MICAL2 enhances branched actin network disassembly by oxidizing Arp3B-containing Arp2/3 complexes

The mechanisms regulating the disassembly of branched actin networks formed by the Arp2/3 complex still remain to be fully elucidated. In addition, the impact of Arp3 isoforms on the properties of Arp2/3 are also unexplored. We now demonstrate that Arp3 and Arp3B isocomplexes promote actin assembly equally efficiently but generate branched actin networks with different disassembly rates. Arp3B dissociates significantly faster than Arp3 from the network, and its depletion increases actin stability. This difference is due to the oxidation of Arp3B, but not Arp3, by the methionine monooxygenase MICAL2, which is recruited to the actin network by coronin 1C. Substitution of Arp3B Met293 by threonine, the corresponding residue in Arp3, increases actin network stability. Conversely, replacing Arp3 Thr293 with glutamine to mimic Met oxidation promotes disassembly. The ability of MICAL2 to enhance network disassembly also depends on cortactin. Our observations demonstrate that coronin 1C, cortactin, and MICAL2 act together to promote disassembly of branched actin networks by oxidizing Arp3B-containing Arp2/3 complexes.

Fluorochloridone perturbs blood-testis barrier/Sertoli cell barrier function through Arp3-mediated F-actin disruption

There are reports of fluorochloridone (FLC)-induced male reproductive toxicity, but the underlying toxicological mechanisms remain unknown. In this study, we looked at how FLC exposure affected the integrity of the blood-testis barrier (BTB) and the Sertoli cell barrier and studied the molecular mechanisms. Male rats received gavage administration of FLC (30 mg/kg/d) for 14 consecutive days with sample collection at the 7th and 14th day; and primary cultured Sertoli cells were treated with 0-10 μM FLC in vitro for 24 h. Our in vivo findings revealed that FLC exposure caused time-dependent testicular injuries, sperm quality decrease as well as adverse changes in BTB integrity, F-actin organization, and expressions of claudin-11 and Arp3. In Sertoli cells isolated from FLC-treated rat testis, Sertoli cell barrier tightness was increased. In Sertoli cells in vitro exposed to FLC, abnormal changes in the barrier permeability were also observed, and the protein expressions of occludin, claudin-11, ZO-1, connexin-43, and Arp3 were significantly decreased in a dose- and time-dependent manner. Furthermore, the FLC-induced adverse changes in Sertoli cell barrier and F-actin were partly alleviated by the induction of Arp3 overexpression. In conclusion, our findings revealed that FLC perturbed BTB/Sertoli cell barrier function through Arp3-mediated F-actin disorganization.

The role of Arabidopsis Actin-Related Protein 3 in amyloplast sedimentation and polar auxin transport in root gravitropism

Gravitropism is vital for shaping directional plant growth in response to the forces of gravity. Signals perceived in the gravity-sensing cells can be converted into biochemical signals and transmitted. Sedimentation of amyloplasts in the columella cells triggers asymmetric auxin redistribution in root tips, leading to downward root growth. The actin cytoskeleton is thought to play an important role in root gravitropism, although the molecular mechanism has not been resolved. DISTORTED1 (DIS1) encodes the ARP3 subunit of the Arabidopsis Actin-Related Protein 2/3 (ARP2/3) complex, and the ARP3/DIS1 mutant dis1-1 showed delayed root curvature after gravity stimulation. Microrheological analysis revealed that the high apparent viscosity within dis1-1 central columella cells is closely associated with abnormal movement trajectories of amyloplasts. Analysis using a sensitive auxin input reporter DII-VENUS showed that asymmetric auxin redistribution was reduced in the root tips of dis1-1, and the actin-disrupting drug Latrunculin B increased the asymmetric auxin redistribution. An uptake assay using the membrane-selective dye FM4-64 indicated that endocytosis was decelerated in dis1-1 root epidermal cells. Treatment and wash-out with Brefeldin A, which inhibits protein transport from the endoplasmic reticulum to the Golgi apparatus, showed that cycling of the auxin-transporter PIN-FORMED (PIN) proteins to the plasma membrane was also suppressed in dis1-1 roots. The results reveal that ARP3/DIS1 acts in root gravitropism by affecting amyloplast sedimentation and PIN-mediated polar auxin transport through regulation of PIN protein trafficking.

ADF and Cofilin1 Control Actin Stress Fibers, Nuclear Integrity, and Cell Survival

Genetic co-depletion of the actin-severing proteins ADF and CFL1 triggers catastrophic loss of adult homeostasis in multiple tissues. There is impaired cell-cell adhesion in skin keratinocytes with dysregulation of E-cadherin, hyperproliferation of differentiated cells, and ultimately apoptosis. Mechanistically, the primary consequence of depleting both ADF and CFL1 is uncontrolled accumulation of contractile actin stress fibers associated with enlarged focal adhesions at the plasma membrane, as well as reduced rates of membrane protrusions. This generates increased intracellular acto-myosin tension that promotes nuclear deformation and physical disruption of the nuclear lamina via the LINC complex that normally connects regulated actin filaments to the nuclear envelope. We therefore describe a pathway involving the actin-severing proteins ADF and CFL1 in regulating the dynamic turnover of contractile actin stress fibers, and this is vital to prevent the nucleus from being damaged by actin contractility, in turn preserving cell survival and tissue homeostasis.

ARP2/3-Mediated Actin Nucleation Associated With Symbiosome Membrane Is Essential for the Development of Symbiosomes in Infected Cells of Medicago truncatula Root Nodules

The nitrogen-fixing rhizobia in the symbiotic infected cells of root nodules are kept in membrane compartments derived from the host cell plasma membrane, forming what are known as symbiosomes. These are maintained as individual units, with mature symbiosomes having a specific radial position in the host cell cytoplasm. The mechanisms that adapt the host cell architecture to accommodate intracellular bacteria are not clear. The intracellular organization of any cell depends heavily on the actin cytoskeleton. Dynamic rearrangement of the actin cytoskeleton is crucial for cytoplasm organization and intracellular trafficking of vesicles and organelles. A key component of the actin cytoskeleton rearrangement is the ARP2/3 complex, which nucleates new actin filaments and forms branched actin networks. To clarify the role of the ARP2/3 complex in the development of infected cells and symbiosomes, we analyzed the pattern of actin microfilaments and the functional role of ARP3 in Medicago truncatula root nodules. In infected cells, ARP3 protein and actin were spatially associated with maturing symbiosomes. Partial ARP3 silencing causes defects in symbiosome development; in particular, ARP3 silencing disrupts the final differentiation steps in functional maturation into nitrogen-fixing units.

The actin-related Protein2/3 complex regulates mitochondrial-associated calcium signaling during salt stress in Arabidopsis

Microfilament and Ca(2+) dynamics play important roles in stress signaling in plants. Through genetic screening of Arabidopsis thaliana mutants that are defective in stress-induced increases in cytosolic Ca(2+) ([Ca(2+)]cyt), we identified Actin-Related Protein2 (Arp2) as a regulator of [Ca(2+)]cyt in response to salt stress. Plants lacking Arp2 or other proteins in the Arp2/3 complex exhibited enhanced salt-induced increases in [Ca(2+)]cyt, decreased mitochondria movement, and hypersensitivity to salt. In addition, mitochondria aggregated, the mitochondrial permeability transition pore opened, and mitochondrial membrane potential Ψm was impaired in the arp2 mutant, and these changes were associated with salt-induced cell death. When opening of the enhanced mitochondrial permeability transition pore was blocked or increases in [Ca(2+)]cyt were prevented, the salt-sensitive phenotype of the arp2 mutant was partially rescued. These results indicate that the Arp2/3 complex regulates mitochondrial-dependent Ca(2+) signaling in response to salt stress.

Identification of Shigella flexneri IcsA residues affecting interaction with N-WASP, and evidence for IcsA-IcsA co-operative interaction

The Shigella flexneri IcsA (VirG) protein is a polarly distributed outer membrane protein that is a fundamental virulence factor which interacts with neural Wiskott-Aldrich syndrome protein (N-WASP). The activated N-WASP then activates the Arp2/3 complex which initiates de novo actin nucleation and polymerisation to form F-actin comet tails and allows bacterial cell-to-cell spreading. In a previous study, IcsA was found to have three N-WASP interacting regions (IRs): IR I (aa 185-312), IR II (aa 330-382) and IR III (aa 508-730). The aim of this study was to more clearly define N-WASP interacting regions II and III by site-directed mutagenesis of specific amino acids. Mutant IcsA proteins were expressed in both smooth lipopolysaccharide (S-LPS) and rough LPS (R-LPS) S. flexneri strains and characterised for IcsA production level, N-WASP recruitment and F-actin comet tail formation. We have successfully identified new amino acids involved in N-WASP recruitment within different N-WASP interacting regions, and report for the first time using co-expression of mutant IcsA proteins, that N-WASP activation involves interactions with different regions on different IcsA molecules as shown by Arp3 recruitment. In addition, our findings suggest that autochaperone (AC) mutant protein production was not rescued by another AC region provided in trans, differing to that reported for two other autotransporters, PrtS and BrkA autotransporters.

The reorganization of actin filaments is required for vacuolar fusion of guard cells during stomatal opening in Arabidopsis

The reorganization of actin filaments (AFs) and vacuoles in guard cells is involved in the regulation of stomatal movement. However, it remains unclear whether there is any interaction between the reorganization of AFs and vacuolar changes during stomatal movement. Here, we report the relationship between the reorganization of AFs and vacuolar fusion revealed in pharmacological experiments, and characterizing stomatal opening in actin-related protein 2 (arp2) and arp3 mutants. Our results show that cytochalasin-D-induced depolymerization or phalloidin-induced stabilization of AFs leads to an increase in small unfused vacuoles during stomatal opening in wild-type (WT) Arabidopsis plants. Light-induced stomatal opening is retarded and vacuolar fusion in guard cells is impaired in the mutants, in which the reorganization and the dynamic parameters of AFs are aberrant compared with those of the WT. In WT, AFs tightly surround the small separated vacuoles, forming a ring that encircles the boundary membranes of vacuoles partly fused during stomatal opening. In contrast, in the mutants, most AFs and actin patches accumulate abnormally around the nuclei of the guard cells, which probably further impair vacuolar fusion and retard stomatal opening. Our results suggest that the reorganization of AFs regulates vacuolar fusion in guard cells during stomatal opening.

The ARP2/3 complex mediates guard cell actin reorganization and stomatal movement in Arabidopsis

Guard cell actin reorganization has been observed in stomatal responses to a wide array of stimuli. However, how the guard cell signaling machinery regulates actin dynamics is poorly understood. Here, we report the identification of an allele of the Arabidopsis thaliana ACTIN-RELATED PROTEIN C2/DISTORTED TRICHOMES2 (ARPC2) locus (encoding the ARPC2 subunit of the ARP2/3 complex) designated high sugar response3 (hsr3). The hsr3 mutant showed increased transpirational water loss that was mainly due to a lesion in stomatal regulation. Stomatal bioassay analyses revealed that guard cell sensitivity to external stimuli, such as abscisic acid (ABA), CaCl(2), and light/dark transition, was reduced or abolished in hsr3. Analysis of a nonallelic mutant of the ARP2/3 complex suggested no pleiotropic effect of ARPC2 beyond its function in the complex in regard to stomatal regulation. When treated with ABA, guard cell actin filaments underwent fast disruption in wild-type plants, whereas those in hsr3 remained largely bundled. The ABA insensitivity phenotype of hsr3 was rescued by cytochalasin D treatment, suggesting that the aberrant stomatal response was a consequence of bundled actin filaments. Our work indicates that regulation of actin reassembly through ARP2/3 complex activity is crucial for stomatal regulation.

The p40/ARPC1 subunit of Arp2/3 complex performs multiple essential roles in WASp-regulated actin nucleation

The Arp2/3 complex is a conserved seven-subunit actin-nucleating machine activated by WASp (Wiskott Aldrich syndrome protein). Despite its central importance in a broad range of cellular processes, many critical aspects of the mechanism of the Arp2/3 complex have yet to be resolved. In particular, some of the individual subunits in the complex have not been assigned clear functional roles, including p40/ARPC1. Here, we dissected the structure and function of Saccharomyces cerevisiae p40/ARPC1, which is encoded by the essential ARC40 gene, by analyzing 39 integrated alleles that target its conserved surfaces. We identified three distinct sites on p40/ARPC1 required for function in vivo: one site contacts p19/ARPC4, one contacts p15/ARPC5, and one site resides in an extended structural "arm" of p40/ARPC1. Using a novel strategy, we purified the corresponding lethal mutant Arp2/3 complexes from yeast and compared their actin nucleation activities. Lethal mutations at the contact with p19/ARPC4 specifically impaired WASp-induced nucleation. In contrast, lethal mutations at the contact with p15/ARPC5 led to unregulated ("leaky") nucleation in the absence of WASp. Lethal mutations in the extended arm drastically reduced nucleation, and the same mutations disrupted the ability of the purified p40/ARPC1 arm domain to bind the VCA domain of WASp. Together, these data indicate that p40/ARPC1 performs at least three distinct, essential functions in regulating Arp2/3 complex-mediated actin assembly: 1) suppression of spontaneous nucleation by the Arp2/3 complex, which requires proper contacts with p15/ARPC5; 2) propagation of WASp activation signals via contacts with p19/ARPC2; and 3) direct facilitation of actin nucleation through interactions of the extended arm with the VCA domain of WASp.

Spatial and temporal relationships between actin-filament nucleation, capping, and disassembly

BACKGROUND

The leading actin network in motile cells is composed of two compartments, the lamellipod and the lamellum. Construction of the lamellipod requires a set of conserved proteins that form a biochemical cycle. The timing of this cycle and the roles of its components in determining actin network architecture in vivo, however, are not well understood.

RESULTS

We performed fluorescent speckle microscopy on spreading Drosophila S2 cells by using labeled derivatives of actin, the Arp2/3 complex, capping protein, and tropomyosin. We find that capping protein and the Arp2/3 complex both incorporate at the cell edge but that capping protein dissociates after covering less than half the width of the lamellipod, whereas the Arp2/3 complex dissociates after crossing two thirds of the lamellipod. The lamellipodial actin network itself persists long after the loss of the Arp2/3 complex. Depletion of capping protein by RNAi results in the displacement of the Arp2/3 complex and disappearance of the lamellipod. In contrast, depletion of cofilin, slingshot, twinfilin, and tropomyosin, all factors that control the stability of actin filaments, dramatically expanded the lamellipod at the expense of the lamellum.

CONCLUSIONS

The Arp2/3 complex is incorporated into the lamellipodial network at the cell edge but debranches well before the lamellipodial network itself is disassembled. Capping protein is required for the formation of a lamellipodial network but dissociates from the network precisely when filament disassembly is first detected. Cofilin, twinfilin, and tropomyosin appear to play no role in lamellipodial network assembly but function to limit its size.

Correlation between liver metastasis of the colocalization of actin-related protein 2 and 3 complex and WAVE2 in colorectal carcinoma

Directed movement of normal cells occurs when actin-related protein 2 and 3 complex (Arp2/3 complex) triggers the actin polymerization that forms lamellipodia immediately after binding to WAVE2. In order to determine whether the same mechanism correlates with liver metastasis from colorectal cancer, paired mirror sections of 154 cancer specimens (29 cases with liver metastasis and 125 cases without liver metastasis in which T factor, gender, primary tumor site, and age at operation were matched) were examined immunohistochemically for the localization of Arp2 and WAVE2. Expression of both Arp2 and WAVE2 was detected in the same cancer cells in 55 (35.7%) of the 154 cases, but not detected in the normal colonic epithelial cells. Univariate analysis showed that the colocalization was significantly predictive of liver metastasis (risk ratio [RR] 8.760. Likewise, histological grade (RR 2.46), lymphatic invasion (RR 9.95), and tumor budding (RR 4.00) were significant predictors. Among these, colocalization and lymphatic invasion were shown to be independent risk factors by multivariate analysis. Another 59 colorectal specimens were examined for mRNA expression of Arp2 by real time polymerase chain reaction. High mRNA levels of Arp2, that in situ hybridization revealed to be expressed by the cancer cells, were significantly associated with liver metastasis. However, its effect was absorbed by the influence of risk of the colocalization that is closely related to high expression of Arp2. These results indicate that the colocalization of Arp2 and WAVE2 is an independent risk factor for liver metastasis of colorectal carcinoma.

The actin multigene family of Paramecium tetraurelia

BACKGROUND

A Paramecium tetraurelia pilot genome project, the subsequent sequencing of a Megabase chromosome as well as the Paramecium genome project aimed at gaining insight into the genome of Paramecium. These cells display a most elaborate membrane trafficking system, with distinct, predictable pathways in which actin could participate. Previously we had localized actin in Paramecium; however, none of the efforts so far could proof the occurrence of actin in the cleavage furrow of a dividing cell, despite the fact that actin is unequivocally involved in cell division. This gave a first hint that Paramecium may possess actin isoforms with unusual characteristics. The genome project gave us the chance to search the whole Paramecium genome, and, thus, to identify and characterize probably all actin isoforms in Paramecium.

RESULTS

The ciliated protozoan, P. tetraurelia, contains an actin multigene family with at least 30 members encoding actin, actin-related and actin-like proteins. They group into twelve subfamilies; a large subfamily with 10 genes, seven pairs and one trio with > 82% amino acid identity, as well as three single genes. The different subfamilies are very distinct from each other. In comparison to actins in other organisms, P. tetraurelia actins are highly divergent, with identities topping 80% and falling to 30%. We analyzed their structure on nucleotide level regarding the number and position of introns. On amino acid level, we scanned the sequences for the presence of actin consensus regions, for amino acids of the intermonomer interface in filaments, for residues contributing to ATP binding, and for known binding sites for myosin and actin-specific drugs. Several of those characteristics are lacking in several subfamilies. The divergence of P. tetraurelia actins and actin-related proteins between different P. tetraurelia subfamilies as well as with sequences of other organisms is well represented in a phylogenetic tree, where P. tetraurelia sequences only partially cluster.

CONCLUSION

Analysis of different features on nucleotide and amino acid level revealed striking differences in isoforms of actin and actin-related proteins in P. tetraurelia, both within the organism and in comparison to other organisms. This diversification suggests unprecedented specification in localization and function within a unicellular eukaryote.

Arp2/3 ATP hydrolysis-catalysed branch dissociation is critical for endocytic force generation

The Arp2/3 complex, which is crucial for actin-based motility, nucleates actin filaments and organizes them into y-branched networks. The Arp2 subunit has been shown to hydrolyse ATP, but the functional importance of Arp2/3 ATP hydrolysis is not known. Here, we analysed an Arp2 mutant in Saccharomyces cerevisiae that is defective in ATP hydrolysis. Arp2 ATP hydrolysis and Arp2/3-dependent actin nucleation occur almost simultaneously. However, ATP hydrolysis is not required for nucleation. In addition, Arp2 ATP hydrolysis is not required for the release of a WASP-like activator from y-branches. ATP hydrolysis by Arp2, and possibly Arp3, is essential for efficient y-branch dissociation in vitro. In living cells, both Arp2 and Arp3 ATP-hydrolysis mutants exhibit defects in endocytic internalization and actin-network disassembly. Our results suggest a critical feature of dendritic nucleation in which debranching and subsequent actin-filament remodelling and/or depolymerization are important for endocytic vesicle morphogenesis.

Strain-dependent regulation of neurotransmission and actin-remodelling proteins in the mouse hippocampus

Individual mouse strains differ significantly in terms of behaviour, cognitive function and long-term potentiation. Hippocampal gene expression profiling of eight different mouse strains points towards strain-specific regulation of genes involved in neuronal information storage. Protein expression with regard to strain- dependent expression of structures related to neuronal information storage has not been investigated yet. Herein, a proteomic approach based on two-dimensional gel electrophoresis coupled with mass spectrometry (MALDI-TOF/TOF) has been chosen to address this question by determining strain-dependent expression of proteins involved in neurotransmission and activity-induced actin remodelling in hippocampal tissue of five mouse strains. Of 31 spots representing 16 different gene products analysed and quantified, N-ethylmaleimide-sensitive fusion protein, N-ethylmaleimide-sensitive factor attachment protein-alpha, actin-like protein 3, profilin and cofilin were expressed in a strain-dependent manner. By treating protein expression as a phenotype, we have shown significant genetic variation in brain protein expression. Further experiments in this direction may provide an indication of the genetic elements that contribute to the phenotypic differences between the selected strains through the expressional level of the translated protein. In view of this, we propose that proteomic analysis enabling to concomitantly survey the expression of a large number of proteins could serve as a valuable tool for genetic and physiological studies of central nervous system function.

DISTORTED3/SCAR2 is a putative arabidopsis WAVE complex subunit that activates the Arp2/3 complex and is required for epidermal morphogenesis

In a plant cell, a subset of actin filaments function as a scaffold that positions the endomembrane system and acts as a substrate on which organelle motility occurs. Other actin filament arrays appear to be more dynamic and reorganize in response to growth signals and external cues. The distorted group of trichome morphology mutants provides powerful genetic tools to study the control of actin filament nucleation in the context of morphogenesis. In this article, we report that DISTORTED3 (DIS3) encodes a plant-specific SCAR/WAVE homolog. Null alleles of DIS3, like those of other Arabidopsis thaliana WAVE and Actin-Related Protein (ARP) 2/3 subunit genes, cause trichome distortion, defects in cell-cell adhesion, and reduced hypocotyl growth in etiolated seedlings. DIS3 efficiently activates the actin filament nucleation and branching activity of vertebrate Arp2/3 and functions within a WAVE-ARP2/3 pathway in vivo. DIS3 may assemble into a WAVE complex via a physical interaction with a highly diverged Arabidopsis Abi-1-like bridging protein. These results demonstrate the utility of the Arabidopsis trichome system to understand how the WAVE and ARP2/3 complexes translate signaling inputs into a coordinated morphogenetic response.