Characterization and identification of the protein partners of Fn3 domain in FnTm2

Recently, a novel transmembrane protein was found to be up-regulated in the auditory learning pathway of birds and mammals. The protein, FnTm2, was predicted to have an extracellular fibronectin III (Fn3) domain and a single transmembrane domain. By contrast to other studied Fn3 domains the extracellular domain of FnTm2 bears several cysteine residues, which are predicted to form disulfide bonds. The Fn3 domain of the FnTm2 protein was expressed in DH5-α Escherichia coli (E. coli) cells, purified and characterized by circular dichroism (CD). In order to identify binding partners to Fn3, the isolated protein was incubated with bird brain lysate for a pull down treatment. Of the proteins recognized, myelin basic protein (MBP) was identified as a bona fide partner; it was further characterized for binding to Fn3 in vitro via fluorescence spectroscopy and confirmed via isothermal calorimetry (ITC).

Beta- and gamma-cytoplasmic actins are required for meiosis in mouse oocytes

In mammals, female meiosis consists of two asymmetric cell divisions, which generate a large haploid oocyte and two small polar bodies. Asymmetric partitioning of the cytoplasm results from migration of the meiotic spindle toward the cortex and requires actin filaments. However, the subcellular localization and the role of the existing two cytoplasmic actin (CYA) isoforms, beta and gamma, have not been characterized. We show that beta- and gamma-CYA are differentially distributed in the maturing oocyte from late metaphase I as well as in preimplantation embryos. Gamma-CYA is preferentially enriched in oocyte cortices and is absent from all cell-cell contact areas from metaphase II until the blastocyst stage. Beta-CYA is enriched in contractile structures, at cytokinesis, at cell-cell contacts, and around the forming blastocoel. Alteration of beta- or gamma-CYA function by isoform-specific antibody microinjection suggests that gamma-CYA holds a major and specific role in the establishment and/or maintenance of asymmetry in meiosis I and in the maintenance of overall cortical integrity. In contrast, beta- and gamma-CYA, together, appear to participate in the formation and the cortical anchorage of the second meiotic spindle in waiting for fertilization. Finally, differences in gamma-CYA expression are amongst the earliest markers of cell fate determination in development.

Actin as target for modification by bacterial protein toxins

Various bacterial protein toxins and effectors target the actin cytoskeleton. At least three groups of toxins/effectors can be identified, which directly modify actin molecules. One group of toxins/effectors causes ADP-ribosylation of actin at arginine-177, thereby inhibiting actin polymerization. Members of this group are numerous binary actin-ADP-ribosylating exotoxins (e.g. Clostridium botulinum C2 toxin) as well as several bacterial ADP-ribosyltransferases (e.g. Salmonella enterica SpvB) which are not binary in structure. The second group includes toxins that modify actin to promote actin polymerization and the formation of actin aggregates. To this group belongs a toxin from the Photorhabdus luminescens Tc toxin complex that ADP-ribosylates actin at threonine-148. A third group of bacterial toxins/effectors (e.g. Vibrio cholerae multifunctional, autoprocessing RTX toxin) catalyses a chemical crosslinking reaction of actin thereby forming oligomers, while blocking the polymerization of actin to functional filaments. Novel findings about members of these toxin groups are discussed in detail.

Biochemical and immunohistochemical characterization of the isoforms of myosin and actin in human placenta

Human placenta has long been known to contain large quantities of smooth muscle-type myosin and actin, while precise isoform compositions of its contractile proteins are not known. To determine the isoform compositions, myosin and actin were extracted from human term placentas and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting by using isoform-specific monoclonal anti-myosin and anti-actin antibodies. The placental myosin was found to be composed of about 65% of a nonmuscle-type heavy chain isoform (MIIA), each about 15% of two smooth muscle-type heavy chain isoforms (SM1 and SM2) and about 5% of a brain/fetus-type heavy chain isoform (MIIB2). Whereas the MIIA isoform was present in both vascular and extravascular tissues, the SM1 isoform was localized almost only in the vascular tissue. Similarly, human term placenta was found to contain approximately 60, 30, and 10% of β-nonmuscle, α-smooth muscle, γ-smooth muscle actin isoforms, respectively. The β-nonmuscle actin was located primarily in the extravascular tissue, while the α-smooth muscle actin was located mostly in the vascular tissue. The extravascular tissue of the human term placenta thus appears to be composed of almost only nonmuscle-type isoforms of contractile proteins. The vascular tissue appears to be composed of both smooth muscle-type and nonmuscle-type isoforms of contractile proteins.

Sendai virus induced cytoplasmic actin remodeling correlates with efficient virus particle production

Cytoplasmic actins have been found interacting with viral proteins and identified in virus particles. We analyzed by confocal microscopy the cytoplasmic β- and γ-actin patterns during the course of Sendai virus infections in polarized cells. We observed a spectacular remodeling of the β-cytoplasmic actin which correlated with productive viral multiplication. Conversely, suppression of M during the course of a productive infection resulted in the decrease of particle production and the absence of β-actin remodeling. As concomitant suppression of β- and γ-actins resulted as well in reduction of virus particle production, we propose that Sendai virus specifically induces actin remodeling in order to promote efficient virion production. Beta- and γ-cytoplasmic actin recruitment could substitute for that of the endosomal sorting complex required for transport (ESCRT) mobilized by other enveloped viruses but apparently not used by Sendai virus.

De novo ACTA2 mutation causes a novel syndrome of multisystemic smooth muscle dysfunction

Smooth muscle cells (SMCs) contract to perform many physiological functions, including regulation of blood flow and pressure in arteries, contraction of the pupils, peristalsis of the gut, and voiding of the bladder. SMC lineage in these organs is characterized by cellular expression of the SMC isoform of α-actin, encoded by the ACTA2 gene. We report here on a unique and de novo mutation in ACTA2, R179H, that causes a syndrome characterized by dysfunction of SMCs throughout the body, leading to aortic and cerebrovascular disease, fixed dilated pupils, hypotonic bladder, malrotation, and hypoperistalsis of the gut and pulmonary hypertension.

Conformational dynamics of actin: effectors and implications for biological function

Actin is a protein abundant in many cell types. Decades of investigations have provided evidence that it has many functions in living cells. The diverse morphology and dynamics of actin structures adapted to versatile cellular functions is established by a large repertoire of actin-binding proteins. The proper interactions with these proteins assume effective molecular adaptations from actin, in which its conformational transitions play essential role. This review attempts to summarise our current knowledge regarding the coupling between the conformational states of actin and its biological function.

Complete nucleotide sequence of a soybean actin gene

Soybean contains a small multigene family of actin-related sequences. We have determined the complete nucleotide sequence of a soybean actin gene carried on the recombinant plasmid pSAc3. As deduced from the nucleotide sequence, this soybean actin is composed of 376 amino acids. Compared to other eukaryotic actins, pSAc3 actin has a deletion of one amino acid between residues 118 and 122. The initiator methionine is followed by alanine, which is not found at this position in other eukaryotic actins. pSAc3 actin differs, in primary sequence, more from fungal and animal actins than any of the known nonplant actins differ from each other. pSAc3 actin appears to be related to both cytoplasmic and muscle specific actins in the location of specific NH(2)-terminal amino acids. The coding sequence is interrupted by three small introns, each less than 90 base pairs long. The splice junctions are similar to those found in other eukaryotic genes, suggesting the presence of a similar splicing apparatus in higher plants. Introns 1 and 3 interrupt the reading frame after codons 20 and 355, respectively. Intron 2 splits a glycine codon at position 151. None of these intron positions is conserved relative to the positions of introns in other actin genes examined.

Isolation and characterization of the actin gene from Tetrahymena thermophila

The macronucleus of Tetrahymena thermophila contains a single actin gene. We have isolated this gene from a partial plasmid library by using the yeast actin gene as a probe. The nucleotide sequence of the gene has been determined and the amino acid sequence of the potential protein deduced. The encoded protein is 375 amino acids long, one amino acid longer than the yeast actin. It is one of the most divergent actins sequenced to date, being only 75% homologous to yeast actin. Unlike the actin genes from most other organisms, it does not contain introns. The coding region contains TAA and TAG codons; the translation termination codon is TGA. Comparison of the amino acid sequence of the Tetrahymena actin with that of actins from other organisms suggests that TAG may code for glutamic acid. The gene is transcribed from multiple initiation sites between 57 and 98 nucleotides upstream of the translation start codon. The 5' flanking region is very A+T-rich and contains numerous "TATA-like" sequences upstream of the transcription start sites.

Isolation of Tanichthys albonubes beta actin gene and production of transgenic Tanichthys albonubes

A beta actin cDNA of Tanichthys albonubes was isolated through the RT-PCR and RACE approach. The cDNA was 1,787-bp in length, including a 1,128-bp CDS, a 95-bp 5'UTR and a 564-bp 3'UTR. Genomic DNA containing the transcription region and 5'-flanking region was cloned based on the beta actin cDNA by Genome walker. A 3,000-bp beta actin gene promoter was then produced by PCR according to the sequences of the 5'-flanking region and the first intron. This promoter consisted of a 1,800-bp 5'-flanking region, and a 1,200-bp 5'-UTR. 3 transcription elements, CAAT box, CArG motif and TATA box were found in the 5'-flanking region. This promoter was inserted into the vector pDsRed2-1 and microinjected into fertilized eggs of Tanichthys albonubes to prove its transcription activity. The beta actin promoter and GH CDS of Tanichthys albonubes were then fused to construct an expression vector pTLA-GH. GH-transgenic Tanichthys albonubes was obtained by microinjection of the pTLA-GH into the fertilized eggs. Fast-growth individuals were observed in the transgenic group and the body weight of the largest individual was 2.1-fold that of the maximum in its non-transgenic siblings in 100 dph. In addition, a co-injection strategy was employed with pTLA-DsRed and pTLA-GH vector and proven to enhance the efficiency of GH-transgenic fish detection.

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.

Ion-dependent polymerization differences between mammalian beta- and gamma-nonmuscle actin isoforms

beta- and gamma-nonmuscle actins differ by 4 amino acids at or near the N terminus and distant from polymerization interfaces. beta-Actin contains an Asp(1)-Asp(2)-Asp(3) and Val(10) whereas gamma-actin has a Glu(1)-Glu(2)-Glu(3) and Ile(10). Despite these small changes, conserved across mammals, fish, and birds, their differential localization in the same cell suggests they may play different roles reflecting differences in their biochemical properties. To test this hypothesis, we established a baculovirus-driven expression system for producing these actins in isoform-pure populations although contaminated with 20-25% insect actin. Surprisingly, Ca-gamma-actin exhibits a slower monomeric nucleotide exchange rate, a much longer nucleation phase, and a somewhat slower elongation rate than beta-actin. In the Mg-form, this difference between the two is much smaller. Ca-gamma-actin depolymerizes half as fast as does beta-actin. Mixing experiments with Ca-actins reveal the two will readily co-polymerize. In the Ca-form, phosphate release from polymerizing beta-actin occurs much more rapidly and extensively than polymerization, whereas phosphate release lags behind polymerization with gamma-actin. Phosphate release during treadmilling is twice as fast with beta- as with gamma-actin. With Mg-actin in the initial stages, phosphate release for both actins correlates much more closely with polymerization. Calcium bound in the high affinity binding site of gamma-actin may cause a selective energy barrier relative to beta-actin that retards the equilibration between G- and F-monomer conformations resulting in a slower polymerizing actin with greater filament stability. This difference may be particularly important in sites such as the gamma-actin-rich cochlear hair cell stereocilium where local mm calcium concentrations may exist.

Unique contractile and structural protein expression in dog ileal inner circular smooth muscle

This study was designed to test the hypothesis that there is heterogeneous expression of contractile and structural proteins between the smooth muscle cells (SMCs) in the inner and outer circular muscle (ICM and OCM) layers of the ileum. Immunohistochemical staining and quantitation of fresh frozen sections of the dog ileum was performed using protein specific antibodies. Smooth muscle (SM) SMA myosin heavy chain (MHC), alpha- and gamma-SM actin, and vinculin all show greater expression in the ICM relative to the OCM. SMB MHC and fibronectin show the opposite pattern, with greater expression in the OCM relative to the ICM. Differences in expression of these proteins are consistent with proposed differences in function of these muscle layers. Hypotheses regarding muscle tone and the coordination and regulation of peristalsis via these different muscle layers based on this data can now be made and tested.

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

Accumulation of TDP-43 and alpha-actin in an amyotrophic lateral sclerosis patient with the K17I ANG mutation

A K17I mutation in the ANG gene encoding angiogenin has been identified in a case that we previously published as ALS with neuronal intranuclear protein inclusions (Seilhean et al. in Acta Neuropathol 108:81-87, 2004). These inclusions were immunoreactive for smooth muscle alpha-actin but not for angiogenin. Moreover, they were not labeled by anti-TDP-43 antibodies, while numerous cytoplasmic inclusions immunoreactive for ubiquitin, p62 and TDP-43 were detected in both oligodendrocytes and neurons in various regions of the central nervous system. In addition, expression of smooth muscle alpha-actin was increased in the liver where severe steatosis was observed. This is the first neuropathological description of a case with an ANG mutation. Angiogenin is known to interact with actin. Like other proteins involved in ALS pathogenesis, such as senataxin, TDP-43 and FUS/TLS, it plays a role in RNA maturation.

Re-expression of alpha skeletal actin as a marker for dedifferentiation in cardiac pathologies

Differentiation of foetal cardiomyocytes is accompanied by sequential actin isoform expression, i.e. down-regulation of the ‘embryonic’ alpha smooth muscle actin, followed by an up-regulation of alpha skeletal actin (αSKA) and a final predominant expression of alpha cardiac actin (αCA). Our objective was to detect whether re-expression of αSKA occurred during cardiomyocyte dedifferentiation, a phenomenon that has been observed in different pathologies characterized by myocardial dysfunction. Immunohistochemistry of αCA, αSKA and cardiotin was performed on left ventricle biopsies from human patients after coronary bypass surgery. Furthermore, actin isoform expression was investigated in left ventricle samples of rabbit hearts suffering from pressure- and volume-overload and in adult rabbit ventricular cardiomyocytes during dedifferentiation in vitro. Atrial goat samples up to 16 weeks of sustained atrial fibrillation (AF) were studied ultrastructurally and were immunostained for αCA and αSKA. Up-regulation of αSKA was observed in human ventricular cardiomyocytes showing down-regulation of αCA and cardiotin. A patchy re-expression pattern of αSKA was observed in rabbit left ventricular tissue subjected to pressure- and volume-overload. Dedifferentiating cardiomyocytes in vitro revealed a degradation of the contractile apparatus and local re-expression of αSKA. Comparable αSKA staining patterns were found in several areas of atrial goat tissue during 16 weeks of AF together with a progressive glycogen accumulation at the same time intervals. The expression of αSKA in adult dedifferentiating cardiomyocytes, in combination with PAS-positive glycogen and decreased cardiotin expression, offers an additional tool in the evaluation of myocardial dysfunction and indicates major changes in the contractile properties of these cells.

Myocardin-dependent activation of the CArG box-rich smooth muscle gamma-actin gene: preferential utilization of a single CArG element through functional association with the NKX3.1 homeodomain protein

Serum response factor (SRF) is a ubiquitously expressed transcription factor that binds a 10-bp element known as the CArG box, located in the proximal regulatory region of hundreds of target genes. SRF activates target genes in a cell- and context-dependent manner by assembling unique combinations of cofactors over CArG elements. One particularly strong SRF cofactor, myocardin (MYOCD), acts as a component of a molecular switch for smooth muscle cell (SMC) differentiation by activating cytoskeletal and contractile genes harboring SRF-binding CArG elements. Here we report that the human ACTG2 promoter, containing four conserved CArG elements, displays SMC-specific basal activity and is highly induced in the presence of MYOCD. Stable transfection of a non-SMC cell type with Myocd elicits elevations in endogenous Actg2 mRNA. Gel shift and luciferase assays reveal a strong bias for MYOCD-dependent transactivation through CArG2 of the human ACTG2 promoter. Substitution of CArG2 with other CArGs, including a consensus CArG element, fails to reconstitute full MYOCD-dependent ACTG2 promoter stimulation. Mutation of an adjacent binding site for NKX3.1 reduces MYOCD-dependent transactivation of the ACTG2 promoter. Co-immunoprecipitation, glutathione S-transferase pulldown, and luciferase assays show a physical and functional association between MYOCD and NKX3.1; no such functional relationship is evident with the related NKX2.5 transcription factor despite its interaction with MYOCD. These results demonstrate the ability of MYOCD to discriminate among several juxtaposed CArG elements, presumably through its novel partnership with NKX3.1, to optimally transactivate the human ACTG2 promoter.

Mutations and polymorphisms of the skeletal muscle alpha-actin gene (ACTA1)

The ACTA1 gene encodes skeletal muscle alpha-actin, which is the predominant actin isoform in the sarcomeric thin filaments of adult skeletal muscle, and essential, along with myosin, for muscle contraction. ACTA1 disease-causing mutations were first described in 1999, when a total of 15 mutations were known. In this article we describe 177 different disease-causing ACTA1 mutations, including 85 that have not been described before. ACTA1 mutations result in five overlapping congenital myopathies: nemaline myopathy; intranuclear rod myopathy; actin filament aggregate myopathy; congenital fiber type disproportion; and myopathy with core-like areas. Mixtures of these histopathological phenotypes may be seen in a single biopsy from one patient. Irrespective of the histopathology, the disease is frequently clinically severe, with many patients dying within the first year of life. Most mutations are dominant and most patients have de novo mutations not present in the peripheral blood DNA of either parent. Only 10% of mutations are recessive and they are genetic or functional null mutations. To aid molecular diagnosis and establishing genotype-phenotype correlations, we have developed a locus-specific database for ACTA1 variations (http://waimr.uwa.edu.au).

Beta and gamma-cytoplasmic actins display distinct distribution and functional diversity

Using newly generated monoclonal antibodies, we have compared the distribution of beta- and gamma-cytoplasmic actin in fibroblastic and epithelial cells, in which they play crucial roles during various key cellular processes. Whereas beta-actin is preferentially localized in stress fibers, circular bundles and at cell-cell contacts, suggesting a role in cell attachment and contraction, gamma-actin displays a more versatile organization, according to cell activities. In moving cells, gamma-actin is mainly organized as a meshwork in cortical and lamellipodial structures, suggesting a role in cell motility; in stationary cells, gamma-actin is also recruited into stress fibers. beta-actin-depleted cells become highly spread, display broad protrusions and reduce their stress-fiber content; by contrast, gamma-actin-depleted cells acquire a contractile phenotype with thick actin bundles and shrinked lamellar and lamellipodial structures. Moreover, beta- and gamma-actin depleted fibroblasts exhibit distinct changes in motility compared with their controls, suggesting a specific role for each isoform in cell locomotion. Our results reveal new aspects of beta- and gamma-actin organization that support their functional diversity.

Proteome and system ontology of hemorrhagic shock: exploring early constitutive changes in postshock mesenteric lymph

BACKGROUND

Postshock mesenteric lymph (PSML) is the mechanistic link between splanchnic ischemia reperfusion (IR) and remote organ injury. We hypothesize that an unbiased inspection of the proteome of PSML will reveal previously unrecognized aberrations in systems biology provoked by hemorrhage-induced mesenteric IR injury in vivo.

METHODS

Shock was induced in male Sprague-Dawley rats by controlled hemorrhage, and the mesenteric duct was cannulated for lymph collection. Preshock and postshock lymph were collected for differential in-gel electrophoresis (DIGE)-based proteomics. Proteins that increased or decreased in relative concentration > or =1.5-fold were selected for trypsin digestion and analysis by mass spectrometry (MS).

RESULTS

Evidence of tissue injury was detected by an increase in cell/tissue proteins in PSML. Components of coagulation were depleted, whereas products of hemolysis were increased. Haptoglobin was decreased, which supports an early postshock hemolytic process. Interestingly, several protective protease inhibitors were decreased in PSML. The unexpected findings were an increase in alpha-enolase (a key glycolitic enzyme and cell-surface plasminogen binding receptor, +2.4-fold change) and increased major urinary protein (MUP, a sex-specific lipid-binding protein, +17.1-fold change) in PSML.

CONCLUSION

A proteomic evaluation of PSML revealed evidence of several shock-associated processes: protein release from tissue injury, depletion of coagulation factors and evidence of hemolysis, depletion of protective protease inhibitors, and an increase in abundance of lipid carriers. These results suggest that constitutive changes in the proteome of PSML may provide novel insights into the complex pathophysiology of postshock systems biology.

Actin dynamics and functions in the interphase nucleus: moving toward an understanding of nuclear polymeric actin

Actin exists as a dynamic equilibrium of monomers and polymers within the nucleus of living cells. It is utilized by the cell for many aspects of gene regulation, including mRNA processing, chromatin remodelling, and global gene expression. Polymeric actin is now specifically linked to transcription by RNA polymerase I, II, and III. An active process, requiring both actin polymers and myosin, appears to drive RNA polymerase I transcription, and is also implicated in long-range chromatin movement. This type of mechanism brings activated genes from separate chromosomal territories together, and then participates in their compartmentalization near nuclear speckles. Nuclear speckle formation requires polymeric actin, and factors promoting polymerization, such as profilin and PIP2, are concentrated there. A review of the literature shows that a functional population of G-actin cycles between the cytoplasm and the nucleoplasm. Its nuclear concentration is dependent on the cytoplasmic G-actin pool, as well as on the activity of import and export mechanisms and the availability of interactions that sequester it within the nucleus. The N-WASP-Arp2/3 actin polymer-nucleating mechanism functions in the nucleus, and its mediators, including NCK, PIP2, and Rac1, can be found in the nucleoplasm, where they likely influence the kinetics of polymer formation. The actin polymer species produced are tightly regulated, and may take on conformations not easily recognized by phalloidin. Many of the factors that cleave F-actin in the cytoplasm are present at high levels in the nucleoplasm, and are also likely to affect actin dynamics there. The absolute and relative G-actin content in the nucleoplasm and the cytoplasm of a cell contains information about the homeostatic state of that cell. We propose that the cycling of G-actin between the nucleus and cytoplasm represents a signal transduction mechanism that can function through both extremes of global cellular G-actin content. MAL signalling within the serum response factor pathway, when G-actin levels are low, represents a well-studied example of actin functioning in signal transduction. The translocation of NCK into the nucleus, along with G-actin, during dissolution of the cytoskeleton in response to DNA damage represents another instance of a unique signalling mechanism operating when G-actin levels are high.

Rescue of skeletal muscle alpha-actin-null mice by cardiac (fetal) alpha-actin

Skeletal muscle alpha-actin (ACTA1) is the major actin in postnatal skeletal muscle. Mutations of ACTA1 cause mostly fatal congenital myopathies. Cardiac alpha-actin (ACTC) is the major striated actin in adult heart and fetal skeletal muscle. It is unknown why ACTC and ACTA1 expression switch during development. We investigated whether ACTC can replace ACTA1 in postnatal skeletal muscle. Two ACTC transgenic mouse lines were crossed with Acta1 knockout mice (which all die by 9 d after birth). Offspring resulting from the cross with the high expressing line survive to old age, and their skeletal muscles show no gross pathological features. The mice are not impaired on grip strength, rotarod, or locomotor activity. These findings indicate that ACTC is sufficiently similar to ACTA1 to produce adequate function in postnatal skeletal muscle. This raises the prospect that ACTC reactivation might provide a therapy for ACTA1 diseases. In addition, the mouse model will allow analysis of the precise functional differences between ACTA1 and ACTC.

Barx2 controls myoblast fusion and promotes MyoD-mediated activation of the smooth muscle alpha-actin gene

Remodeling of the actin cytoskeleton is a critical early step in skeletal muscle differentiation. Smooth muscle alpha-actin (SMA) is one of the earliest markers of myoblast differentiation and is important for the migration and cell shape changes that precede fusion. We have found that satellite cell-derived primary myoblasts from mice lacking the Barx2 homeobox gene show altered patterns of actin remodeling, reduced cell migration, and delayed differentiation. Consistent with the role of SMA in these processes, Barx2(-)(/)(-) myoblasts also show reduced expression of SMA mRNA and protein. The proximal SMA promoter contains binding sites for muscle regulatory factors and serum response factor as well as a conserved homeodomain binding site (HBS). We found that Barx2 binds to the HBS element and potentiates up-regulation of SMA promoter activity by MyoD. We also show that Barx2, MyoD, and serum response factor simultaneously occupy the SMA promoter in cells and that Barx2 interacts with MyoD. Overall these data indicate that Barx2 cooperates with other muscle-expressed transcription factors to regulate the early cytoskeletal remodeling events that underlie efficient myoblast differentiation.

In vivo and in vitro effects of two novel gamma-actin (ACTG1) mutations that cause DFNA20/26 hearing impairment

Here we report the functional assessment of two novel deafness-associated gamma-actin mutants, K118N and E241K, in a spectrum of different situations with increasing biological complexity by combining biochemical and cell biological analysis in yeast and mammalian cells. Our in vivo experiments showed that while the K118N had a very mild effect on yeast behaviour, the phenotype caused by the E241K mutation was very severe and characterized by a highly compromised ability to grow on glycerol as a carbon source, an aberrant multi-vacuolar pattern and the deposition of thick F-actin bundles randomly in the cell. The latter feature is consistent with the highly unusual spontaneous tendency of the E241K mutant to form bundles in vitro, although this propensity to bundle was neutralized by tropomyosin and the E241K filament bundles were hypersensitive to severing in the presence of cofilin. In transiently transfected NIH3T3 cells both mutant actins were normally incorporated into cytoskeleton structures, although cytoplasmic aggregates were also observed indicating an element of abnormality caused by the mutations in vivo. Interestingly, gene-gun mediated expression of these mutants in cochlear hair cells results in no gross alteration in cytoskeletal structures or the morphology of stereocilia. Our results provide a more complete picture of the biological consequences of deafness-associated gamma-actin mutants and support the hypothesis that the post-lingual and progressive nature of the DFNA20/26 hearing loss is the result of a progressive deterioration of the hair cell cytoskeleton over time.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Insoluble, speckled cytosolic distribution of retinoic acid receptor alpha protein as a marker of hepatic stellate cell activation in vitro

Hepatic stellate cells (HSCs) are the major site of retinoid storage, and their activation is a key process in liver fibrogenesis. We have previously shown that expression of the retinoic acid receptor alpha (RARalpha) is upregulated in activated rat HSCs at a posttranscriptional level and that these RARalpha proteins showed a speckled distribution in the cytosol, despite their possession of a nuclear localization signal (NLS). In this report, we further characterize these cytosolic RARalpha proteins by using exogenously expressed RARalpha protein fragments or mutants tagged with a green fluorescent protein. Substitution of four amino acids, 161-164 from lysine to alanine, abolished the NLS. Exogenously expressed RARalpha protein fragments containing an NLS were localized exclusively in the nuclei of activated rat HSCs and never colocalized with the endogenous RARalpha proteins in the cytosol, suggesting that the NLS of endogenous RARalpha proteins is masked. Biochemical analysis showed that 65% of RARalpha proteins in activated HSCs were insoluble in a mixture of detergents. The insolubility of RARalpha proteins makes it difficult to identify RARalpha proteins in activated HSCs. Therefore, we propose that insoluble, speckled cytosolic distribution of RARalpha proteins represents a new marker of HSC activation.

Cloning of black carp beta-actin gene and primarily detecting the function of its promoter region

A 3 338 bp DNA fragment including the open reading frame and 5'-flanking region of beta-actin gene for black carp genome was obtained through PCR amplification. Analysis of the sequencing results indicated the ORF of black carp beta-actin gene encoding a 375 amino acid protein that shares a high degree of conservation to other known actins. The black carp beta-actin sequence showed 100% identity to common carp, grass carp, and zebrafish, 99.2% identity to human and Norway rat beta-actin gene, 98.9% and 98.1% identity to chicken and Kenyan clawed frog beta-actin gene, respectively. The promoter region of black carp beta-actin gene was inserted into the promoterless pEGFP1 vector. The recombinant plasmid was microinjected into the fertilized eggs of mud loach before two-cell stage as well as transfected into HeLa cell line. GFP expression was found in 50% of mud loach embryos and 2/3 HeLa cells. The GFP expression could be observed in every part of the mud loach embryos, and in some embryos, the GFP was expressed in the whole body. Thus, the usefulness of black carp beta-actin promoter as a ubiquitous expression promoter was confirmed using the EGFP as a reporter gene.

Recombinant alpha-actin for specific fluorescent labeling

Until recently, actin was thought to act merely as a passive track for its motility partner, myosin, during actomyosin interactions. Yet a recent report having observed dynamical conformational changes in labeled skeletal muscle alpha-actin suggests that actin has a more active role. Because the labeling technique was still immature, however, conclusions regarding the significance of the different conformations are difficult to make. Here, we describe the preparation of fully active alpha-actin obtained from a baculovirus expression system. We developed alpha-actin recombinants, of which subdomains 1 and 2 have specific sites for fluorescent probes. This specific labeling technique offers to significantly expand the information acquired from actin studies.