MuRF-1 is Involved in Laryngeal Muscle Denervation Atrophy by Regulating G-Actin Ubiquitination

OBJECTIVE

Muscle RING-finger protein-1 (MuRF-1), an E3 ubiquitin ligase, has been reported to aggravate skeletal muscle denervated atrophy by mediating the ubiquitination degradation of multiple proteins, whereas the molecular mechanism underlying MuRF-1-mediated internal laryngeal muscle denervated atrophy remains unknown.

METHODS

A rat unilateral recurrent laryngeal nerve (RLN) transection model was established to evaluate denervated muscle atrophy of the larynx. The expression of MuRF-1, G- and F-actin in thyroarytenoid muscle (TA) myocytes before and after RLN injury was analyzed by immunofluorescence and Western blotting. Coimmunoprecipitation experiments detected molecular interactions between MuRF-1 and G-actin. Immunoprecipitation tested MuRF-1-mediated ubiquitination of G-actin in denervated and innervated TA muscle tissues. The shRNA-MuRF-1 AAV was used to suppress MuRF-1 expression in denervated TA muscles in vivo.

RESULTS

First, MuRF-1 expression was significantly elevated in denervated TA muscle compared to innervated TA muscle (p < 0.001). Second, there was a progressive increase in the G/F-actin ratio in TA myocytes from day 3 to 14 after RLNI (p < 0.01). Furthermore, colocalization of MuRF-1 and G-actin in denervated TA myocytes was observed. Moreover, the upregulation of MuRF-1 was closely associated with the ubiquitination of G-actin in denervated TA myocytes and muscle tissues. Knockdown of MuRF-1 decelerated the degree of TA muscle atrophy compared with that in the Blank and NC groups (p < 0.001) but seemed to promote the compensatory movement of the healthy side.

CONCLUSION

Collectively, we illustrate a novel molecular mechanism underlying MuRF-1-mediated internal laryngeal muscle denervated atrophy in that MuRF-1 could promote disequilibrium of the G/F-actin ratio by regulating G-actin ubiquitination.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:855-864, 2024.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

Dynamic coalescence of yeast Heat Shock Protein genes bypasses the requirement for actin

Nuclear actin has been implicated in dynamic chromatin rearrangements in diverse eukaryotes. In mammalian cells, it is required to reposition double-strand DNA breaks to enable homologous recombination repair and to enhance transcription by facilitating RNA Pol II recruitment to gene promoters. In the yeast Saccharomyces cerevisiae, nuclear actin modulates interphase chromosome dynamics and is required to reposition the induced INO1 gene to the nuclear periphery. Here, we have investigated the role of actin in driving intergenic interactions between Heat Shock Factor 1 (Hsf1)-regulated Heat Shock Protein (HSP) genes in budding yeast. These genes, dispersed on multiple chromosomes, dramatically reposition following exposure of cells to acute thermal stress, leading to their clustering within dynamic biomolecular condensates. Using an auxin-induced degradation strategy, we found that conditional depletion of nucleators of either linear or branched F-actin (Bni1/Bnr1 and Arp2, respectively) had little or no effect on heat shock-induced HSP gene coalescence or transcription. In addition, we found that pretreatment of cells with latrunculin A, an inhibitor of both filamentous and monomeric actin, failed to affect intergenic interactions between activated HSP genes and their heat shock-induced intragenic looping and folding. Moreover, latrunculin A pretreatment had little effect on HSP gene expression at either RNA or protein levels. In notable contrast, we confirmed that repositioning of activated INO1 to the nuclear periphery and its proper expression do require actin. Collectively, our work suggests that transcriptional activation and 3D genome restructuring of thermally induced, Hsf1-regulated genes can occur in the absence of actin.

MRTFA: A critical protein in normal and malignant hematopoiesis and beyond

Myocardin-related transcription factor A (MRTFA) is a coactivator of serum response factor, a transcription factor that participates in several critical cellular functions including cell growth and apoptosis. MRTFA couples transcriptional regulation to actin cytoskeleton dynamics, and the transcriptional targets of the MRTFA–serum response factor complex include genes encoding cytoskeletal proteins as well as immediate early genes. Previous work has shown that MRTFA promotes the differentiation of many cell types, including various types of muscle cells and hematopoietic cells, and MRTFA's interactions with other protein partners broaden its cellular roles. However, despite being first identified as part of the recurrent t(1;22) chromosomal translocation in acute megakaryoblastic leukemia, the mechanisms by which MRTFA functions in malignant hematopoiesis have yet to be defined. In this review, we provide an in-depth examination of the structure, regulation, and known functions of MRTFA with a focus on hematopoiesis. We conclude by identifying areas of study that merit further investigation.

CAPt'n of Actin Dynamics: Recent Advances in the Molecular, Developmental and Physiological Functions of Cyclase-Associated Protein (CAP)

Cyclase-associated protein (CAP) has been discovered three decades ago in budding yeast as a protein that associates with the cyclic adenosine monophosphate (cAMP)-producing adenylyl cyclase and that suppresses a hyperactive RAS2 variant. Since that time, CAP has been identified in all eukaryotic species examined and it became evident that the activity in RAS-cAMP signaling is restricted to a limited number of species. Instead, its actin binding activity is conserved among eukaryotes and actin cytoskeleton regulation emerged as its primary function. However, for many years, the molecular functions as well as the developmental and physiological relevance of CAP remained unknown. In the present article, we will compile important recent progress on its molecular functions that identified CAP as a novel key regulator of actin dynamics, i.e., the spatiotemporally controlled assembly and disassembly of actin filaments (F-actin). These studies unraveled a cooperation with ADF/Cofilin and Twinfilin in F-actin disassembly, a nucleotide exchange activity on globular actin monomers (G-actin) that is required for F-actin assembly and an inhibitory function towards the F-actin assembly factor INF2. Moreover, by focusing on selected model organisms, we will review current literature on its developmental and physiological functions, and we will present studies implicating CAP in human pathologies. Together, this review article summarizes and discusses recent achievements in understanding the molecular, developmental and physiological functions of CAP, which led this protein emerge as a novel CAPt'n of actin dynamics.

Profilin binding couples chloride intracellular channel protein CLIC4 to RhoA-mDia2 signaling and filopodium formation

Chloride intracellular channel 4 (CLIC4) is a cytosolic protein implicated in diverse actin-based processes, including integrin trafficking, cell adhesion, and tubulogenesis. CLIC4 is rapidly recruited to the plasma membrane by RhoA-activating agonists and then partly colocalizes with β1 integrins. Agonist-induced CLIC4 translocation depends on actin polymerization and requires conserved residues that make up a putative binding groove. However, the mechanism and significance of CLIC4 trafficking have been elusive. Here, we show that RhoA activation by either lysophosphatidic acid (LPA) or epidermal growth factor is necessary and sufficient for CLIC4 translocation to the plasma membrane and involves regulation by the RhoA effector mDia2, a driver of actin polymerization and filopodium formation. We found that CLIC4 binds the G-actin–binding protein profilin-1 via the same residues that are required for CLIC4 trafficking. Consistently, shRNA-induced profilin-1 silencing impaired agonist-induced CLIC4 trafficking and the formation of mDia2-dependent filopodia. Conversely, CLIC4 knockdown increased filopodium formation in an integrin-dependent manner, a phenotype rescued by wild-type CLIC4 but not by the trafficking-incompetent mutant CLIC4(C35A). Furthermore, CLIC4 accelerated LPA-induced filopodium retraction. We conclude that through profilin-1 binding, CLIC4 functions in a RhoA–mDia2–regulated signaling network to integrate cortical actin assembly and membrane protrusion. We propose that agonist-induced CLIC4 translocation provides a feedback mechanism that counteracts formin-driven filopodium formation.

The effects of leptin on F-actin remodelling in type 1 diabetes

BACKGROUND

The aim of the current study is to investigate the effect of leptin on cytoskeleton structures in both in vivo and in vitro model of diabetes.

MATERIALS AND METHODS

For in vivo studies, leptin in different doses (240, and 480 mg/kg) was injected to the diabetic rats after 1-week of streptozotocin (STZ, 55 mg/kg) treatment. Leptin levels were analysed in serum, liver, and pancreas samples. Hepatic and pancreatic F- and G-actin expressions were determined by Western blotting. For in vitro studies, hepatic and pancreatic primary cell lines were obtained from the control rats. To these cultures, STZ (15 and 30 mM), leptin (50, 60 and 100 ng/mL), and their combinations were applied for 1, 3, and 4 weeks. After the treatment period, F-actin was visualised by the Alexa-fluor fluorescent dye.

RESULTS

Streptozotocin decreased the G-actin in both tissues in vivo. However, leptin caused a dose-dependent increase in G-actin levels while F-actin decreased in both tissues. Moreover, leptin caused the perimembranous condensation of actin filaments and amelioration of F-actin structures in vivo. A dose-dependent corruption of F-actin filament structures was observed in leptin-treated primary cells in vitro, while STZ also caused corruption of these filaments. Co-exposure of STZ and leptin caused the amelioration of F-actin filaments, while the peri- membranous condensation was also observed as was in vivo study.

CONCLUSIONS

Leptin therapy could be a candidate for diabetes, but it should not be ruled out as being important the severity of diabetes and leptin doses.

The effect of ATP marination on the depolymerization of actin filament in goose muscles during postmortem conditioning

In order to study the tenderization mechanism of ATP treatments by depolymerizing actin filaments, breast muscles of Eastern Zhejiang White Geese were randomly divided into 3 groups: control, 10 and 20 mM groups. Shear force (SF), sarcomere length (SL) and myofibrillar fraction index (MFI), the content of F-actin and G-actin, the expression of actin associated proteins (cofilins and tropomodulins) were investigated during conditioning. In 20 mM group, cofilins content increased from 48 to 168 h, while tropomodulins decreased; the content of F-actin decreased from 24 to 168 h, while the increased G-actin was observed upto 48 h. In the control, the degraded tropomodulins were observed at 168 h, and the increased cofilins and G-actin were detected at the same time; the increase of MFI and decrease of F-actin content were shown at 96 and 168 h. Compared to control group, 20 mM group accelerated the transformation of F-actin into G-actin; it showed higher SL and MFI, and lower SF at 48, 96 and 168 h, respectively. We concluded that depolymerization of actin filaments, which was regulated by cofilins and tropomodulins, contributed to myofibrillar fraction and low SF during conditioning.

Structure, dynamics, and biochemical characterization of ADF/cofilin Twinstar from Drosophilamelanogaster

BACKGROUND

Twinstar is an ADF/cofilin family protein, which is expressed by the tsr gene in Drosophila melanogaster. Twinstar is one of the main regulators of actin cytoskeleton remodelling and is essential for vital cellular processes like cytokinesis and endocytosis.

METHODS

We have characterized the structure and dynamics of Twinstar by solution NMR spectroscopy, the interaction of Twinstar with rabbit muscle actin by ITC, and biochemical activities of Twinstar through different biochemical assays using fluorescence spectroscopy and ultra-centrifugation.

RESULTS

The solution structure of Twinstar shows characteristic ADF-H fold with well-formed G/F-site and F-site for interaction with actin. The structure possesses an extended F-loop, which is rigid at the base, but flexible towards its apical region. Twinstar shares similar dynamics for the G/F-site with C. elegans homologs, UNC-60A and UNC-60B. However, the dynamics of its F-loop are different from its C. elegans homologs. Twinstar shows strong affinity for ADP-G-Actin and ATP-G-Actin with K_ds of ~7.6 nM and ~0.4 μM, respectively. It shows mild F-actin depolymerizing activity and stable interaction with F-actin with a K_d of ~5.0 μM. It inhibits the rate of the nucleotide exchange in a dose dependent manner.

CONCLUSION

On the basis of structure, dynamics, and biochemical activity, Twinstar can be taken to execute its biochemical role by facilitating directional growth and maintenance of length of actin filaments.

GENERAL SIGNIFICANCE

This study characterizes the structure, backbone dynamics, and biochemical activities of Twinstar of Drosophila, which provides an insight into the regulation of actin dynamics in the member of phylum insecta.

Mechanical differences between ATP and ADP actin states: A molecular dynamics study

This paper aims to give a comprehensive atomistic modeling of the nanomechanical behavior of actin monomer. Actin is a ubiquitous and essential component of cytoskeleton which forms many different cellular structures. Despite for several years great effort has been devoted to the investigation of mechanical properties of the actin filament, studies on the nanomechanical behavior of actin monomer are still lacking. These scales are, however, important for a complete understanding of the role of actin as an important component in the cytoskeleton structure. Based on the accuracy of atomistic modeling methods such as molecular dynamics simulations, steered molecular dynamics method is performed to assess tension of monomeric G-actin molecule under different types of mechanical loading including axial and lateral. As a result, stress-strain curves are obtained in aqueous solution, with either ATP or ADP bound in the nucleotide binding pocket. The obtained results yield evaluation of the tensile stiffness of a single actin monomer in lateral and normal direction. In order to compare the behavior of ATP and ADP G-actins, the number of hydrogen bonds and nonbonded interactions between the nucleotide and the protein are analyzed. Moreover, The effect of virtual spring of steered molecular dynamics on the mechanical behavior of actin monomer is investigated. The results reveal increasing the virtual spring constant leads to convergence of the stiffness. Moreover, in this paper, a generalized model is proposed to extend the obtained results for the monomeric G-actin scale to the actin filament. Our modeling estimated a persistence length of actin filament 15.41 µm, close to experimental measurements. Moreover, In this paper, the breaking force actin-actin bond is evaluated using steered molecular dynamics simulation. By applying a tensile force, actin-actin bond ruptured at 4197.5 pN.

A Sephin1-insensitive tripartite holophosphatase dephosphorylates translation initiation factor 2α

The integrated stress response (ISR) is regulated by kinases that phosphorylate the α subunit of translation initiation factor 2 and phosphatases that dephosphorylate it. Genetic and biochemical observations indicate that the eIF2α^P-directed holophosphatase, a therapeutic target in diseases of protein misfolding, is comprised of a regulatory subunit, PPP1R15, and a catalytic subunit, protein phosphatase 1 (PP1). In mammals, there are two isoforms of the regulatory subunit, PPP1R15A and PPP1R15B, with overlapping roles in the essential function of eIF2α^P dephosphorylation. However, conflicting reports have appeared regarding the requirement for an additional co-factor, G-actin, in enabling substrate-specific dephosphorylation by PPP1R15-containing PP1 holoenzymes. An additional concern relates to the sensitivity of the holoenzyme to the [(o-chlorobenzylidene)amino]guanidines Sephin1 or guanabenz, putative small-molecule proteostasis modulators. It has been suggested that the source and method of purification of the PP1 catalytic subunit and the presence or absence of an N-terminal repeat–containing region in the PPP1R15A regulatory subunit might influence the requirement for G-actin and sensitivity of the holoenzyme to inhibitors. We found that eIF2α^P dephosphorylation by PP1 was moderately stimulated by repeat-containing PPP1R15A in an unphysiological low ionic strength buffer, whereas stimulation imparted by the co-presence of PPP1R15A and G-actin was observed under a broad range of conditions, low and physiological ionic strength, regardless of whether the PPP1R15A regulatory subunit had or lacked the N-terminal repeat–containing region and whether it was paired with native PP1 purified from rabbit muscle or recombinant PP1 purified from bacteria. Furthermore, none of the PPP1R15A-containing holophosphatases tested were inhibited by Sephin1 or guanabenz.

ARF GTPases control phenotypic switching of vascular smooth muscle cells through the regulation of actin function and actin dependent gene expression

Vascular smooth muscle cells (VSMC) can exhibit a contractile or a synthetic phenotype depending on the extracellular stimuli present and the composition of the extracellular matrix. Uncontrolled activation of the synthetic VSMC phenotype is however associated with the development of cardiovascular diseases. Here, we aimed to elucidate the role of the ARF GTPases in the regulation of VSMC dedifferentiation. First, we observed that the inhibition of the activation of ARF proteins with SecinH3, a blocker of the cytohesin ARF GEF family, reduced the ability of the cells to migrate and proliferate. In addition, this inhibitor also blocked expression of sm22α and αSMA, two contractile markers, at the transcription level impairing cell contractility. Specific knockdown of ARF1 and ARF6 showed that both isoforms were required for migration and proliferation, but ARF1 only regulated contractility through sm22α and αSMA expression. Expression of these VSMC markers was correlated with the degree of actin polymerization. VSMC treatment with SecinH3 as well as ARF1 depletion was both able to block the formation of stress fibres and focal adhesions, demonstrating the role of this GTPase in actin filament formation. Consequently, we observed that both treatments increased the ratio of G-actin to F-actin in these cells. The elevated amounts of cytoplasmic G-actin, acting as a signaling intermediate, blocked the recruitment of the Mkl1 (MRTF-A) transcription factor in the nucleus, demonstrating its involvement in the regulation of contractile protein expression. Altogether, these findings show for the first time that ARF GTPases are actively involved in VSMC phenotypic switching through the regulation of actin function in migration and proliferation, and the control of actin dependent gene regulation.

Reconsidering an active role for G-actin in cytoskeletal regulation

Globular (G)-actin, the actin monomer, assembles into polarized filaments that form networks that can provide structural support, generate force and organize the cell. Many of these structures are highly dynamic and to maintain them, the cell relies on a large reserve of monomers. Classically, the G-actin pool has been thought of as homogenous. However, recent work has shown that actin monomers can exist in distinct groups that can be targeted to specific networks, where they drive and modify filament assembly in ways that can have profound effects on cellular behavior. This Review focuses on the potential factors that could create functionally distinct pools of actin monomers in the cell, including differences between the actin isoforms and the regulation of G-actin by monomer binding proteins, such as profilin and thymosin β4. Owing to difficulties in studying and visualizing G-actin, our knowledge over the precise role that specific actin monomer pools play in regulating cellular actin dynamics remains incomplete. Here, we discuss some of these unanswered questions and also provide a summary of the methodologies currently available for the imaging of G-actin.

Monomeric G-actin is uniformly distributed in pollen tubes and is rapidly redistributed via cytoplasmic streaming during pollen tube growth

Dynamic assembly and disassembly of the actin cytoskeleton has been implicated in the regulation of pollen germination and subsequent tube growth. It is widely accepted that actin filaments are arrayed into distinct structures within different regions of the pollen tube. Maintenance of the equilibrium between monomeric globular actin (G-actin) and filamentous actin (F-actin) is crucial for actin assembly and array construction, and the local concentration of G-actin thus directly impacts actin assembly. The localization and dynamics of G-actin in the pollen tube, however, remain to be determined conclusively. To address this question, we created a series of fusion proteins between green fluorescent protein (GFP) and the Arabidopsis reproductive actin ACT11. Expression of a fusion protein with GFP inserted after methionine at position 49 within the DNase I-binding loop of ACT11 (GFP_Met49 -ACT11) rescued the phenotypes in act11 mutants. Consistent with the notion that the majority of actin is in its monomeric form, GFP_Met49 -ACT11 and GFP fusion proteins of four other reproductive actins generated with the same strategy do not obviously label filamentous structures. In further support of the functionality of these fusion proteins, we found that they can be incorporated into filamentous structures in jasplakinolide (Jasp)-treated pollen tubes. Careful observations showed that G-actin is distributed uniformly in the pollen tube and is rapidly redistributed via cytoplasmic streaming during pollen tube growth. Our study suggests that G-actin is readily available in the cytoplasm to support continuous actin polymerization during rapid pollen tube growth.

The effect of CaCl2 marination on the tenderizing pathway of goose meat during conditioning

In order to figure out the effect of CaCl₂ on the tenderizing pathway of goose meat, breast muscles of thirty-two Eastern Zhejiang White Geese were divided into three treatments: the control, 150 and 300mM CaCl₂. Shear force, myofibrillar fraction index (MFI), actin filaments and F-actin, G-actin and tropomodulins (Tmods) levels were investigated during 168h. Results showed that 300mM treatment had lower shear force at 48, 96 and 168h and higher MFI at 24, 48, 96 and 168h than the control. The rate of actin filaments disruption, the decrease of F-actin, the degradation of Tmods, the increase of G-actin in 300mM treatment was faster than 150mM treatment; the rate in the control was the slowest among treatments. CaCl₂ accelerated the transformation of F-actin into G-actin. We concluded that CaCl₂ tenderized goose meat by depolymerizing actin filaments and cleaving Tmods.

The effect of microwave on the interaction of flavour compounds with G-actin from grass carp (Catenopharyngodon idella)

BACKGROUND

In order to investigate the influence of non-thermal effects of microwaves on the flavour of fish and meat products, the G-actin of grass carp in ice baths was exposed to different microwave powers (0, 100, 300 or 500 W); the surface hydrophobicity, sulfhydryl contents, secondary structures and adsorption capacity of G-actin to ketones were determined.

RESULTS

As microwave power increased from 0 to 300 W, the surface hydrophobicity, total and reactive sulfhydryls increased; α-helix, β-sheet and random coil fractions turned into β-turn fractions. As microwave power increased from 300 to 500 W, however, hydrophobicity and sulfhydryl contents decreased; β-turn and random coil fractions turned into α-helix and β-sheet fractions. The tendencies of adsorbed capacity of ketones were similar to hydrophobicity and sulfhydryl contents.

CONCLUSION

The increased adsorbing of ketones could be attributed to the unfolding of secondary structures by revealing new binding sites, including thiol groups and hydrophobic binding sites. The decreased binding capacity was related to the refolding and aggregation of protein. The results suggested that microwave powers had obvious effects on the flavour retention and proteins structures in muscle foods. © 2017 Society of Chemical Industry.

The effect of oxidation on the structure of G-actin and its binding ability with aroma compounds in carp grass skeletal muscle

To investigate the influence of oxidative modifications of G-actin on its binding ability with aroma compounds, the influence of H₂O₂ treatments on G-actin structure and the absorption for alcohols and aldehydes was investigated. Raman spectroscopy and scanning electron microscopy were used to evaluate structural changes of G-actin; GC-MS was used to analyze the binding with alcohols and aldehydes. Results showed that 0-5mM H₂O₂ enhanced the absorption of G-actin toward alcohols involved in the formation of hydrogen bonds by increasing α-helix and carbonyl values. 0-1mM H₂O₂ caused the release of aldehydes with decreased sulfhydryl sites. 1-20mM H₂O₂ increased the retention of aldehydes, due to the increased hydrophobic sites by G-actin rebuilding and aggregating. The aggregated G-actin favoured the hydrophobic interactions with aroma compounds, forming the protein-aroma compound complex, thus enhancing the resultant binding ability, as evidenced by scanning electron microscopy and GC/MS analysis.

Interactions of histatin-3 and histatin-5 with actin

Background

Histatins are histidine rich polypeptides produced in the parotid and submandibular gland and secreted into the saliva. Histatin-3 and −5 are the most important polycationic histatins. They possess antimicrobial activity against fungi such as Candida albicans. Histatin-5 has a higher antifungal activity than histatin-3 while histatin-3 is mostly involved in wound healing in the oral cavity. We found that these histatins, like other polycationic peptides and proteins, such as LL-37, lysozyme and histones, interact with extracellular actin.

Results

Histatin-3 and −5 polymerize globular actin (G-actin) to filamentous actin (F-actin) and bundle F-actin filaments. Both actin polymerization and bundling by histatins is pH sensitive due to the high histidine content of histatins. In spite of the equal number of net positive charges and histidine residues in histatin-3 and −5, less histatin-3 is needed than histatin-5 for polymerization and bundling of actin. The efficiency of actin polymerization and bundling by histatins greatly increases with decreasing pH. Histatin-3 and −5 induced actin bundles are dissociated by 100 and 50 mM NaCl, respectively. The relatively low NaCl concentration required to dissociate histatin-induced bundles implies that the actin-histatin filaments bind to each other mainly by electrostatic forces. The binding of histatin-3 to F-actin is stronger than that of histatin-5 showing that hydrophobic forces have also some role in histatin-3- actin interaction. Histatins affect the fluorescence of probes attached to the D-loop of G-actin indicating histatin induced changes in actin structure. Transglutaminase cross-links histatins to actin. Competition and limited proteolysis experiments indicate that the main histatin cross-linking site on actin is glutamine-49 on the D-loop of actin.

Conclusions

Both histatin-3 and −5 interacts with actin, however, histatin 3 binds stronger to actin and affects actin structure at lower concentration than histatin-5 due to the extra 8 amino acid sequence at the C-terminus of histatin-3. Extracellular actin might regulate histatin activity in the oral cavity, which should be the subject of further investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-017-0078-0) contains supplementary material, which is available to authorized users.

The effect of Cytochalasin B and Jasplakinolide on depolymerization of actin filaments in goose muscles during postmortem conditioning

Breast muscles of twenty-four Eastern Zhejiang White Geese were randomly divided into three groups: control, Cytochalasin B (Cyt B) and Jasplakinolide (Jasp) treatments during postmortem conditioning. The myofibrillar fraction index (MFI), actin filaments and the levels of F-actin, G-actin and actin associated proteins (cofilins and tropomodulins) during conditioning were investigated. In control, the degraded tropomodulins, increased G-actin and disrupted actin filaments were observed at 4 and 7days; the increase of MFI and decrease of F-actin content were shown during conditioning. Cyt B treatments accelerated the transformation from F-actin to G-actin, weakened actin filaments and increased MFI compared to the control, while Jasp gained the opposite effect against Cyt B. We concluded that depolymerization of actin filaments regulated by tropomodulins contributed to myofibrillar fraction during conditioning. This work provided a new pathway of tenderization by the depolymerization of actin filaments.

Study on physisorption between G-actin and amphiphilic polymer-coated gold nanoparticles

Physisorptions occurs everywhere and constantly in living organisms and between nanomaterials and biomolecules. In this study, one of the most important proteins, G-actin, was selected to investigate its bio-nano physisorption with a model nanoparticle coated with a amphiphilic polymer. Using a photoluminescence quenching method, both the binding constant and the Hill constant were determined as 1.79 × 10⁷  M^-1 and 0.84, respectively. Thermodynamic calculations proved that such a physisorption was a spontaneous procedure. The physisorption-mediated protein-nanoparticle conjugates were robust enough to resist gel electrophoresis, and protein conformation was kept intact, as revealed using circular dichroism. This conjugate might be a promising candidate for nanofabrication or could play a significant role in actin-related bioactivities.

Isolation of Actin and Actin-Binding Proteins

Actin-binding proteins mediate and regulate the dynamics of actin and the organization of highly ordered structures of F-actin. Villin is generally expressed in plant cells and is associated with G-actin or F-actin dependent on Ca²⁺ concentrations. Using a DNase I affinity column chromatography approach, the villin and the G-actin can be isolated from plant material. An outline of this method including the preparation of crude protein extract from plant material, its application on the affinity column, and the successive elution of villin with a solution containing EGTA and then of G-actin with denatured reagents is presented.

Single-cell analysis reveals differential regulation of the alveolar macrophage actin cytoskeleton by surfactant proteins A1 and A2: implications of sex and aging

Background

Surfactant protein A (SP-A) contributes to lung immunity by regulating inflammation and responses to microorganisms invading the lung. The huge genetic variability of SP-A in humans implies that this protein is highly important in tightly regulating the lung immune response. Proteomic studies have demonstrated that there are differential responses of the macrophages to SP-A1 and SP-A2 and that there are sex differences implicated in these responses.

Methods

Purified SP-A variants were used for administration to alveolar macrophages from SP-A knockout (KO) mice for in vitro studies, and alveolar macrophages from humanized SP-A transgenic mice were isolated for ex vivo studies. The actin cytoskeleton was examined by fluorescence and confocal microscopy, and the macrophages were categorized according to the distribution of polymerized actin.

Results

In accordance with previous data, we report that there are sex differences in the response of alveolar macrophages to SP-A1 and SP-A2. The cell size and F-actin content of the alveolar macrophages are sex- and age-dependent. Importantly, there are different subpopulations of cells with differential distribution of polymerized actin. In vitro, SP-A2 destabilizes actin in female, but not male, mice, and the same tendency is observed by SP-A1 in cells from male mice. Similarly, there are differences in the distribution of AM subpopulations isolated from SP-A transgenic mice depending on sex and age.

Conclusions

There are marked sex- and age-related differences in the alveolar macrophage phenotype as illustrated by F-actin staining between SP-A1 and SP-A2. Importantly, the phenotypic switch caused by the different SP-A variants is subtle, and pertains to the frequency of the observed subpopulations, demonstrating the need for single-cell analysis approaches. The differential responses of alveolar macrophages to SP-A1 and SP-A2 highlight the importance of genotype in immune regulation and the susceptibility to lung disease and the need for development of individualized treatment options.

Plasma levels of F-actin and F:G-actin ratio as potential new biomarkers in patients with septic shock

OBJECTIVE

To compare plasma levels of F-actin, G-actin and thymosin beta 4 (TB4) in humans with septic shock, noninfectious systemic inflammatory response syndrome (SIRS) and healthy controls.

RESULTS

F-actin was significantly elevated in septic shock as compared with noninfectious SIRS and healthy controls. G-actin levels were greatest in the noninfectious SIRS group but significantly elevated in septic shock as compared with healthy controls. TB4 was not detectable in the septic shock or noninfectious SIRS group above the assay's lowest detection range (78 ng/ml).

CONCLUSIONS

F-actin is significantly elevated in patients with septic shock as compared with noninfectious SIRS. F-actin and the F:G-actin ratio are potential biomarkers for the diagnosis of septic shock.

Native globular actin has a thermodynamically unstable quasi-stationary structure with elements of intrinsic disorder

The native form of globular actin, G-actin, is formed in vivo as a result of complex post-translational folding processes that require ATP energy expenditure and are assisted by the 70 kDa heat shock protein, prefoldin and chaperonin containing TCP-1. G-actin is stabilized by the binding of one ATP molecule and one Ca(2+) ion (or Mg(2+) in vivo). Chemical denaturants, heating or Ca(2+) removal transform native actin (N) into 'inactivated actin' (I), a compact oligomer comprising 14-16 subunits. Viscogenic and crowding agents slow this process but do not stop it. The lack of calcium in the solution accelerates the spontaneous N → I transition. Thus, native G-actin has a kinetically stable (as a result of the high free energy barrier between the N and I states) but thermodynamically unstable structure, which, in the absence of Ca(2+) or other bivalent metal ions, spontaneously converts to the thermodynamically stable I state. It was noted that native actin has much in common with intrinsically disordered proteins: it has functionally important disordered regions; it is constantly in complex with one of its numerous partners; and it plays key roles in many cellular processes, in a manner similar to disordered hub proteins. By analyzing actin folding in vivo and unfolding in vitro, we advanced the hypothesis that proteins in a native state may have a thermodynamically unstable quasi-stationary structure. The kinetically stable native state of these proteins appears forcibly under the influence of intracellular folding machinery. The denaturation of such proteins is always irreversible because the inactivated state, for which the structure is determined by the amino acid sequence of a protein, comprises the thermodynamically stable state under physiological conditions.

Effect of serine phosphorylation and Ser25 phospho-mimicking mutations on nuclear localisation and ligand interactions of annexin A2

Annexin A2 (AnxA2) interacts with numerous ligands, including calcium, lipids, mRNAs and intracellular and extracellular proteins. Different post-translational modifications participate in the discrimination of the functions of AnxA2 by modulating its ligand interactions. Here, phospho-mimicking mutants (AnxA2-S25E and AnxA2-S25D) were employed to investigate the effects of Ser25 phosphorylation on the structure and function of AnxA2 by using AnxA2-S25A as a control. The overall α-helical structure of AnxA2 is not affected by the mutations, since the thermal stabilities and aggregation tendencies of the mutants differ only slightly from the wild-type (wt) protein. Unlike wt AnxA2, all mutants bind the anxA2 3' untranslated region and β-γ-G-actin with high affinity in a Ca(2+)-independent manner. AnxA2-S25E is not targeted to the nucleus in transfected PC12 cells. In vitro phosphorylation of AnxA2 by protein kinase C increases its affinity to mRNA and inhibits its nuclear localisation, in accordance with the data obtained with the phospho-mimicking mutants. Ca(2+)-dependent binding of wt AnxA2 to phosphatidylinositol, phosphatidylinositol-3-phosphate, phosphatidylinositol-4-phosphate and phosphatidylinositol-5-phosphate, as well as weaker but still Ca(2+)-dependent binding to phosphatidylserine and phosphatidylinositol-3,5-bisphosphate, was demonstrated by a protein-lipid overlay assay, whereas binding of AnxA2 to these lipids, as well as its binding to liposomes, is inhibited by the Ser25 mutations. Thus, introduction of a modification (mutation or phosphorylation) at Ser25 appears to induce a conformational change leading to increased accessibility of the mRNA- and G-actin-binding sites in domain IV independent of Ca(2+) levels, while the Ca(2+)-dependent binding of AnxA2 to phospholipids is attenuated.

Effect of methionine sulfoxide reductase B1 (SelR) gene silencing on peroxynitrite-induced F-actin disruption in human lens epithelial cells

F-actin plays a crucial role in fundamental cellular processes, and is extremely susceptible to peroxynitrite attack due to the high abundance of tyrosine in the peptide. Methionine sulfoxide reductase (Msr) B1 is a selenium-dependent enzyme (selenoprotein R) that may act as a reactive oxygen species (ROS) scavenger. However, its function in coping with reactive nitrogen species (RNS)-mediated stress and the physiological significance remain unclear. Thus, the present study was conducted to elucidate the role and mechanism of MsrB1 in protecting human lens epithelial (hLE) cells against peroxynitrite-induced F-actin disruption. While exposure to high concentrations of peroxynitrite and gene silencing of MsrB1 by siRNA alone caused disassembly of F-actin via inactivation of extracellular signal-regulated kinase (ERK) in hLE cells, the latter substantially aggravated the disassembly of F-actin triggered by the former. This aggravation concurred with elevated nitration of F-actin and inactivation of ERK compared with that induced by the peroxynitrite treatment alone. In conclusion, MsrB1 protected hLE cells against the peroxynitrite-induced F-actin disruption, and the protection was mediated by inhibiting the resultant nitration of F-actin and inactivation of ERKs.

Mechanotransduction down to individual actin filaments

The actin cytoskeleton plays an essential role in a cell's ability to generate and sense forces, both internally and in interaction with the outside world. The transduction of mechanical cues into biochemical reactions in cells, in particular, is a multi-scale process which requires a variety of approaches to be understood. This review focuses on understanding how mechanical stress applied to an actin filament can affect its assembly dynamics. Today, experiments addressing this issue at the scale of individual actin filaments are emerging and bring novel insight into mechanotransduction. For instance, recent data show that actin filaments can act as mechanosensors, as an applied tension or curvature alters their conformation and their affinity for regulatory proteins. Filaments can also transmit mechanical tension to other proteins, which consequently change the way they interact with the filaments to regulate their assembly. These results provide evidence for mechanotransduction at the scale of individual filaments, showing that forces participate in the regulation of filament assembly and organization. They bring insight into the elementary events coupling mechanics and biochemistry in cells. The experiments presented here are linked to recent technical developments, and certainly announce the advent of more exciting results in the future.

Structure and function of palladin's actin binding domain

Here, we report the NMR structure of the actin-binding domain contained in the cell adhesion protein palladin. Previously, we demonstrated that one of the immunoglobulin domains of palladin (Ig3) is both necessary and sufficient for direct filamentous actin binding in vitro. In this study, we identify two basic patches on opposite faces of Ig3 that are critical for actin binding and cross-linking. Sedimentation equilibrium assays indicate that the Ig3 domain of palladin does not self-associate. These combined data are consistent with an actin cross-linking mechanism that involves concurrent attachment of two actin filaments by a single palladin molecule by an electrostatic mechanism. Palladin mutations that disrupt actin binding show altered cellular distributions and morphology of actin in cells, revealing a functional requirement for the interaction between palladin and actin in vivo.

Interference targeting the G-actin binding domain of N-WASP inhibits the invasion and metastasis of colon cancer cells

AIM: To examine the effect of intervention targeting the G-actin binding domain of N-WASP on the invasion and metastasis of colon cancer cells.

METHODS: Recombinant expression vector encoding the V motif of N-WASP was designed, constructed and transfected into human colon cancer cells (LoVo) to obtain stably expressed cell strain. Transwell assay was used to examine the change in the invasive ability of LoVo cells. Capture-ELISA assay was used to measure transferrin (Tfn) internalization, a hallmark of cell endocytosis. Furthermore, a mouse model of metastatic colon cancer was generated by injection of LoVo cells into the spleen of mice to observe liver metastasis.

RESULTS: The recombinant plasmid was successfully constructed and transfected into LoVo cells. Cells transfected with the recombinant plasmid showed a low invasive ability when compared with untransfected cells or cells transfected with empty plasmid. The number of recombinant plasmid-transfected cells passing through the Transwell membrane were significantly lower than those of untransfected cells or cells transfected with empty plasmid (42.15 ± 7.35 vs 85.25 ± 6.16 and 84.35 ± 7.99, both P < 0.05). Capture-ELISA assay showed that internalization of transferrin in cells transfected with recombinant plasmid was attenuated dramatically when compared to control cells. Animal experiments showed that the number of mice developing liver metastasis in mice injected with recombinant plasmid-transfected cells was significantly lower than that in mice injected with untransfected or empty plasmid-transfected cells (2/12 vs 10/12 and 10/12).

CONCLUSION: Interference targeting the G-actin binding domain of N-WASP is able to inhibit the invasion and metastasis of colon caner cells.