Actin monomer binding proteins

Parallel import mechanisms ensure the robust nuclear localization of actin in Drosophila

Actin, as an ancient and fundamental protein, participates in various cytoplasmic as well as nuclear functions in eukaryotic cells. Based on its manifold tasks in the nucleus, it is a reasonable assumption that the nuclear presence of actin is essential for the cell, and consequently, its nuclear localization is ensured by a robust system. However, today only a single nuclear import and a single nuclear export pathway is known which maintain the dynamic balance between cytoplasmic and nuclear actin pools. In our work, we tested the robustness of the nuclear import of actin, and investigated whether the perturbations of nuclear localization affect the viability of the whole organism. For this aim, we generated a genetic system in Drosophila, in which we rescued the lethal phenotype of the null mutation of the Actin5C gene with transgenes that express different derivatives of actin, including a Nuclear Export Signal (NES)-tagged isoform which ensures forced nuclear export of the protein. We also disrupted the SUMOylation site of actin, suggested earlier to be responsible for nuclear retention, and eliminated the activity of the single nuclear import factor dedicated to actin. We found that, individually, none of the above mentioned manipulations led to a notable reduction in nuclear actin levels and thus, fully rescued lethality. However, the NES tagging of actin, together with the knock out of its importin, significantly reduced the amount of nuclear actin and induced lethality, confirming that the presence of actin in the nucleus is essential, and thereby, over-secured. Supporting this, we identified novel nuclear importins specific to actin, which sheds light on the mechanism behind the robustness of nuclear localization of actin, and supports the idea of essentiality of its nuclear functions.

Actin-Associated Proteins and Small Molecules Targeting the Actin Cytoskeleton

Actin-associated proteins (AAPs) act on monomeric globular actin (G-actin) and polymerized filamentous actin (F-actin) to regulate their dynamics and architectures which ultimately control cell movement, shape change, division; organelle localization and trafficking. Actin-binding proteins (ABPs) are a subset of AAPs. Since actin was discovered as a myosin-activating protein (hence named actin) in 1942, the protein has also been found to be expressed in non-muscle cells, and numerous AAPs continue to be discovered. This review article lists all of the AAPs discovered so far while also allowing readers to sort the list based on the names, sizes, functions, related human diseases, and the dates of discovery. The list also contains links to the UniProt and Protein Atlas databases for accessing further, related details such as protein structures, associated proteins, subcellular localization, the expression levels in cells and tissues, mutations, and pathology. Because the actin cytoskeleton is involved in many pathological processes such as tumorigenesis, invasion, and developmental diseases, small molecules that target actin and AAPs which hold potential to treat these diseases are also listed.

AimB Is a Small Protein Regulator of Cell Size and MreB Assembly

The MreB actin-like cytoskeleton assembles into dynamic polymers that coordinate cell shape in many bacteria. In contrast to most other cytoskeleton systems, few MreB-interacting proteins have been well characterized. Here, we identify a small protein from Caulobacter crescentus, an assembly inhibitor of MreB (AimB). AimB overexpression mimics inhibition of MreB polymerization, leading to increased cell width and MreB delocalization. Furthermore, aimB appears to be essential, and its depletion results in decreased cell width and increased resistance to A22, a small-molecule inhibitor of MreB assembly. Molecular dynamics simulations suggest that AimB binds MreB at its monomer-monomer protofilament interaction cleft and that this interaction is favored for C. crescentus MreB over Escherichia coli MreB because of a closer match in the degree of opening with AimB size, suggesting coevolution of AimB with MreB conformational dynamics in C. crescentus. We support this model through functional analysis of point mutants in both AimB and MreB, photo-cross-linking studies with site-specific unnatural amino acids, and species-specific activity of AimB. Together, our findings are consistent with AimB promoting MreB dynamics by inhibiting monomer-monomer assembly interactions, representing a new mechanism for regulating actin-like polymers and the first identification of a non-toxin MreB assembly inhibitor. Because AimB has only 104 amino acids and small proteins are often poorly characterized, our work suggests the possibility of more bacterial cytoskeletal regulators to be found in this class. Thus, like FtsZ and eukaryotic actin, MreB may have a rich repertoire of regulators to tune its precise assembly and dynamics.

Single-cell proteomics reveals changes in expression during hair-cell development

Hearing and balance rely on small sensory hair cells that reside in the inner ear. To explore dynamic changes in the abundant proteins present in differentiating hair cells, we used nanoliter-scale shotgun mass spectrometry of single cells, each ~1 picoliter, from utricles of embryonic day 15 chickens. We identified unique constellations of proteins or protein groups from presumptive hair cells and from progenitor cells. The single-cell proteomes enabled the de novo reconstruction of a developmental trajectory using protein expression levels, revealing proteins that greatly increased in expression during differentiation of hair cells (e.g., OCM, CRABP1, GPX2, AK1, GSTO1) and those that decreased during differentiation (e.g., TMSB4X, AGR3). Complementary single-cell transcriptome profiling showed corresponding changes in mRNA during maturation of hair cells. Single-cell proteomics data thus can be mined to reveal features of cellular development that may be missed with transcriptomics.

Stochastic models of polymerization-based axonal actin transport

Recent advances in live cell imaging of F-actin structures, combined with pulse-chase imaging and computational modeling have suggested that actin is transported along the axon via biased polymerization of metastable actin fibers (actin trails). This mechanism is distinct from motor driven polymer transport, such as for neurofilaments and can be best described as molecular hitchhiking, where G-actin molecules are intermittently incorporated into actin fibers which grow preferentially in the anterograde direction. In this paper, we discuss how various axonal and actin trail parameters like axon diameter, trail nucleation rates, basal G-actin concentration, and trail length influence the transport rate. These predictions can help guide future experiments to verify this novel protein transport mechanism. We introduce a simplified, analytically solvable model of actin transport which relates these parameters to experimentally measurable quantities. We also discuss why a simple diffusion-based transport mechanism cannot explain bulk actin transport in the axon.

Processive flow by biased polymerization mediates the slow axonal transport of actin

Chakrabarty et al. propose a model in which slow axonal transport of actin can occur by a biased polymerization of actin filaments along the axon shaft without the involvement of microtubules (MTs) or MT-based motors. These dynamics are distinct from polymer sliding—the canonical mechanism thought to convey cytoskeletal cargoes in slow transport.

Classic pulse-chase studies have shown that actin is conveyed in slow axonal transport, but the mechanistic basis for this movement is unknown. Recently, we reported that axonal actin was surprisingly dynamic, with focal assembly/disassembly events (“actin hotspots”) and elongating polymers along the axon shaft (“actin trails”). Using a combination of live imaging, superresolution microscopy, and modeling, in this study, we explore how these dynamic structures can lead to processive transport of actin. We found relatively more actin trails elongated anterogradely as well as an overall slow, anterogradely biased flow of actin in axon shafts. Starting with first principles of monomer/filament assembly and incorporating imaging data, we generated a quantitative model simulating axonal hotspots and trails. Our simulations predict that the axonal actin dynamics indeed lead to a slow anterogradely biased flow of the population. Collectively, the data point to a surprising scenario where local assembly and biased polymerization generate the slow axonal transport of actin without involvement of microtubules (MTs) or MT-based motors. Mechanistically distinct from polymer sliding, this might be a general strategy to convey highly dynamic cytoskeletal cargoes.

Solution structure and dynamics of glia maturation factor from Caenorhabditis elegans

BACKGROUND

The GMF class of the ADF-H domain family proteins regulate actin dynamics by binding to the Arp2/3 complex and F-actin through their Site-1 and Site-2, respectively. CeGMF of C. elegans is analogous to GMFγ of human and mouse and is 138 amino acids in length.

METHODS

We have characterized the solution structure and dynamics of CeGMF by solution NMR spectroscopy and its thermal stability by DSC.

RESULTS

The solution structure of CeGMF shows canonical ADF-H fold with two additional β-strands in the β4-β5 loop region. The Site-1 of CeGMF is well formed and residues of all three regions of Site-1 show dynamic flexibility. However, the β4-β5 loop of Site-2 is less inclined towards the C-terminal, as the latter is truncated by four residues in comparison to GMF isoforms of human and mouse. Regions of Site-2 show motions on ns-ps timescale, but dynamic flexibility of β4-β5 loop is low in comparison to corresponding F-loop region of ADF/cofilin UNC-60B. A general difference in packing of α3 and α1 between GMF and ADF/cofilins was noticed. Additionally, thermal stability of CeGMF was significantly higher than its ADF/cofilin homologs.

CONCLUSION

We have presented the first solution structure of GMF from C. elegans, which highlights the structural differences between the Site-2 of CeGMF and mammalian GMF isoforms. Further, we have seen the differences in structure, dynamics, and thermal stability of GMF and ADF/cofilin.

GENERAL SIGNIFICANCE

This study provides a useful insight to structural and dynamics factors that define the specificity of GMF towards Arp2/3 complex.

Transgelin-2 in immunity: Its implication in cell therapy

Transgelin-2 is a small 22-kDa actin-binding protein implicated in actin dynamics, which stabilizes actin structures and participates in actin-associated signaling pathways. Much curiosity regarding transgelin-2 has centered around its dysregulation in tumor development and associated diseases. However, recent studies have shed new light on the functions of transgelin-2, the only transgelin family member present in leukocytes, in the context of various immune responses. In this review, we outlined the biochemical properties of transgelin-2 and its physiological functions in T cells, B cells, and macrophages. Transgelin-2 regulates T cell activation by stabilizing the actin cytoskeleton at the immunological synapse. Transgelin-2 in B cells also participates in the stabilization of T cell-B cell conjugates. While transgelin-2 is expressed at trace levels in macrophages, its expression is highly upregulated upon lipopolysaccharide stimulation and plays an essential role in macrophage phagocytosis. Since transgelin-2 increases T cell adhesion to target cells via boosting the "inside-out" costimulatory activation of leukocyte function-associated antigen 1, transgelin-2 could be a suitable candidate to potentiate the antitumor response of cytotoxic T cells by compensating for the lack of costimulation in tumor microenvironment. We discussed the feasibility of using native or engineered transgelin-2 as a synergistic molecule in cell-based immunotherapies, without inducing off-target disturbance in actin dynamics in other cells.

VEGF Triggers the Activation of Cofilin and the Arp2/3 Complex within the Growth Cone

A crucial neuronal structure for the development and regeneration of neuronal networks is the axonal growth cone. Affected by different guidance cues, it grows in a predetermined direction to reach its final destination. One of those cues is the vascular endothelial growth factor (VEGF), which was identified as a positive effector for growth cone movement. These positive effects are mainly mediated by a reorganization of the actin network. This study shows that VEGF triggers a tight colocalization of cofilin and the Arp2/3 complex to the actin cytoskeleton within chicken dorsal root ganglia (DRG). Live cell imaging after microinjection of GFP (green fluorescent protein)-cofilin and RFP (red fluorescent protein)-LifeAct revealed that both labeled proteins rapidly redistributed within growth cones, and showed a congruent distribution pattern after VEGF supplementation. Disruption of signaling upstream of cofilin via blocking LIM-kinase (LIMK) activity resulted in growth cones displaying regressive growth behavior. Microinjection of GFP-p16b (a subunit of the Arp2/3 complex) and RFP-LifeAct revealed that both proteins redistributed into lamellipodia of the growth cone within minutes after VEGF stimulation. Disruption of the signaling to the Arp2/3 complex in the presence of VEGF by inhibition of N-WASP (neuronal Wiskott–Aldrich–Scott protein) caused retraction of growth cones. Hence, cofilin and the Arp2/3 complex appear to be downstream effector proteins of VEGF signaling to the actin cytoskeleton of DRG growth cones. Our data suggest that VEGF simultaneously affects different pathways for signaling to the actin cytoskeleton, since activation of cofilin occurs via inhibition of LIMK, whereas activation of Arp2/3 is achieved by stimulation of N-WASP.

Across intra-mammalian stages of the liver f luke Fasciola hepatica: a proteomic study

Fasciola hepatica is the agent of fasciolosis, a foodborne zoonosis that affects livestock production and human health. Although flukicidal drugs are available, re-infection and expanding resistance to triclabendazole demand new control strategies. Understanding the molecular mechanisms underlying the complex interaction with the mammalian host could provide relevant clues, aiding the search for novel targets in diagnosis and control of fasciolosis. Parasite survival in the mammalian host is mediated by parasite compounds released during infection, known as excretory/secretory (E/S) products. E/S products are thought to protect parasites from host responses, allowing them to survive for a long period in the vertebrate host. This work provides in-depth proteomic analysis of F. hepatica intra-mammalian stages, and represents the largest number of proteins identified to date for this species. Functional classification revealed the presence of proteins involved in different biological processes, many of which represent original findings for this organism and are important for parasite survival within the host. These results could lead to a better comprehension of host-parasite relationships, and contribute to the development of drugs or vaccines against this parasite.

A differential screen for putative targets of the bHLH transcription factor Hand1 in cardiac morphogenesis

The bHLH transcription factor, Hand1 has been implicated in cardiac looping in the mouse, however its function in the developing heart remains unknown. To investigate the mechanism(s) through which Hand1 might function, we screened for potential downstream target genes using representational difference analysis. Thymosin beta4 was found to be down-regulated whereas cystatin C and alphaCA were up-regulated in Hand1-null embryoid bodies. Whole-mount in situ hybridisation on wild type embryos (E8.0-E10.5) and Hand1 homozygous-mutant embryos (E8.0) confirmed co-expression of the putative targets with Hand1 in the heart and their aberrant expression in a Hand1-null background.

Hypoxia/reoxygenation-experienced cancer cell migration and metastasis are regulated by Rap1- and Rac1-GTPase activation via the expression of thymosin beta-4

Signaling by small guanosine triphosphatases (GTPase), Rap1/Rac1, is one of the major pathways controlling cancer cell migration and tumor metastasis. Thymosin beta-4 (Tβ4), an actin-sequestering protein, has been shown to increase migration of cancer cells. Episodes of hypoxia and re-oxygenation (H/R) are an important phenomenon in tumor microenvironment (TME). We investigated whether Tβ4 could play as an intermediary to crosstalk between Rac1- and Rap1- GTPase activation under hypoxia/reoxygenation (H/R) conditions. Inhibition of Tβ4 expression using transcription activator-like effector nucleases (TALEN) significantly decreased lung metastasis of B16F10 cells. Rac1 and Rap1 activity, as well as cancer cell migration, increased following induction of Tβ4 expression in normoxia- or H/R-experienced cells, but were barely detectable in Tβ4-depleted cells. Rap1-regulated Rac1 activity was decreased by a dominant negative Rap1 (Rap1N17), and increased by 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (CPT), a Rap1 activator. In contrast, a Rac1-specific inhibitor, NSC23766, and dominant negative Rac1 (Rac1N17) enhanced Tβ4 expression and aberrant Rap1 activity. While NSC23766 and Rac1N17 incompletely inhibited tumor metastasis in vivo, and H/R-experienced cancer cell migration in vitro, more efficient attenuation of cancer cell migration was accomplished by simultaneous inactivation of Rap1 and Rac1 with Rap1N17 and Rac1N17, respectively. These data suggest that a combination therapy targeting both Rap1 and Rac1 activity may be an effective method of inhibiting tumor metastasis.

Tropomyosin as a Regulator of Actin Dynamics

Tropomyosin is a major regulatory protein of contractile systems and cytoskeleton, an actin-binding protein that positions laterally along actin filaments and modulates actin-myosin interaction. About 40 tropomyosin isoforms have been found in a variety of cytoskeleton systems, not necessarily connected with actin-myosin interaction and contraction. Involvement of specific tropomyosin isoforms in the regulation of key cell processes was shown, and specific features of tropomyosin genes and protein structure have been investigated with molecular biology and genetics approaches. However, the mechanisms underlying the effects of tropomyosin on cytoskeleton dynamics are still unclear. As tropomyosin is primarily an F-actin-binding protein, it is important to understand how it interacts both with actin and actin-binding proteins functioning in muscles and cytoskeleton to regulate actin dynamics. This review focuses on biochemical data on the effects of tropomyosin on actin assembly and dynamics, as well as on the modulation of these effects by actin-binding proteins. The data indicate that tropomyosin can efficiently regulate actin dynamics via allosteric conformational changes within actin filaments.

Cell adhesion proteins: roles in periodontal physiology and discovery by proteomics

Adhesion molecules expressed by periodontal connective tissue cells are involved in cell migration, matrix remodeling and inflammatory responses to infection. Currently, the processes by which the biologic activity of these molecules are appropriately regulated in time and space to preserve tissue homeostasis, and to control inflammatory responses and tissue regeneration, are not defined. As cell adhesions are heterogeneous, dynamic, contain a complex group of interacting molecules and are strongly influenced by the type of substrate to which they adhere, we focus on how cell adhesions in periodontal connective tissues contribute to information generation and processing that regulate periodontal structure and function. We also consider how proteomic methods can be applied to discover novel cell-adhesion proteins that could potentially contribute to the form and function of periodontal tissues.

MICAL-family proteins: Complex regulators of the actin cytoskeleton

SIGNIFICANCE

The molecules interacting with CasL (MICAL) family members participate in a multitude of activities, including axonal growth cone repulsion, membrane trafficking, apoptosis, and bristle development in flies. An interesting feature of MICAL proteins is the presence of an N-terminal flavo-mono-oxygenase domain. This mono-oxygenase domain generates redox potential with which MICALs can either oxidize proteins or produce reactive oxygen species (ROS). Actin is one such protein that is affected by MICAL function, leading to dramatic cytoskeletal rearrangements. This review describes the MICAL-family members, and discusses their mechanisms of actin-binding and regulation of actin cytoskeleton organization.

RECENT ADVANCES

Recent studies show that MICALs directly induce oxidation of actin molecules, leading to actin depolymerization. ROS production by MICALs also causes oxidation of collapsin response mediator protein-2, a microtubule assembly promoter, which subsequently undergoes phosphorylation.

CRITICAL ISSUES

MICAL proteins oxidize proteins through two mechanisms: either directly by oxidizing methionine residues or indirectly via the production of ROS. It remains unclear whether MICAL proteins employ both mechanisms or whether the activity of MICAL-family proteins might vary with different substrates.

FUTURE DIRECTIONS

The identification of additional substrates oxidized by MICAL will shed new light on MICAL protein function. Additional directions include expanding studies toward the MICAL-like homologs that lack flavin adenine dinucleotide domains and oxidation activity.

A genome-wide over-expression screen identifies genes involved in phagocytosis in the human protozoan parasite, Entamoeba histolytica

Functional genomics and forward genetics seek to assign function to all known genes in a genome. Entamoeba histolytica is a protozoan parasite for which forward genetics approaches have not been extensively applied. It is the causative agent of amoebic dysentery and liver abscess, and infection is prevalent in developing countries that cannot prevent its fecal-oral spread. It is responsible for considerable global morbidity and mortality. Given that the E. histolytica genome has been sequenced, it should be possible to apply genomic approaches to discover gene function. We used a genome-wide over-expression screen to uncover genes regulating an important virulence function of E. histolytica, namely phagocytosis. We developed an episomal E. histolytica cDNA over-expression library, transfected the collection of plasmids into trophozoites, and applied a high-throughput screen to identify phagocytosis mutants in the population of over-expressing cells. The screen was based on the phagocytic uptake of human red blood cells loaded with the metabolic toxin, tubercidin. Expression plasmids were isolated from trophozoites that survived exposure to tubercidin-charged erythrocytes (phagocytosis mutants), and the cDNAs were sequenced. We isolated the gene encoding profilin, a well-characterized cytoskeleton-regulating protein with a known role in phagocytosis. This supports the validity of our approach. Furthermore, we assigned a phagocytic role to several genes not previously known to function in this manner. To our knowledge, this is the first genome-wide forward genetics screen to be applied to this pathogen. The study demonstrates the power of forward genetics in revealing genes regulating virulence in E. histolytica. In addition, the study validates an E. histolytica cDNA over-expression library as a valuable tool for functional genomics.

Flightless I is a focal adhesion-associated actin-capping protein that regulates cell migration

The role of adhesion-associated actin-binding proteins in cell migration is not well defined. In mouse fibroblasts we screened for focal adhesion-associated proteins that were isolated with collagen-coated beads and detected by tandem mass spectrometry. We identified flightless I (FliI) as an actin-binding protein in focal adhesion fractions, which was verified by immunoblotting. By confocal microscopy most FliI was distributed throughout the cytosol and in focal adhesions. By sedimentation assays and in vitro binding assays, we found that FliI associates with actin filaments and actin monomers. Assays using purified proteins showed that FliI inhibits actin polymerization and caps but does not sever actin filaments. Cells with FliI knockdown or cells overexpressing FliI migrated more or less rapidly, respectively, than wild-type controls. Compared with controls, cells with FliI knockdown were less adherent than wild-type cells, exhibited reduced numbers of focal adhesions containing activated β1 integrins and vinculin, and exhibited increased incorporation of actin monomers into nascent filaments at focal adhesions. These data indicate that FliI regulates cell migration through its localization to focal adhesions and its ability to cap actin filaments, which collectively affect focal adhesion maturation.

Trichinella spiralis infection induces β-actin co-localized with thymosin β4

Trichinella spiralis (T. spiralis) infection in muscle is characterized by the vascular network for the nurse cell-larva complex. We showed in a previous report that thymosin β4 was up-regulated during nurse cell formation by T. spiralis. As thymosin β4 (Tβ4) is the actin-sequestering protein that regulates actin polymerization, the expression pattern of β-actin during the nurse cell formation was analyzed. The protein level of β-actin in muscle fibers 10 days after infection was significantly increased, and its expression remained high in the nurse cells for six weeks. In order to investigate the functional relationship between Tβ4 and β-actin, localization of two proteins was analyzed. Immunofluorescence showed that Tβ4 and β-actin were co-localized in the T. spiralis-infected nurse cells from 10 days to six weeks. The expression patterns of other actin-binding proteins, including thymosin β10 (Tβ10), subunits of the Arp2/3 complex, subunits of Capping protein, profilin, and cofilin, were also analyzed at the mRNA level. Tβ10 expression was also increased during nurse cell formation. Expressions of the Arp2/3 complex was increased at 21 days after infection and Capping proteins was increased during nurse cell formation but shows different expression patterns, depending on the subunit. Profilin and cofilin were specifically increased in the muscle fibers from 14 days after infection. These data show that Tβ4 and β-actin are over-expressed during nurse cell formation upon T. spiralis infection and may be involved in nurse cell formation along with other actin-binding proteins.

Roles of the actin cytoskeleton and an actin-binding protein in wheat resistance against Puccinia striiformis f. sp. tritici

Elucidating resistance mechanisms of plant cells against pathogens is essential to develop novel strategies of disease control. The actin cytoskeleton was found intimately involved in plant defense. In order to reveal how actin would be involved in the interaction between wheat and the stripe rust Puccinia striiformis f. sp. tritici, prior to fungal inoculation, wheat leaves were treated with cytochalasin A, an inhibitor of actin polymerization. Our results showed reduced incidence of hypersensitive cell death and delayed accumulation of H(2)O(2) in wheat leaves treated with cytochalasin A compared to the control. We also found that the TaPRO profilin gene exhibited significantly different expression levels in host leaves when comparing compatible and incompatible interactions. Real-time PCR analysis revealed that the expression transcript of TaPRO was lower at each time point in incompatible interactions when compared to compatible ones, and the largest difference between the two interactions occurred at 12 h post-inoculation. Both pharmacological and gene expression results collectively support the notion that the compromise of the actin microfilament is linked to the compatible interaction between the stripe rust fungus and the leaves of its wheat host.

EST sequencing and gene expression profiling of defence-related genes from Persea americana infected with Phytophthora cinnamomi

BACKGROUND

Avocado (Persea americana) belongs to the Lauraceae family and is an important commercial fruit crop in over 50 countries. The most serious pathogen affecting avocado production is Phytophthora cinnamomi which causes Phytophthora root rot (PRR). Root pathogens such as P. cinnamomi and their interactions with hosts are poorly understood and despite the importance of both the avocado crop and the effect Phytophthora has on its cultivation, there is a lack of molecular knowledge underpinning our understanding of defence strategies against the pathogen. In order to initiate a better understanding of host-specific defence we have generated EST data using 454 pyrosequencing and profiled nine defence-related genes from Pc-infected avocado roots.

RESULTS

2.0 Mb of data was generated consisting of ~10,000 reads on a single lane of the GS FLX platform. Using the Newbler assembler 371 contigs were assembled, of which 367 are novel for Persea americana. Genes were classified according to Gene Ontology terms. In addition to identifying root-specific ESTs we were also able to identify and quantify the expression of nine defence-related genes that were differentially regulated in response to P. cinnamomi. Genes such as metallothionein, thaumatin and the pathogenesis related PsemI, mlo and profilin were found to be differentially regulated.

CONCLUSIONS

This is the first study in elucidating the avocado root transcriptome as well as identifying defence responses of avocado roots to the root pathogen P. cinnamomi. Our data is currently the only EST data that has been generated for avocado rootstocks, and the ESTs identified in this study have already been useful in identifying defence-related genes as well as providing gene information for other studies looking at processes such as ROS regulation as well as hypoxia in avocado roots. Our EST data will aid in the elucidation of the avocado transcriptome and identification of markers for improved rootstock breeding and screening. The characterization of the avocado transcriptome will furthermore form a basis for functional genomics of basal angiosperms.

Effect of toll-like receptor activation on thymosin beta-4 production by chicken macrophages

Thymosin beta-4 (Tβ4) is an actin-binding intracellular peptide that promotes wound healing, tissue remodeling, and angiogenesis. The mechanism of Tβ4 secretion to the extracellular environment is not understood. The macrophage is a rich source of Tβ4 which also participates in wound healing process. The objective of this study was to find how Tβ4 may be externalized. Using activation of macrophage through their toll-like receptors (TLR), the changes in cellular Tβ4 was studied. A naturally transformed chicken macrophage cell line HTC was treated with different TLR agonists and the cellular Tβ4 changes was determined at 6 and 24 h after stimulations using stable isotope labelling of amino acids in cell culture (SILAC) and mass spectrometry. Real time PCR was used to determine changes in gene expression. The results showed that TLR agonists such as peptidoglycan (PGN) or lipopolysacharide (LPS) caused depletions in cellular Tβ4 peptide along with its detection in the cell culture supernatant at 24 h. These TLR agonists also induced the expression of interleukins-1β, -6, and nitric oxide synthase genes at 6 h but failed to modulate Tβ4 gene at that time point indicating that the Tβ4 externalization was not associated with its production. To find whether Tβ4 externalization was associated with cell death, we measured the lactate dehydrogenase (LDH) activity of the conditioned media as an indicator of cell damage. The results showed that the TLR agonists which induced depletion of intracellular Tβ4 at 24 h also increased the LDH content of the conditioned media, suggesting that the Tβ4 in the extracellular media most likely originated from dying macrophages.

Photorhabdus luminescens toxins ADP-ribosylate actin and RhoA to force actin clustering

The bacterium Photorhabdus luminescens is mutualistically associated with entomopathogenetic nematodes. These nematodes invade insect larvae and release the bacteria from their intestine, which kills the insects through the action of toxin complexes. We elucidated the mode of action of two of these insecticidal toxins from P. luminescens. We identified the biologically active components TccC3 and TccC5 as adenosine diphosphate (ADP)-ribosyltransferases, which modify unusual amino acids. TccC3 ADP-ribosylated threonine-148 of actin, resulting in actin polymerization. TccC5 ADP-ribosylated Rho guanosine triphosphatase proteins at glutamine-61 and glutamine-63, inducing their activation. The concerted action of both toxins inhibited phagocytosis of target insect cells and induced extensive intracellular polymerization and clustering of actin. Several human pathogenic bacteria produce related toxins.

NMR solution structures of actin depolymerizing factor homology domains

Actin is one of the most conserved proteins in nature. Its assembly and disassembly are regulated by many proteins, including the family of actin-depolymerizing factor homology (ADF-H) domains. ADF-H domains can be divided into five classes: ADF/cofilin, glia maturation factor (GMF), coactosin, twinfilin, and Abp1/drebrin. The best-characterized class is ADF/cofilin. The other four classes have drawn much less attention and very few structures have been reported. This study presents the solution NMR structure of the ADF-H domain of human HIP-55-drebrin-like protein, the first published structure of a drebrin-like domain (mammalian), and the first published structure of GMF beta (mouse). We also determined the structures of mouse GMF gamma, the mouse coactosin-like domain and the C-terminal ADF-H domain of mouse twinfilin 1. Although the overall fold of the five domains is similar, some significant differences provide valuable insights into filamentous actin (F-actin) and globular actin (G-actin) binding, including the identification of binding residues on the long central helix. This long helix is stabilized by three or four residues. Notably, the F-actin binding sites of mouse GMF beta and GMF gamma contain two additional beta-strands not seen in other ADF-H structures. The G-actin binding site of the ADF-H domain of human HIP-55-drebrin-like protein is absent and distorted in mouse GMF beta and GMF gamma.

Different thymosin Beta 4 immunoreactivity in foetal and adult gastrointestinal tract

Background

Thymosin beta 4 (Tβ₄) is a member of beta-thymosins, a family of peptides that play essential roles in many cellular functions. A recent study from our group suggested a role for Tβ₄ in the development of human salivary glands. The aim of this study was to analyze the expression of Tβ₄ in the human gut during development, and in the adult.

Methodology/Principal Findings

Immunolocalization of Tβ₄ was studied in autoptic samples of tongue, oesophagus, stomach, ileum, colon, liver and pancreas obtained from two human foetuses and two adults. Tβ₄ appeared unevenly distributed, with marked differences between foetuses and adults. In the stomach, superficial epithelium was positive in foetuses and negative in adults. Ileal enterocytes were strongly positive in the adult and weakly positive in the foetuses. An increase in reactivity for Tβ₄ was observed in superficial colon epithelium of adults as compared with the foetuses. Striking differences were found between foetal and adult liver: the former showed a very low reactivity for Tβ₄ while in the adult we observed a strong reactivity in the vast majority of the hepatocytes. A peculiar pattern was found in the pancreas, with the strongest reactivity observed in foetal and adult islet cells.

Significance

Our data show a strong expression of Tβ₄ in the human gut and in endocrine pancreas during development. The observed differential expression of Tβ₄ suggests specific roles of the peptide in the gut of foetuses and adults. The observed heterogeneity of Tβ₄ expression in the foetal life, ranging from a very rare detection in liver cells up to a diffuse reactivity in endocrine pancreas, should be taken into account when the role of Tβ₄ in the development of human embryo is assessed. Future studies are needed to shed light on the link between Tβ₄ and organogenesis.

Thymosin beta4 is involved in stabilin-2-mediated apoptotic cell engulfment

Stabilin-2 was recently identified as a novel receptor for membrane phosphatidylserine of apoptotic cells. To identify proteins that were candidates for stabilin-2 cytoplasmic domain binding, we screened a human spleen cDNA library using the yeast two-hybrid system. We found that thymosin beta4 interacts with the stabilin-2 cytoplasmic domain and is co-localized with stabilin-2 at the phagocytic cup. Knockdown of thymosin beta4 significantly decreased the phagocytic activity of stabilin-2, whereas overexpression of thymosin beta4 increased this activity. Additionally, amino acids 2504-2514 of stabilin-2 cytoplasmic domain were found to be responsible for the interaction with thymosin beta4. Taken together, these results suggest that thymosin beta4 is a downstream molecule of stabilin-2 that plays a role in stabilin-2-mediated cell corpse clearance.

Abnormal cytoskeletal protein expression in cultured skin fibroblasts from type 1 diabetes mellitus patients with nephropathy: A proteomic approach

Diabetic nephropathy (DN) develops in about 40% of insulin-dependent type 1 diabetes mellitus (T1DM) patients, and is associated not only with diabetes duration and metabolic control, but also with a genetic predisposition. Constitutive alterations of cytoskeletal proteins may play a role in the development of DN. We investigated the expression of these proteins in cultured skin fibroblasts, obtained from long-term T1DM patients with and without DN but comparable metabolic control, and from matched healthy subjects, by means of 2-DE electrophoresis and MS-MALDI analyses. In T1DM with DN, compared to the other two groups, quantitative analyses revealed an altered expression of 17 spots (p<0.05-p<0.01), corresponding to 12 unique proteins. In T1DM with DN, beta-actin and three isoforms of tubulin beta-2 chain, tropomodulin-3, and LASP-1 were decreased, whereas two tubulin beta-4 chain isoforms, one alpha actinin-4 isoform, membrane-organizing extension spike protein (MOESIN), FLJ00279 (corresponding to a fragment of myosin heavy chain, non-muscle type A), vinculin, a tropomyosin isoform, and the macrophage capping protein were increased. A shift in caldesmon isoforms was also detected. These results demonstrate an association between DN and the constitutive expression of cytoskeleton proteins in cultured skin fibroblasts from T1DM with DN, which may retain pathophysiologycal implications.

Laser capture microdissection (LCM) and comparative microarray expression analysis of syncytial cells isolated from incompatible and compatible soybean (Glycine max) roots infected by the soybean cyst nematode (Heterodera glycines)

Syncytial cells in soybean (Glycine max cultivar [cv.] Peking) roots infected by incompatible and compatible populations of soybean cyst nematode (SCN [Heterodera glycines]) were collected using laser capture microdissection (LCM). Gene transcript abundance was assayed using Affymetrix soybean GeneChips, each containing 37,744 probe sets. Our analyses identified differentially expressed genes in syncytial cells that are not differentially expressed in the whole root analyses. Therefore, our results show that the mass of transcriptional activity occurring in the whole root is obscuring identification of transcriptional events occurring within syncytial cells. In syncytial cells from incompatible roots at three dpi, genes encoding lipoxygenase (LOX), heat shock protein (HSP) 70, superoxidase dismutase (SOD) were elevated almost tenfold or more, while genes encoding several transcription factors and DNA binding proteins were also elevated, albeit at lower levels. In syncytial cells formed during the compatible interaction at three dpi, genes encoding prohibitin, the epsilon chain of ATP synthase, allene oxide cyclase and annexin were more abundant. By 8 days, several genes of unknown function and genes encoding a germin-like protein, peroxidase, LOX, GAPDH, 3-deoxy-D-arabino-heptolosonate 7-phosphate synthase, ATP synthase and a thioesterase were abundantly expressed. These observations suggest that gene expression is different in syncytial cells as compared to whole roots infected with nematodes. Our observations also show that gene expression is different between syncytial cells that were isolated from incompatible and compatible roots and that gene expression is changing over the course of syncytial cell development as it matures into a functional feeding site.

Molecular cloning and characterization of a profilin gene BnPFN from Brassica nigra that expressing in a pollen-specific manner

Brassica nigra is a newly found invasive species in Zhejiang Province, China. It distributes alongside the roads, in vegetable fields and on riversides. When it blooms, some natives there will suffer from allergic rhinitis. We designed gene-specific primer pairs according to reported profilin genes and successfully isolated their homolog from flower bud cDNA of B. nigra. The gene, designated BnPFN, was submitted to GenBank under accession number EU004073. BnPFN was 405 bp in length encoding 134 amino acids. Expression analysis of BnPFN gene was carried out by means of RT-PCR. The results showed that BnPFN express only in anthers and pollens, and there was no detection in roots, leaves, stems, sepals, petals and pistils. We suggest that BnPFN is a pollen-specific gene and may be responsible for pollen anaphylactic reactions in those invading areas when B. nigra blooms.

One-trial in vitro conditioning regulates an association between the beta-thymosin repeat protein Csp24 and actin

One-trial conditioning in Hermissenda results in enhanced intrinsic cellular excitability of sensory neurons in the conditioned stimulus pathway, and the phosphorylation of several proteins. Previous results demonstrated that the development of enhanced intrinsic excitability was dependent on the expression of conditioned stimulus pathway phosphoprotein-24 (Csp24), an intracellular protein containing four repeated beta-thymosin homology domains. Consistent with this, antisense oligonucleotide-mediated inhibition of Csp24 expression prevents the reduction in amplitude of the A-type transient K+ current (I(A)) and the depolarized shift in the steady-state activation curve normally produced by one-trial in vitro conditioning of isolated photoreceptors. One-trial conditioning also regulates Csp24 phosphorylation. We now show that purified recombinant Csp24 sequesters G-actin in vitro with an approximate K(d) value of 2.8 microM. We also observed a significant increase in the coprecipitation of actin with Csp24 after one-trial in vitro conditioning using antibodies directed toward either Csp24 or phospho-Csp24. Preincubation with protein kinase C (PKC) selective inhibitors attenuated the increase in Csp24 phosphorylation and coprecipitated actin observed after one-trial conditioning. Our findings indicate that the PKC signaling pathway contributes to the phosphorylation of Csp24 after one-trial conditioning, and that PKC activity modulates an association between Csp24 and actin. These data suggest Csp24 may influence intrinsic excitability by regulating cytoskeletal dynamics.

Thymosin beta4 and angiogenesis: modes of action and therapeutic potential

Here we review the mechanisms by which Thymosin beta4 (Tbeta4) regulates angiogenesis, its role in processes, such as wound healing and tumour progression and we discuss in more detail the role of Tbeta4 in the cardiovascular system and significant recent findings implicating Tbeta4 as a potential therapeutic agent for ischaemic heart disease.

The beta-Thymosin Enigma

Actin dynamics in nonmuscle cells is controlled by the availability of actin nucleating sites and actin monomers. Thymosin beta-4 (Tbeta-4) has been implicated in modulating the availability of actin monomers in a large variety of cells. It together with actin nucleating, severing, and uncapping proteins, harnesses the intrinsic dynamic properties of actin to regulate the actin polymerization response in cells. Overexpression or addition of exogenous Tbeta-4 or its homolog, Tbeta-10, alters the actin cytoskeleton, and has multiple effects on cellular functions related to motility. Some of these effects are consistent with beta-thymosins functioning exclusively as monomer-binding proteins, while others are not. Therefore, the complex pleiotropic effects of beta-thymosin in cells may be due to direct and indirect effects on the actin cytoskeleton, as well as modulation of signaling pathways that will impact the cytoskeleton and a variety of cell functions.

Calponin binds G-actin and F-actin with similar affinity

Calponins are actin-binding proteins that are implicated in the regulation of actomyosin. Calponin binds filamentous actin (F-actin) through two distinct sites ABS1 and ABS2, with an affinity in the low micromolar range. We report that smooth muscle calponin binds monomeric actin with a similar affinity (K(d) of 0.15 microM). We show that the arrangement of binding is similar to that of F-actin by a number of criteria, most notably that the distance between Cys273 on calponin and Cys374 of actin is 29A when measured by fluorescent resonance energy transfer, the same distance as previously reported for F-actin.

Comparative proteome analysis of breast cancer and normal breast

Breast cancer is a leading cause of death for women. The underlying molecular mechanism is still not well understood. In this study, two-dimensional gel electrophoresis combined with mass spectrometry was used to analyze changes in the proteome of infiltrating ductal carcinoma compared to normal breast tissue. Ten sets of two-dimensional gels per experimental condition were analyzed and more than 500 spots each were detected. This revealed 39 spots for which expression in breast cancer cells were reproducibly altered more than twofold compared to normal controls (p < 0.01). These spots represented 25 different proteins after identification using the database search after mass spectrometry, comprising cell defense proteins, enzymes involved in glycolytic energy metabolism and homeostasis, protein folding and structural proteins, proteins involved in cytoskeleton and cell motility, and proteins involved in other functions. In addition, 28 nondifferentially expressed proteins with different functions were also mapped and identified, which might help to establish a two-dimensional gel electrophoresis reference map of human breast cancer. Our study shows that proteomics offers a powerful methodology to detect the proteins that show different expression patterns in breast cancer tissue and may provide an accurate molecular classification. The differentially expressed proteins may be used as potential candidate markers for diagnostic purposes or for determination of tumor sensitivity to therapy. The functional implications of the identified proteins are discussed.

Profilin and Rop GTPases are localized at infection sites of plant cells

We have found 5 profilin cDNAs in cultured parsley cells, representing a small gene family of about 5 members in parsley. Specific antibodies were produced using heterologously expressed parsley profilin as antigen. Western blot analysis revealed the occurrence of similar amounts of profilin in roots and green parts of parsley plants. Immunocytochemical staining of parsley cells infected with the oomycetous plant pathogen Phytophthora infestans clearly revealed that profilin accumulates at the site on the plasma membrane subtending the oomycetous appressorium, where the actin cables focus. We also observed the accumulation of Rop GTPases around this site, which might point to a potential function in signaling to the cytoskeleton.

Structural analysis of an Echinococcus granulosus actin-fragmenting protein by small-angle x-ray scattering studies and molecular modeling

The Echinococcus granulosus actin filament-fragmenting protein (EgAFFP) is a three domain member of the gelsolin family of proteins, which is antigenic to human hosts. These proteins, formed by three or six conserved domains, are involved in the dynamic rearrangements of the cytoskeleton, being responsible for severing and capping actin filaments and promoting nucleation of actin monomers. Various structures of six domain gelsolin-related proteins have been investigated, but little information on the structure of three domain members is available. In this work, the solution structure of the three domain EgAFFP has been investigated through small-angle x-ray scattering (SAXS) studies. EgAFFP exhibits an elongated molecular shape. The radius of gyration and the maximum dimension obtained by SAXS were, respectively, 2.52 +/- 0.01 nm and 8.00 +/- 1.00 nm, both in the absence and presence of Ca2+. Two different molecular homology models were built for EgAFFP, but only one was validated through SAXS studies. The predicted structure for EgAFFP consists of three repeats of a central beta-sheet sandwiched between one short and one long alpha-helix. Possible implications of the structure of EgAFFP upon actin binding are discussed.

The V-ATPase Subunit C Binds to Polymeric F-actin as Well as to Monomeric G-actin and Induces Cross-linking of Actin Filaments

Previously, we have shown that the V-ATPase holoenzyme as well as the V1 complex isolated from the midgut of the tobacco hornworm (Manduca sexta) exhibits the ability of binding to actin filaments via the V1 subunits B and C (Vitavska, O., Wieczorek, H., and Merzendorfer,H. (2003) J. Biol. Chem. 278, 18499-18505). Since the recombinant subunit C not only enhances actin binding of the V1 complex but also can bind separately to F-actin, we analyzed the interaction of recombinant subunit C with actin. We demonstrate that it binds not only to F-actin but also to monomeric G-actin. With dissociation constants of approximately 50 nm, the interaction exhibits a high affinity, and no difference could be observed between binding to ATP-G-actin or ADP-G-actin, respectively. Unlike other proteins such as members of the ADF/cofilin family, which also bind to G- as well as to F-actin, subunit C does not destabilize actin filaments. On the contrary, under conditions where the disassembly of F-actin into G-actin usually occurred, subunit C stabilized F-actin. In addition, it increased the initial rate of actin polymerization in a concentration-dependent manner and was shown to cross-link actin filaments to bundles of varying thickness. Apparently bundling is enabled by the existence of at least two actin-binding sites present in the N- and in the C-terminal halves of subunits C, respectively. Since subunit C has the possibility to dimerize or even to oligomerize, spacing between actin filaments could be variable in size.

Regulation of cytoskeletal dynamics by actin-monomer-binding proteins

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes. Many proteins that interact with filamentous and/or monomeric actin regulate the structure and dynamics of the actin cytoskeleton. Actin-filament-binding proteins control the nucleation, assembly, disassembly and crosslinking of actin filaments, whereas actin-monomer-binding proteins regulate the size, localization and dynamics of the large pool of unpolymerized actin in cells. In this article, we focus on recent advances in understanding how the six evolutionarily conserved actin-monomer-binding proteins - profilin, ADF/cofilin, twinfilin, Srv2/CAP, WASP/WAVE and verprolin/WIP - interact with actin monomers and regulate their incorporation into filament ends. We also present a model of how, together, these ubiquitous actin-monomer-binding proteins contribute to cytoskeletal dynamics and actin-dependent cellular processes.

Coupling of Folding and Binding of Thymosin β4 upon Interaction with Monomeric Actin Monitored by Nuclear Magnetic Resonance

Thymosin beta4 is a major actin-sequestering protein, yet the structural basis for its biological function is still unknown. This study provides insight regarding the way this 43-amino acid peptide, mostly unstructured in solution, binds to monomeric actin and prevents its assembly in filaments. We show here that the whole backbone of thymosin beta4 is highly affected upon binding to G-actin. The assignment of all amide protons and nitrogens of thymosin in the bound state, obtained using a combination of NMR experiments and selective labelings, shows that thymosin folds completely upon binding and displays a central extended region flanked by two N- and C-terminal helices. The cleavage of actin by subtilisin in the DNase I binding loop does not modify the structure of thymosin beta4 in the complex, showing that the backbone of the peptide is not in close proximity to segment 42-47 of actin. The combination of our NMR results and previously published mutation and cross-link data allows a better characterization of the binding mode of thymosins on G-actin.

Regulation of the neuronal actin cytoskeleton by ADF/cofilin

Actin and microtubules are major cytoskeletal elements of most cells including neurons. In order for a cell to move and change shape, its cytoskeleton must undergo rearrangements that involve breaking down and reforming filaments. Many recent reviews have focused on the signaling pathways emanating from receptors that ultimately affect axon growth and growth cone steering. This particular review will address changes in the actin cytoskeleton modulated by the family of actin dynamizing proteins known as actin depolymerizing factor (ADF)/cofilin or AC proteins. Though much is known about inactivation of AC proteins through phosphorylation at ser3 by LIM or TES kinases, new mechanisms of regulation of AC have recently emerged. A novel phosphatase, slingshot (SSH), and the 14-3-3 family of regulatory proteins have also been found to affect AC activity. The potential role of AC proteins in modulating the actin organizational changes that accompany neurite initiation, axonogenesis, growth cone guidance, and dendritic spine formation will be discussed.

Mechanism of actin polymerization in cellular ATP depletion

Cellular ATP depletion in diverse cell types results in the net conversion of monomeric G-actin to polymeric F-actin and is an important aspect of cellular injury in tissue ischemia. We propose that this conversion results from altering the ratio of ATP-G-actin and ADP-G-actin, causing a net decrease in the concentration of thymosinactin complexes as a consequence of the differential affinity of thymosin beta4 for ATP- and ADP-G-actin. To test this hypothesis we examined the effect of ATP depletion induced by antimycin A and substrate depletion on actin polymerization, the nucleotide state of the monomer pool, and the association of actin monomers with thymosin and profilin in the kidney epithelial cell line LLC-PK1. ATP depletion for 30 min increased F-actin content to 145% of the levels under physiological conditions, accompanied by a corresponding decrease in G-actin content. Cytochalasin D treatment did not reduce F-actin formation during ATP depletion, indicating that it was predominantly not because of barbed end monomer addition. ATP-G-actin levels decreased rapidly during depletion, but there was no change in the concentration of ADP-G-actin monomers. The decrease in ATP-G-actin levels could be accounted for by dissociation of the thymosin-G-actin binary complex, resulting in a rise in the concentration of free thymosin beta4 from 4 to 11 microm. Increased detection of profilin-actin complexes during depletion indicated that profilin may participate in catalyzing nucleotide exchange during depletion. This mechanism provides a biochemical basis for the accumulation of F-actin aggregates in ischemic cells.

Profilin Functions in Cytokinesis, Nuclear Positioning, and Stomatogenesis in Tetrahymena thermophila

Expression of the actin-binding protein profilin was disrupted in the ciliate Tetrahymena thermophila by an antisense ribosome method. In cells with the antisense disruption no profilin protein was detected. Cultures of cells with the antisense disruption could be maintained, indicating that profilin was not essential for cytokinesis or vegetative growth. Disruption of the expression of profilin resulted in many cells that were large and abnormally shaped. Formation of multiple micronuclei, which divide mitotically, was observed in cells with a single macronucleus, indicating a defect in early cytokinesis. Some cells with the antisense disruption contained multiple macronuclei, which in Tetrahymena may indicate a function late in cytokinesis. The lack of profilin also affected cytokinesis in the cells that could divide. Normal-sized and normal-shaped cells with the antisense disruption took significantly longer to divide than control cell types. The profilin disruption revealed two new processes in which profilin functions. In cells lacking profilin, micronuclei were not positioned at their normal site on the surface of the macronucleus and phagocytosis was defective. The defect in phagocytosis appeared to be due to disruption of the formation of oral apparatuses (stomatogenesis) and a possible failure in the internalization of phagocytic vacuoles.

Regulation of growth cone actin dynamics by ADF/cofilin

Nervous system development is reliant on neuronal pathfinding, the process in which axons are guided to their target cells by specific extracellular cues. The ability of neurons to extend over long distances in response to environmental guidance signals is made possible by the growth cone, a highly motile structure found at the end of neuronal processes. Growth cones detect directional cues and respond with either attractive or repulsive movements. The motility of growth cones is dependent on rapid reorganization of the actin cytoskeleton, presumably mediated by actin-associated proteins under the control of incoming guidance signals. This article reviews how one such family of proteins, the ADF/cofilins, are emerging as key regulators of growth cone actin dynamics. These proteins are essential for rapid actin turnover in a variety of different cell types. ADF/cofilins are heavily co-localized with actin in growth cones and are necessary for neurite outgrowth. ADF/cofilin activities are regulated through reversible phosphorylation by LIM kinases and slingshot phosphatases. LIM kinases are downstream effectors of the Rho GTPases Rho, Rac, and Cdc42. Growing evidence suggests that extracellular guidance cues may locally alter actin dynamics by regulating the activity of LIM kinase and ADF/cofilin phosphatases via the Rho GTPases. In this way, ADF/cofilins and their upstream effectors may be pivotal to our understanding of how guidance information is translated into physical alterations of the growth cone actin cytoskeleton.

Synthesis and angiogenetic activity in the chick chorioallantoic membrane model of thymosin beta-15

Thymosin beta-15 (Tbeta15), a 44 amino acid peptide (MW = 5173) localized in human prostate and breast cancer tissues was successfully synthesized in multigram quantities using Fmoc solid-phase peptide synthesis. The synthesized product was shown to have the right structure by ESI and MALDI mass spectral techniques and amino acid analysis. Relatively high yield was achieved, which might be due to enhanced acid stability of the p-cyanotrityl resin used. The effect of the synthesized Tbeta15 on the angiogenesis process was investigated using the chick chorioallantoic membrane (CAM) in vivo model. At concentrations above 1 microg/10 microl per disc, Tbeta15 exhibited a positive effect on angiogenesis, comparable to the effect of the intense angiogenetic factor phorbol 12-myristate 13-acetate at a standard concentration of 0.1 microg/10 microl per disc. The results of this study contribute to the further elucidation of the biological regulatory role of thymosin peptides and provide helpful information in the investigation of their possible therapeutic potential.

The Ena/VASP enigma

Ena/VASP proteins are actin-binding proteins that localize to actin stress fibres, the tips of filopodia and the lamellipodial leading edge. In the past few years, a number of seemingly conflicting studies have confused the Ena/VASP field, pointing to roles for these proteins in both promotion and inhibition of actin-dependent processes. Recent discoveries resolve these contradictions and suggest a novel mechanism of Ena/VASP function, in which the proteins function as 'anti-capping' proteins that antagonize capping proteins at the barbed end of actin filaments.

Motility determinants in WASP family proteins

In response to upstream signals, proteins in the Wiskott-Aldrich Syndrome protein (WASP) family regulate actin nucleation via the Arp2/3 complex. Despite intensive study of the function of WASP family proteins in nucleation, it is not yet understood how their distinct structural organization contributes to actin-based motility. Herein, we analyzed the activities of WASP and Scar1 truncation derivatives by using a bead-based motility assay. The minimal region of WASP sufficient to direct movement was the C-terminal WCA fragment, whereas the corresponding region of Scar1 was insufficient. In addition, the proline-rich regions of WASP and Scar1 and the Ena/VASP homology 1 (EVH1) domain of WASP independently enhanced motility rates. The contributions of these regions to motility could not be accounted for by their direct effects on actin nucleation with the Arp2/3 complex, suggesting that they stimulate motility by recruiting additional factors. We have identified profilin as one such factor. WASP- and Scar1-coated bead motility rates were significantly reduced by depletion of profilin and VASP and could be more efficiently rescued by a combination of VASP and wild-type profilin than by VASP and a mutant profilin that cannot bind proline-rich sequences. Moreover, motility of WASP WCA beads was not affected by the depletion or addback of VASP and profilin. Our results suggest that recruitment of factors, including profilin, by the proline-rich regions of WASP and Scar1 and the EVH1 domain of WASP stimulates cellular actin-based motility.

Possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation in Ehrlich ascites tumor cells

Osmotic shrinkage of Ehrlich ascites tumor cells (EATC) elicited translocation of myosin II from the cytosol to the cortical region, and swelling elicits concentration of myosin II in the Golgi region. Rho kinase and p38 both appeared to be involved in shrinkage-induced myosin II reorganization. In contrast, the previously reported shrinkage-induced actin polymerization [Pedersen et al. (1999) Exp. Cell Res. 252, 63-74] was independent of Rho kinase, p38, myosin light chain kinase (MLCK), and protein kinase C (PKC), which thus do not exert their effects on the shrinkage-activated transporters via effects on F-actin. The subsequent F-actin depolymerization, however, appeared MLCK- and PKC-dependent, and the initial swelling-induced F-actin depolymerization was MLCK-dependent; both effects were apparently secondary to kinase-mediated effects on cell volume changes. NHE1 in EATC is activated both by osmotic shrinkage and by the serine/threonine phosphatase inhibitor Calyculin A (CL-A). Both stimuli caused Rho kinase-dependent myosin II relocation to the cortical cytoplasm, but in contrast to the shrinkage-induced F-actin polymerization, CL-A treatment elicited a slight F-actin depolymerization. Moreover, Rho kinase inhibition did not significantly affect NHE1 activation, neither by shrinkage nor by CL-A. Implications for the possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation are discussed.

The beta-thymosins, small actin-binding peptides widely expressed in the developing and adult cerebellum

The beta-thymosins are a highly conserved family of small polar peptides known to bind monomeric actin and inhibit its polymerization. The beta-thymosins show a high degree of sequence conservation among all vertebrate classes and they have been also identified in some invertebrate phyla. The most abundant beta-thymosins in mammals are thymosin beta4 (Tbeta4) and thymosin beta10 (Tbeta10), two ubiquitous small (43 amino acids) peptides sharing a high degree of sequence homology. Both beta-thymosins are present in virtually all mammalian tissues and cells studied, showing distinct patterns of expression in several tissues. The beta-thymosins are expressed in the developing and mature nervous system, indicating their participation with other actin-binding peptides in the control of actin polymerization. In the rat cerebellum the temporal and cellular patterns of expression of Tbeta4 and Tbeta10 are different, suggesting that each beta-thymosin could play a specific physiological function during cerebellum development. The possible roles of beta-thymosins in the developing mammalian cerebellum are discussed.

Genomic organization of profilin-III and evidence for a transcript expressed exclusively in testis

Profilins are small, widely expressed actin binding proteins, thought to be key regulators of actin dynamics in living cells. So far, three profilin-genes have been described: profilin-I (PFN1), profilin-II (PFN2) with two splice variants and the recently identified profilin-III (PFN3). Here we describe the genomic organization of the genes encoding human and mouse profilin-III. Both are single exon genes and lie in close vicinity to the renal sodium-phosphate transport gene 2 (SLC34A1, NPT2) which is highly expressed in kidney. Northern hybridization to rat tissues has previously demonstrated expression of an approximately 4.5 kb long profilin-III mRNA transcript in kidney and a mRNA transcript of approximately 1 kb in length in testis. Here we show that mouse profilin-III expression is restricted to testis and that the 4.2 kb profilin-III mRNA in kidney is the result of a slc34a1 transcript which includes the antisense profilin-III open reading frame in its 3'-untranslated region. Finally, we demonstrate by in situ hybridization that profilin-III mRNA is localized to cells in the late stage of spermatogenesis.

Human cofilin forms oligomers exhibiting actin bundling activity

Human cofilin possesses the tendency for self-association, as indicated by the rapid formation of dimers and oligomers when reacted with water-soluble carbodiimide, Ellman's reagent, or glutathione disulfide. Intermolecular disulfide bonds involve Cys(39) and probably Cys(147) of two adjacent cofilin units. The disulfide-linked dimers and oligomers exhibit a biological activity distinct from the monomer. While monomeric cofilin decreased viscosity and light-scattering of F-actin solutions, dimers and oligomers caused an increase in viscosity and light scattering. Electron microscopy revealed that cofilin oligomers induce the formation of highly ordered actin bundles with occasionally blunt ends similar to actin-cofilin rods observed in cells under oxidative stress. Bundling activity of the disulfide-linked oligomers could be completely reversed into severing activity by dithiothreitol. Formation of cofilin oligomers occurred also in the presence of actin at pH 8, but not at pH 6.6, and was significantly enhanced in the presence of phosphatidylinositol 4,5-bisphosphate. Our data are consistent with the idea that cofilin exists in two forms in vivo also: as monomers exhibiting the known severing activity and as oligomers exhibiting actin bundling activity. However, stabilization of cofilin oligomers in cytoplasm is probably achieved not by disulfide bonds but by a local increase in cofilin concentration and/or binding of regulatory proteins.