DAip1, a Dictyostelium homologue of the yeast actin-interacting protein 1, is involved in endocytosis, cytokinesis, and motility

The combination of positive anti‑WDR1 antibodies with negative anti‑CFL1 antibodies in serum is a poor prognostic factor for patients with esophageal carcinoma

WD repeat-containing protein 1 (WDR1) regulates the cofilin 1 (CFL1) activity, promotes cytoskeleton remodeling, and thus, facilitates cell migration and invasion. A previous study reported that autoantibodies against CFL1 and β-actin were useful biomarkers for diagnosing and predicting the prognosis of patients with esophageal carcinoma. Therefore, the present study aimed to evaluate the serum levels of anti-WDR1 antibodies (s-WDR1-Abs) combined with serum levels of anti-CFL1 antibodies (s-CFL1-Abs) in patients with esophageal carcinoma. Serum samples obtained from 192 patients with esophageal carcinoma and other solid cancers. And s-WDR1-Ab and s-CFL1-Ab titers were analyzed using the amplified luminescent proximity homogeneous assay-linked immunosorbent assay. Compared with those of healthy donors, the s-WDR1-Ab levels were significantly higher in the 192 patients with esophageal, whereas these were not significantly higher in the samples from patients with gastric, colorectal, lung, or breast cancer. In 91 patients treated with surgery, sex, tumor depth, lymph node metastasis, stage and C-reactive protein levels were significantly associated with overall survival, as determined using the log-rank test, whereas the squamous cell carcinoma antigen, p53 antibody and s-WDR1-Ab levels tended to be associated with a worse prognosis. Although no significant difference was observed in the survival between the positive and negative groups of s-WDR1-Abs or s-CFL1-Abs alone in the Kaplan-Meier test, the patients in the s-WDR1-Ab-positive and s-CFL1-Ab-negative groups exhibited a significantly poorer prognosis in the overall survival analysis. On the whole, the present study demonstrates that the combination of positive anti-WDR1 antibodies with negative anti-CFL1 antibodies in serum may be a poor prognostic factor for patients with esophageal carcinoma.

The Amoebal Model for Macropinocytosis

Macropinocytosis is a relatively unexplored form of large-scale endocytosis driven by the actin cytoskeleton. Dictyostelium amoebae form macropinosomes from cups extended from the plasma membrane, then digest their contents and absorb the nutrients in the endo-lysosomal system. They use macropinocytosis for feeding, maintaining a high rate of fluid uptake that makes assay and experimentation easy. Mutants collected over the years identify cytoskeletal and signalling proteins required for macropinocytosis. Cups are organized around plasma membrane domains of intense PIP3, Ras and Rac signalling, proper formation of which also depends on the RasGAPs NF1 and RGBARG, PTEN, the PIP3-regulated protein kinases Akt and SGK and their activators PDK1 and TORC2, Rho proteins, plus other components yet to be identified. This PIP3 domain directs dendritic actin polymerization to the extending lip of macropinocytic cups by recruiting a ring of the SCAR/WAVE complex around itself and thus activating the Arp2/3 complex. The dynamics of PIP3 domains are proposed to shape macropinocytic cups from start to finish. The role of the Ras-PI3-kinase module in organizing feeding structures in unicellular organisms most likely predates its adoption into growth factor signalling, suggesting an evolutionary origin for growth factor signalling.

Formation of Cytoplasmic Actin-Cofilin Rods is Triggered by Metabolic Stress and Changes in Cellular pH

Actin dynamics plays a crucial role in regulating essential cell functions and thereby is largely responsible to a considerable extent for cellular energy consumption. Certain pathological conditions in humans, like neurological disorders such as Alzheimer’s disease or amyotrophic lateral sclerosis (ALS) as well as variants of nemaline myopathy are associated with cytoskeletal abnormalities, so-called actin-cofilin rods. Actin-cofilin rods are aggregates consisting mainly of actin and cofilin, which are formed as a result of cellular stress and thereby help to ensure the survival of cells under unfavorable conditions. We have used Dictyostelium discoideum, an established model system for cytoskeletal research to study formation and principles of cytoplasmic actin rod assembly in response to energy depletion. Experimentally, depletion of ATP was provoked by addition of either sodium azide, dinitrophenol, or 2-deoxy-glucose, and the formation of rod assembly was recorded by live-cell imaging. Furthermore, we show that hyperosmotic shock induces actin-cofilin rods, and that a drop in the intracellular pH accompanies this condition. Our data reveal that acidification of the cytoplasm can induce the formation of actin-cofilin rods to varying degrees and suggest that a local reduction in cellular pH may be a cause for the formation of cytoplasmic rods. We hypothesize that local phase separation mechanistically triggers the assembly of actin-cofilin rods and thereby influences the material properties of actin structures.

Counting actin in contractile rings reveals novel contributions of cofilin and type II myosins to fission yeast cytokinesis

Cytokinesis by animals, fungi, and amoebas depends on actomyosin contractile rings, which are stabilized by continuous turnover of actin filaments. Remarkably little is known about the amount of polymerized actin in contractile rings, so we used low concentrations of GFP-Lifeact to count total polymerized actin molecules in the contractile rings of live fission yeast cells. Contractile rings of wild-type cells accumulated polymerized actin molecules at 4900/min to a peak number of ∼198,000 followed by a loss of actin at 5400/min throughout ring constriction. In adf1-M3 mutant cells with cofilin that severs actin filaments poorly, contractile rings accumulated polymerized actin at twice the normal rate and eventually had almost twofold more actin along with a proportional increase in type II myosins Myo2, Myp2, and formin Cdc12. Although 30% of adf1-M3 mutant cells failed to constrict their rings fully, the rest lost actin from the rings at the wild-type rates. Mutations of type II myosins Myo2 and Myp2 reduced contractile ring actin filaments by half and slowed the rate of actin loss from the rings.

Arabidopsis AIP1-1 regulates the organization of apical actin filaments by promoting their turnover in pollen tubes

Apical actin filaments are highly dynamic structures that are crucial for rapid pollen tube growth, but the mechanisms regulating their dynamics and spatial organization remain incompletely understood. We here identify that AtAIP1-1 is important for regulating the turnover and organization of apical actin filaments in pollen tubes. AtAIP1-1 is distributed uniformly in the pollen tube and loss of function of AtAIP1-1 affects the organization of the actin cytoskeleton in the pollen tube. Specifically, actin filaments became disorganized within the apical region of aip1-1 pollen tubes. Consistent with the role of apical actin filaments in spatially restricting vesicles in pollen tubes, the apical region occupied by vesicles becomes enlarged in aip1-1 pollen tubes compared to WT. Using ADF1 as a representative actin-depolymerizing factor, we demonstrate that AtAIP1-1 enhances ADF1-mediated actin depolymerization and filament severing in vitro, although AtAIP1-1 alone does not have an obvious effect on actin assembly and disassembly. The dynamics of apical actin filaments are reduced in aip1-1 pollen tubes compared to WT. Our study suggests that AtAIP1-1 works together with ADF to act as a module in regulating the dynamics of apical actin filaments to facilitate the construction of the unique "apical actin structure" in the pollen tube.

An actin-depolymerizing factor from the halophyte smooth cordgrass, Spartina alterniflora (SaADF2), is superior to its rice homolog (OsADF2) in conferring drought and salt tolerance when constitutively overexpressed in rice

Actin-depolymerizing factors (ADFs) maintain the cellular actin network dynamics by regulating severing and disassembly of actin filaments in response to environmental cues. An ADF isolated from a monocot halophyte, Spartina alterniflora (SaADF2), imparted significantly higher level of drought and salinity tolerance when expressed in rice than its rice homologue OsADF2. SaADF2 differs from OsADF2 by a few amino acid residues, including a substitution in the regulatory phosphorylation site serine-6, which accounted for its weak interaction with OsCDPK6 (calcium-dependent protein kinase), thus resulting in an increased efficacy of SaADF2 and enhanced cellular actin dynamics. SaADF2 overexpression preserved the actin filament organization better in rice protoplasts under desiccation stress. The predicted tertiary structure of SaADF2 showed a longer F-loop than OsADF2 that could have contributed to higher actin-binding affinity and rapid F-actin depolymerization in vitro by SaADF2. Rice transgenics constitutively overexpressing SaADF2 (SaADF2-OE) showed better growth, relative water content, and photosynthetic and agronomic yield under drought conditions than wild-type (WT) and OsADF2 overexpressers (OsADF2-OE). SaADF2-OE preserved intact grana structure after prolonged drought stress, whereas WT and OsADF2-OE presented highly damaged and disorganized grana stacking. The possible role of ADF2 in transactivation was hypothesized from the comparative transcriptome analyses, which showed significant differential expression of stress-related genes including interacting partners of ADF2 in overexpressers. Identification of a complex, differential interactome decorating or regulating stress-modulated cytoskeleton driven by ADF isoforms will lead us to key pathways that could be potential target for genome engineering to improve abiotic stress tolerance in agricultural crops.

Ate1-mediated posttranslational arginylation affects substrate adhesion and cell migration in Dictyostelium discoideum

The highly conserved enzyme arginyl-tRNA-protein transferase (Ate1) mediates arginylation, a posttranslational modification that is only incompletely understood at its molecular level. To investigate whether arginylation affects actin-dependent processes in a simple model organism, Dictyostelium discoideum, we knocked out the gene encoding Ate1 and characterized the phenotype of ate1-null cells. Visualization of actin cytoskeleton dynamics by live-cell microscopy indicated significant changes in comparison to wild-type cells. Ate1-null cells were almost completely lacking focal actin adhesion sites at the substrate-attached surface and were only weakly adhesive. In two-dimensional chemotaxis assays toward folate or cAMP, the motility of ate1-null cells was increased. However, in three-dimensional chemotaxis involving more confined conditions, the motility of ate1-null cells was significantly reduced. Live-cell imaging showed that GFP-tagged Ate1 rapidly relocates to sites of newly formed actin-rich protrusions. By mass spectrometric analysis, we identified four arginylation sites in the most abundant actin isoform of Dictyostelium, in addition to arginylation sites in other actin isoforms and several actin-binding proteins. In vitro polymerization assays with actin purified from ate1-null cells revealed a diminished polymerization capacity in comparison to wild-type actin. Our data indicate that arginylation plays a crucial role in the regulation of cytoskeletal activities.

WDR1 predicts poor prognosis and promotes cancer progression in hepatocellular carcinoma

WDR1, an activator of cofilin-mediated actin depolymerization, is involved in various actin-dependent processes of living cells including cell migration and cytokinesis. Recently, several studies have found that WDR1 is dysregulated in several types of cancer and is associated with cancer metastasis. However, its role in hepatocellular carcinoma (HCC) remains unknown. In this study, we found that WDR1 expression was aberrantly upregulated at the mRNA and protein levels in HCC cell lines and HCC tissues. WDR1 overexpression was highly correlated with tumor aggressive phenotypes such as capsulation formation, microvascular invasion (MVI), tumor node metastasis (TNM) stage, and was an independent poor prognostic factor for overall survival (OS) and disease-free survival (DFS) for HCC patients after curative surgery. Furthermore, WDR1 overexpression significantly promoted HCC cell migration, invasion and proliferation. In contrast, WDR1 downregulation inhibited HCC cell migration, invasion and proliferation. Conclusion: This study indicates that WDR1 could be used as a new useful prognostic marker and may be a potential therapeutic target for HCC.

Real-Time Single-Molecule Kinetic Analyses of AIP1-Enhanced Actin Filament Severing in the Presence of Cofilin

Rearrangement of actin filaments by polymerization, depolymerization, and severing is important for cell locomotion, membrane trafficking, and many other cellular functions. Cofilin and actin-interacting protein 1 (AIP1; also known as WDR1) are evolutionally conserved proteins that cooperatively sever actin filaments. However, little is known about the biophysical basis of the actin filament severing by these proteins. Here, we performed single-molecule kinetic analyses of fluorescently labeled AIP1 during the severing process of cofilin-decorated actin filaments. Results demonstrated that binding of a single AIP molecule was sufficient to enhance filament severing. After AIP1 binding to a filament, severing occurred with a delay of 0.7 s. Kinetics of binding and dissociation of a single AIP1 molecule to/from actin filaments followed a second-order and a first-order kinetics scheme, respectively. AIP1 binding and severing were detected preferentially at the boundary between the cofilin-decorated and bare regions on actin filaments. Based on the kinetic parameters explored in this study, we propose a possible mechanism behind the enhanced severing by AIP1.

Functions of actin-interacting protein 1 (AIP1)/WD repeat protein 1 (WDR1) in actin filament dynamics and cytoskeletal regulation

Actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), are conserved among eukaryotes and play critical roles in dynamic reorganization of the actin cytoskeleton. AIP1 preferentially promotes disassembly of ADF/cofilin-decorated actin filaments but exhibits minimal effects on bare actin filaments. Therefore, AIP1 has been often considered to be an ancillary co-factor of ADF/cofilin that merely boosts ADF/cofilin activity level. However, genetic and cell biological studies show that AIP1 deficiency often causes lethality or severe abnormalities in multiple tissues and organs including muscle, epithelia, and blood, suggesting that AIP1 is a major regulator of many biological processes that depend on actin dynamics. This review summarizes recent progress in studies on the biochemical mechanism of actin filament severing by AIP1 and in vivo functions of AIP1 in model organisms and human diseases.

Actin-Interacting Protein 1 Contributes to Intranuclear Rod Assembly in Dictyostelium discoideum

Intranuclear rods are aggregates consisting of actin and cofilin that are formed in the nucleus in consequence of chemical or mechanical stress conditions. The formation of rods is implicated in a variety of pathological conditions, such as certain myopathies and some neurological disorders. It is still not well understood what exactly triggers the formation of intranuclear rods, whether other proteins are involved, and what the underlying mechanisms of rod assembly or disassembly are. In this study, Dictyostelium discoideum was used to examine appearance, stages of assembly, composition, stability, and dismantling of rods. Our data show that intranuclear rods, in addition to actin and cofilin, are composed of a distinct set of other proteins comprising actin-interacting protein 1 (Aip1), coronin (CorA), filactin (Fia), and the 34 kDa actin-bundling protein B (AbpB). A finely tuned spatio-temporal pattern of protein recruitment was found during formation of rods. Aip1 is important for the final state of rod compaction indicating that Aip1 plays a major role in shaping the intranuclear rods. In the absence of both Aip1 and CorA, rods are not formed in the nucleus, suggesting that a sufficient supply of monomeric actin is a prerequisite for rod formation.

A Diaphanous-related formin links Ras signaling directly to actin assembly in macropinocytosis and phagocytosis

Phagocytosis and macropinocytosis are Ras-regulated and actin-driven processes that depend on the dynamic rearrangements of the plasma membrane that protrudes and internalizes extracellular material by cup-shaped structures. However, the regulatory mechanisms underlying actin assembly in large-scale endocytosis remain elusive. Here, we show that the Diaphanous-related formin G (ForG) from the professional phagocyte Dictyostelium discoideum localizes to endocytic cups. Biochemical analyses revealed that ForG is a rather weak nucleator but efficiently elongates actin filaments in the presence of profilin. Notably, genetic inactivation of ForG is associated with a strongly impaired endocytosis and a markedly diminished F-actin content at the base of the cups. By contrast, ablation of the Arp2/3 (actin-related protein-2/3) complex activator SCAR (suppressor of cAMP receptor) diminishes F-actin mainly at the cup rim, being consistent with its known localization. These data therefore suggest that ForG acts as an actin polymerase of Arp2/3-nucleated filaments to allow for efficient membrane expansion and engulfment of extracellular material. Finally, we show that ForG is directly regulated in large-scale endocytosis by RasB and RasG, which are highly related to the human proto-oncogene KRas.

Noisy Oscillations in the Actin Cytoskeleton of Chemotactic Amoeba

Biological systems with their complex biochemical networks are known to be intrinsically noisy. Here we investigate the dynamics of actin polymerization of amoeboid cells, which are close to the onset of oscillations. We show that the large phenotypic variability in the polymerization dynamics can be accurately captured by a generic nonlinear oscillator model in the presence of noise. We determine the relative role of the noise with a single dimensionless, experimentally accessible parameter, thus providing a quantitative description of the variability in a population of cells. Our approach, which rests on a generic description of a system close to a Hopf bifurcation and includes the effect of noise, can characterize the dynamics of a large class of noisy systems close to an oscillatory instability.

Platelet WDR1 suppresses platelet activity and is associated with cardiovascular disease

Platelet activity plays a major role in hemostasis with increased platelet activity likely contributing to the pathogenesis of atherothrombosis. We sought to identify associations between platelet activity variability and platelet-related genes in healthy controls. Transcriptional profiling of platelets revealed that WD-40 repeat domain 1 (WDR1), an enhancer of actin-depolymerizing factor activity, is downregulated in platelet messenger RNA (mRNA) from subjects with a hyperreactive platelet phenotype. We used the human megakaryoblastic cell line MEG-01 as an in vitro model for human megakaryocytes and platelets. Stimulation of MEG-01 with thrombin reduced levels of WDR1 transcripts and protein. WDR1 knockdown (KD) in MEG-01 cells increased adhesion and spreading in both the basal and activated states, increased F-actin content, and increased the basal intracellular calcium concentration. Platelet-like particles (PLPs) produced by WDR1 KD cells were fewer in number but larger than PLPs produced from unmodified MEG-01 cells, and had significantly increased adhesion in the basal state and upon thrombin activation. In contrast, WDR1 overexpression reversed the WDR1 KD phenotype of megakaryocytes and PLPs. To translate the clinical significance of these findings, WDR1 expression was measured in platelet RNA from subjects with established cardiovascular disease (n = 27) and age- and sex-matched controls (n = 10). The WDR1 mRNA and protein level was significantly lower in subjects with cardiovascular disease. These data suggest that WDR1 plays an important role in suppressing platelet activity, where it alters the actin cytoskeleton dynamics, and downregulation of WDR1 may contribute to the platelet-mediated pathogenesis of cardiovascular disease.

Aip1 promotes actin filament severing by cofilin and regulates constriction of the cytokinetic contractile ring

Aip1 (actin interacting protein 1) is ubiquitous in eukaryotic organisms, where it cooperates with cofilin to disassemble actin filaments, but neither its mechanism of action nor its biological functions have been clear. We purified both fission yeast and human Aip1 and investigated their biochemical activities with or without cofilin. Both types of Aip1 bind actin filaments with micromolar affinities and weakly nucleate actin polymerization. Aip1 increases up to 12-fold the rate that high concentrations of yeast or human cofilin sever actin filaments, most likely by competing with cofilin for binding to the side of actin filaments, reducing the occupancy of the filaments by cofilin to a range favorable for severing. Aip1 does not cap the barbed ends of filaments severed by cofilin. Fission yeast lacking Aip1 are viable and assemble cytokinetic contractile rings normally, but rings in these Δaip1 cells accumulate 30% less myosin II. Further, these mutant cells initiate the ingression of cleavage furrows earlier than normal, shortening the stage of cytokinetic ring maturation by 50%. The Δaip1 mutation has negative genetic interactions with deletion mutations of both capping protein subunits and cofilin mutations with severing defects, but no genetic interaction with deletion of coronin.

Small GTPases and phosphoinositides in the regulatory mechanisms of macropinosome formation and maturation

Macropinosome formation requires the sequential activation of numerous signaling pathways that coordinate the actin-driven formation of plasma membrane protrusions (ruffles) and circular ruffles (macropinocytic cups), followed by the closure of these macropinocytic cups into macropinosomes. In the process of macropinosome formation, localized productions of phosphoinositides such as PI(4,5)P₂ and PI(3,4,5)P₃ spatiotemporally orchestrate actin polymerization and rearrangement through recruiting and activating a variety of actin-associated proteins. In addition, the sequential activation of small GTPases, which are known to be master regulators of the actin cytoskeleton, plays a pivotal role in parallel with phosphoinositides. To complete macropinosome formation, phosphoinositide breakdown and Rho GTPase deactivation must occur in appropriate timings. After the nascent macropinosomes are formed, phosphoinositides and several Rab GTPases control macropinosome maturation by regulating vesicle trafficking and membrane fusion. In this review, we summarize recent advances in our understanding of the critical functions of phosphoinositide metabolism and small GTPases in association with their downstream effectors in macropinocytosis.

Two actin-interacting protein 1 isoforms function redundantly in the somatic gonad and are essential for reproduction in Caenorhabditis elegans

The somatic gonad of the nematode Caenorhabditis elegans exhibits highly regulated contractility during ovulation, which is essential for successful reproduction. Nonstriated actin filament networks in the myoepithelial sheath at the proximal ovary provide contractile forces to push a mature oocyte for ovulation, but the mechanism of assembly and regulation of the contractile actin networks is poorly understood. Here, we show that actin-interacting protein 1 (AIP1) is essential for the assembly of the contractile actin networks in the myoepithelial sheath. AIP1 promotes disassembly of actin filaments in the presence of actin depolymerizing factor (ADF)/cofilin. C. elegans has two AIP1 genes, unc-78 and aipl-1. Mutation or RNA interference of a single AIP1 isoform causes only minor impacts on reproduction. However, simultaneous depletion of the two AIP1 isoforms causes sterility. AIP1-depleted animals show very weak contractility of the myoepithelial sheath and fail to ovulate a mature oocyte, which results in accumulation of endomitotic oocytes in the ovary. Depletion of AIP1 prevents assembly of actin networks and causes abnormal aggregation of actin as well as ADF/cofilin in the myoepithelial sheath. These results indicate that two AIP1 isoforms have redundant roles in assembly of the contractile apparatuses necessary for C. elegans reproduction.

Oryza sativa actin-interacting protein 1 is required for rice growth by promoting actin turnover

Rapid actin turnover is essential for numerous actin-based processes. However, how it is precisely regulated remains poorly understood. Actin-interacting protein 1 (AIP1) has been shown to be an important factor by acting coordinately with actin-depolymerizing factor (ADF)/cofilin in promoting actin depolymerization, the rate-limiting factor in actin turnover. However, the molecular mechanism by which AIP1 promotes actin turnover remains largely unknown in plants. Here, we provide a demonstration that AIP1 promotes actin turnover, which is required for optimal growth of rice plants. Specific down-regulation of OsAIP1 increased the level of filamentous actin and reduced actin turnover, whereas over-expression of OsAIP1 induced fragmentation and depolymerization of actin filaments and enhanced actin turnover. In vitro biochemical characterization showed that, although OsAIP1 alone does not affect actin dynamics, it enhances ADF-mediated actin depolymerization. It also caps the filament barbed end in the presence of ADF, but the capping activity is not required for their coordinated action. Real-time visualization of single filament dynamics showed that OsAIP1 enhanced ADF-mediated severing and dissociation of pointed end subunits. Consistent with this, the filament severing frequency and subunit off-rate were enhanced in OsAIP1 over-expressors but decreased in RNAi protoplasts. Importantly, OsAIP1 acts coordinately with ADF and profilin to induce massive net actin depolymerization, indicating that AIP1 plays a major role in the turnover of actin, which is required to optimize F-actin levels in plants.

Actin interacting protein1 and actin depolymerizing factor drive rapid actin dynamics in Physcomitrella patens

The remodeling of actin networks is required for a variety of cellular processes in eukaryotes. In plants, several actin binding proteins have been implicated in remodeling cortical actin filaments (F-actin). However, the extent to which these proteins support F-actin dynamics in planta has not been tested. Using reverse genetics, complementation analyses, and cell biological approaches, we assessed the in vivo function of two actin turnover proteins: actin interacting protein1 (AIP1) and actin depolymerizing factor (ADF). We report that AIP1 is a single-copy gene in the moss Physcomitrella patens. AIP1 knockout plants are viable but have reduced expansion of tip-growing cells. AIP1 is diffusely cytosolic and functions in a common genetic pathway with ADF to promote tip growth. Specifically, ADF can partially compensate for loss of AIP1, and AIP1 requires ADF for function. Consistent with a role in actin remodeling, AIP1 knockout lines accumulate F-actin bundles, have fewer dynamic ends, and have reduced severing frequency. Importantly, we demonstrate that AIP1 promotes and ADF is essential for cortical F-actin dynamics.

The professional phagocyte Dictyostelium discoideum as a model host for bacterial pathogens

The use of simple hosts such as Dictyostelium discoideum in the study of host pathogen interactions offers a number of advantages and has steadily increased in recent years. Infection-specific genes can often only be studied in a very limited way in man and even in the mouse model their analysis is usually expensive, time consuming and technically challenging or sometimes even impossible. In contrast, their functional analysis in D. discoideum and other simple model organisms is often easier, faster and cheaper. Because host-pathogen interactions necessarily involve two organisms, it is desirable to be able to genetically manipulate both the pathogen and its host. Particularly suited are those hosts, like D. discoideum, whose genome sequence is known and annotated and for which excellent genetic and cell biological tools are available in order to dissect the complex crosstalk between host and pathogen. The review focusses on host-pathogen interactions of D. discoideum with Legionella pneumophila, mycobacteria, and Salmonella typhimurium which replicate intracellularly.

The two actin-interacting protein 1 genes have overlapping and essential function for embryonic development in Caenorhabditis elegans

Disassembly of actin filaments by actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1) is a conserved mechanism to promote reorganization of the actin cytoskeleton. We previously reported that unc-78, an AIP1 gene in the nematode Caenorhabditis elegans, is required for organized assembly of sarcomeric actin filaments in the body wall muscle. unc-78 functions in larval and adult muscle, and an unc-78-null mutant is homozygous viable and shows only weak phenotypes in embryos. Here we report that a second AIP1 gene, aipl-1 (AIP1-like gene-1), has overlapping function with unc-78, and that depletion of the two AIP1 isoforms causes embryonic lethality. A single aipl-1-null mutation did not cause a detectable phenotype. However, depletion of both unc-78 and aipl-1 arrested development at late embryonic stages due to severe disorganization of sarcomeric actin filaments in body wall muscle. In vitro, both AIPL-1 and UNC-78 preferentially cooperated with UNC-60B, a muscle-specific ADF/cofilin isoform, in actin filament disassembly but not with UNC-60A, a nonmuscle ADF/cofilin. AIPL-1 is expressed in embryonic muscle, and forced expression of AIPL-1 in adult muscle compensated for the function of UNC-78. Thus our results suggest that enhancement of actin filament disassembly by ADF/cofilin and AIP1 proteins is critical for embryogenesis.

Paxillin and phospholipase D interact to regulate actin-based processes in Dictyostelium discoideum

The actin cytoskeleton forms a membrane-associated network whose proper regulation is essential for numerous processes, including cell differentiation, proliferation, adhesion, chemotaxis, endocytosis, exocytosis, and multicellular development. In this report, we show that in Dictyostelium discoideum, paxillin (PaxB) and phospholipase D (PldB) colocalize and coimmunoprecipitate, suggesting that they interact physically. Additionally, the phenotypes observed during development, cell sorting, and several actin-required processes, including cyclic AMP (cAMP) chemotaxis, cell-substrate adhesion, actin polymerization, phagocytosis, and exocytosis, reveal a genetic interaction between paxB and pldB, suggesting a functional interaction between their gene products. Taken together, our data point to PldB being a required binding partner of PaxB during processes involving actin reorganization.

Genetic evidence for concerted control of actin dynamics in cytokinesis, endocytic traffic, and cell motility by coronin and Aip1

Coronin and actin-interacting protein 1 (Aip1) are actin-binding proteins that by different mechanisms inhibit actin polymerization or enhance the disassembly of actin filaments. Cells of Dictyostelium discoideum lacking both proteins are retarded in growth and early development and often fail to proceed to fruiting body formation. Coronin/Aip1-null cells show numerous surface protrusions enriched in filamentous actin and cofilin. We show that the double-null cells are characterized by an increase in filamentous actin that causes a thickening of the cell cortex. This imbalance has severe consequences for processes that rely on the dynamic reorganization of the actin cytoskeleton, such as cell motility, cytokinesis and endocytosis. Although cell motility is considerably slowed down, the double-mutant cells are still capable of orientating in a gradient of chemoattractant. The cytokinesis defect is caused by the lack of proper cleavage furrow formation, a defect that is partially rescued by low concentrations of latrunculin A, an inhibitor of actin polymerization. Furthermore, we demonstrate that the disassembly of the actin coat after phagocytic or macropinocytic uptake is significantly delayed in the double-mutant cells. Our results prove that coronin and Aip1 are important effectors that act together in maintaining the balance of actin polymerization and depolymerization in living cells.

Expression of actin-interacting protein 1 suppresses impaired chemotaxis of Dictyostelium cells lacking the Na+-H+ exchanger NHE1

Dictyostelium cells lacking the intracellular pH regulator NHE1 have defective chemotaxis. A modifier screen and reconstitution studies show expression of recombinant actin interacting protein 1 (Aip1) suppresses the Ddnhe1-phenotype. Aip1 promotes cofilin-dependent actin remodeling, which is likely a major determinant in pH-dependent chemotaxis.

Increased intracellular pH is an evolutionarily conserved signal necessary for directed cell migration. We reported previously that in Dictyostelium cells lacking H⁺ efflux by a Na⁺-H⁺ exchanger (NHE; Ddnhe1⁻), chemotaxis is impaired and the assembly of filamentous actin (F-actin) is attenuated. We now describe a modifier screen that reveals the C-terminal fragment of actin-interacting protein 1 (Aip1) enhances the chemotaxis defect of Ddnhe1⁻ cells but has no effect in wild-type Ax2 cells. However, expression of full-length Aip1 mostly suppresses chemotaxis defects of Ddnhe1⁻ cells and restores F-actin assembly. Aip1 functions to promote cofilin-dependent actin remodeling, and we found that although full-length Aip1 binds cofilin and F-actin, the C-terminal fragment binds cofilin but not F-actin. Because pH-dependent cofilin activity is attenuated in mammalian cells lacking H⁺ efflux by NHE1, our current data suggest that full-length Aip1 facilitates F-actin assembly when cofilin activity is limited. We predict the C-terminus of Aip1 enhances defective chemotaxis of Ddnhe1⁻ cells by sequestering the limited amount of active cofilin without promoting F-actin assembly. Our findings indicate a cooperative role of Aip1 and cofilin in pH-dependent cell migration, and they suggest defective chemotaxis in Ddnhe1⁻ cells is determined primarily by loss of cofilin-dependent actin dynamics.

The STE group kinase SepA controls cleavage furrow formation in Dictyostelium

During a REMI screen for proteins regulating cytokinesis in Dictyostelium discoideum we isolated a mutant forming multinucleate cells. The gene affected in this mutant encoded a kinase, SepA, which is an ortholog of Cdc7, a serine-threonine kinase essential for septum formation in Schizosaccharomyces pombe. Localization of SepA-GFP in live cells and its presence in isolated centrosomes indicated that SepA, like its upstream regulator Spg1, is associated with centrosomes. Knockout mutants of SepA showed a severe cytokinesis defect and a delay in development. In multinucleate SepA-null cells nuclear division proceeded normally and synchronously. However, often cleavage furrows were either missing or atypical: they were extremely asymmetric and constriction was impaired. Cortexillin-I, a marker localizing strictly to the furrow in wild-type cells, demonstrated that large, crescent-shaped furrows expanded and persisted long after the spindle regressed and nuclei returned to the interphase state. Outside the furrow the filamentous actin system of the cell cortex showed strong ruffling activity. These data suggest that SepA is involved in the spatial and temporal control system organizing cortical activities in mitotic and postmitotic cells.

Self-organizing actin waves as planar phagocytic cup structures

Actin waves that travel on the planar membrane of a substrate-attached cell underscore the capability of the actin system to assemble into dynamic structures by the recruitment of proteins from the cytoplasm. The waves have no fixed shape, can reverse their direction of propagation and can fuse or divide. Actin waves separate two phases of the plasma membrane that are distinguished by their lipid composition. The area circumscribed by a wave resembles in its phosphoinositide content the interior of a phagocytic cup, leading us to explore the possibility that actin waves are in-plane phagocytic structures generated without the localized stimulus of an attached particle. Consistent with this view, wave-forming cells were found to exhibit a high propensity for taking up particles. Cells fed rod-shaped particles produced elongated phagocytic cups that displayed a zonal pattern that reflected in detail the actin and lipid pattern of free-running actin waves. Neutrophils and macrophages are known to spread on surfaces decorated with immune complexes, a process that has been interpreted as "frustrated" phagocytosis. We suggest that actin waves enable a phagocyte to scan a surface for particles that might be engulfed.

Critical roles of actin-interacting protein 1 in cytokinesis and chemotactic migration of mammalian cells

Cofilin regulates actin filament dynamics by stimulating actin filament disassembly and plays a critical role in cytokinesis and chemotactic migration. Aip1 (actin-interacting protein 1), also called WDR1 (WD-repeat protein 1), is a highly conserved WD-repeat protein in eukaryotes and promotes cofilin-mediated actin filament disassembly in vitro; however, little is known about the mechanisms by which Aip1 functions in cytokinesis and cell migration in mammalian cells. In the present study, we investigated the roles of Aip1 in cytokinesis and chemotactic migration of human cells by silencing the expression of Aip1 using siRNA (small interfering RNA). Knockdown of Aip1 in HeLa cells increased the percentage of multinucleate cells; this effect was reversed by expression of an active form of cofilin. In Aip1-knockdown cells, the cleavage furrow ingressed normally from anaphase to early telophase; however, an excessive accumulation of actin filaments was observed on the contractile ring in late telophase. These results suggest that Aip1 plays a crucial role in the completion of cytokinesis by promoting cofilin-mediated actin filament disassembly in telophase. We have also shown that Aip1 knockdown significantly suppressed chemokine-induced chemotactic migration of Jurkat T-lymphoma cells, and this was blocked by expression of an active form of cofilin. Whereas control cells mostly formed a single lamellipodium in response to chemokine stimulation, Aip1 knockdown cells abnormally exhibited multiple protrusions around the cells before and after cell stimulation. This indicates that Aip1 plays an important role in directional cell migration by restricting the stimulus-induced membrane protrusion to one direction via promoting cofilin activity.

Live imaging of theDictyosteliumcell cycle reveals widespread S phase during development, a G2 bias in spore differentiation and a premitotic checkpoint

The regulation of the Dictyostelium cell cycle has remained ambiguous owing to difficulties in long-term imaging of motile cells and a lack of markers for defining cell cycle phases. There is controversy over whether cells replicate their DNA during development, and whether spores are in G1 or G2 of the cell cycle. We have introduced a live-cell S-phase marker into Dictyostelium cells that allows us to precisely define cycle phase. We show that during multicellular development, a large proportion of cells undergo nuclear DNA synthesis. Germinating spores enter S phase only after their first mitosis, indicating that spores are in G2. In addition, we demonstrate that Dictyostelium heterochromatin is copied late in S phase and replicates via accumulation of replication factors, rather than recruitment of DNA to pre-existing factories. Analysis of variability in cycle times indicates that regulation of the cycle manifests at a single random transition in G2, and we present the first identified checkpoint in Dictyostelium, which operates at the G2-M transition in response to DNA damage.

WDR1 presence in the songbird basilar papilla

WD40 repeat 1 protein (WDR1) was first reported in the acoustically injured chicken inner ear, and bioinformatics revealed that WDR1 has numerous WD40 repeats, important for protein-protein interactions. It has significant homology to actin interacting protein 1 (Aip1) in several lower species such as yeast, roundworm, fruitfly and frog. Several studies have shown that Aip1 binds cofilin/actin depolymerizing factor, and that these interactions are pivotal for actin disassembly via actin filament severing and actin monomer capping. However, the role of WDR1 in auditory function has yet to be determined. WDR1 is typically restricted to hair cells of the normal avian basilar papilla, but is redistributed towards supporting cells after acoustic overstimulation, suggesting that WDR1 may be involved in inner ear response to noise stress. One aim of the present study was to resolve the question as to whether stress factors, other than intense sound, could induce changes in WDR1 presence in the affected avian inner ear. Several techniques were used to assess WDR1 presence in the inner ears of songbird strains, including Belgian Waterslager (BW) canary, an avian strain with degenerative hearing loss thought to have a genetic basis. Reverse transcription, followed by polymerase chain reactions with WDR1-specific primers, confirmed WDR1 presence in the basilar papillae of adult BW, non-BW canaries, and zebra finches. Confocal microscopy examinations, following immunocytochemistry with anti-WDR1 antibody, localized WDR1 to the hair cell cytoplasm along the avian sensory epithelium. In addition, little, if any, staining by anti-WDR1 antibody was observed among supporting cells in the chicken or songbird ear. The present observations confirm and extend the early findings of WDR1 localization in hair cells, but not in supporting cells, in the normal avian basilar papilla. However, unlike supporting cells in the acoustically damaged chicken basilar papilla, the inner ear of the BW canary showed little, if any, WDR1 up-regulation in supporting cells. This may be due to the fact that the BW canary already has established hearing loss and/or to the possibility that the mechanism(s) involved in BW hearing loss may not be related to WDR1.

Phagocytosis and host-pathogen interactions in Dictyostelium with a look at macrophages

Research into phagocytosis and host-pathogen interactions in the lower eukaryote Dictyostelium discoideum has flourished in recent years. This chapter presents a glimpse of where this research stands, with emphasis on the cell biology of the phagocytic process and on the wealth of molecular genetic data that have been gathered. The basic mechanistic machinery and most of the underlying genes appear to be evolutionarily conserved, reflecting the fact that phagocytosis arose as an efficient way to ingest food in single protozoan cells devoid of a rigid cell wall. In spite of some differences, the signal transduction pathways regulating phagosome biogenesis are also emerging as ultimately similar between Dictyostelium and macrophages. Both cell types are hosts for many pathogenic invasive bacteria, which exploit phagocytosis to grow intracellularly. We present an overwiew, based on the analysis of mutants, on how Dictyostelium contributes as a genetic model system to decipher the complexity of host-pathogen interactions.

Competition between targeting signals in hybrid proteins provides information on their relative in vivo affinities for subcellular compartments

After their translation and folding in the cytoplasm, proteins may be imported into an organelle, associate with a membrane, or rather become part of large, highly localised cytoplasmic structures such as the cytoskeleton. The localisation of a protein is governed by the strength of binding to its immediate target, such as an import receptor for an organelle or a major component of the cytoskeleton, e.g. actin. We have experimentally provided a set of actin-binding proteins with competing targeting information and expressed them at various concentrations to analyse the strength of the signal that governs their subcellular localisation. Our microscopic observations indicate that organellar sorting signals override the targeting preference of most cytoskeletal proteins. Among these signals, the nuclear localisation signal of SV40 is strongest, followed by the oligomerised PHB domain that targets vacuolin to the endosomal surface, and finally the tripeptide SKL mediating transport into the peroxisome. The actin-associated protein coronin, however, can only be misled by the nuclear localisation signal. Interestingly, the targeting behaviour of this model set of hybrid proteins in living Dictyostelium amoebae correlates surprisingly well with the affinities of their constituent signals derived from in vitro experiments conducted in various other organisms. Accordingly, this approach allows estimating the in vivo affinity of a protein to its target even if the latter is not known, as in the case of vacuolin.

Asymmetric elastic properties of Dictyostelium discoideum in relation to chemotaxis

In this study we used an AFM to investigate the cytoskeletal properties of live Dictyostelium discoideum cells by measuring the local stiffness across individual living cells. We have examined differences in elastic properties of polarized and unpolarized AX3 wild type and the mutant DAip1- cells, as well as the differences in the front and rear of the cells in relation to organization of the actin cytoskeleton. We found that the average Young's modulus increases upon polarization for the thin regions of the cell and that in polarized cells, the cell front was stiffer than the cell back. We also found that AX3 cells were stiffer than DAip1- cells. This finding suggests that actin polymerization is one of the major determinants of cell motility in Dictyostelium. In addition, a thin agarose film was studied as a model system to examine the influence of the substrate of thin materials probed with the AFM.

The localization of inner centromeric protein (INCENP) at the cleavage furrow is dependent on Kif12 and involves interactions of the N terminus of INCENP with the actin cytoskeleton

The inner centromeric protein (INCENP) and other chromosomal passenger proteins are known to localize on the cleavage furrow and to play a role in cytokinesis. However, it is not known how INCENP localizes on the furrow or whether this localization is separable from that at the midbody. Here, we show that the association of Dictyostelium INCENP (DdINCENP) with the cortex of the cleavage furrow involves interactions with the actin cytoskeleton and depends on the presence of the kinesin-6-related protein Kif12. We found that Kif12 is found on the central spindle and the cleavage furrow during cytokinesis. Kif12 is not required for the redistribution of DdINCENP from centromeres to the central spindle. However, in the absence of Kif12, DdINCENP fails to localize on the cleavage furrow. Domain analysis indicates that the N terminus of DdINCENP is necessary and sufficient for furrow localization and that it binds directly to the actin cytoskeleton. Our data suggest that INCENP moves from the central spindle to the furrow of a dividing cell by a Kif12-dependent pathway. Once INCENP reaches the equatorial cortex, it associates with the actin cytoskeleton where it then concentrates toward the end of cytokinesis.

The flare gene, which encodes the AIP1 protein of Drosophila, functions to regulate F-actin disassembly in pupal epidermal cells

Adult Drosophila are decorated with several types of polarized cuticular structures, such as hairs and bristles. The morphogenesis of these takes place in pupal cells and is mediated by the actin and microtubule cytoskeletons. Mutations in flare (flr) result in grossly abnormal epidermal hairs. We report here that flr encodes the Drosophila actin interacting protein 1 (AIP1). In other systems this protein has been found to promote cofilin-mediated F-actin disassembly. In Drosophila cofilin is encoded by twinstar (tsr). We show that flr mutations result in increased levels of F-actin accumulation and increased F-actin stability in vivo. Further, flr is essential for cell proliferation and viability and for the function of the frizzled planar cell polarity system. All of these phenotypes are similar to those seen for tsr mutations. This differs from the situation in yeast where cofilin is essential while aip1 mutations result in only subtle defects in the actin cytoskeleton. Surprisingly, we found that mutations in flr and tsr also result in greatly increased tubulin staining, suggesting a tight linkage between the actin and microtubule cytoskeleton in these cells.

Mutations in the cofilin partner Aip1/Wdr1 cause autoinflammatory disease and macrothrombocytopenia

A pivotal mediator of actin dynamics is the protein cofilin, which promotes filament severing and depolymerization, facilitating the breakdown of existing filaments, and the enhancement of filament growth from newly created barbed ends. It does so in concert with actin interacting protein 1 (Aip1), which serves to accelerate cofilin's activity. While progress has been made in understanding its biochemical functions, the physiologic processes the cofilin/Aip1 complex regulates, particularly in higher organisms, are yet to be determined. We have generated an allelic series for WD40 repeat protein 1 (Wdr1), the mammalian homolog of Aip1, and report that reductions in Wdr1 function produce a dramatic phenotype gradient. While severe loss of function at the Wdr1 locus causes embryonic lethality, macrothrombocytopenia and autoinflammatory disease develop in mice carrying hypomorphic alleles. Macrothrombocytopenia is the result of megakaryocyte maturation defects, which lead to a failure of normal platelet shedding. Autoinflammatory disease, which is bone marrow-derived yet nonlymphoid in origin, is characterized by a massive infiltration of neutrophils into inflammatory lesions. Cytoskeletal responses are impaired in Wdr1 mutant neutrophils. These studies establish an essential requirement for Wdr1 in megakaryocytes and neutrophils, indicating that cofilin-mediated actin dynamics are critically important to the development and function of both cell types.

Biochemical and genetic analyses provide insight into the structural and mechanistic properties of actin filament disassembly by the Aip1p cofilin complex in Saccharomyces cerevisiae

Explication of the Aip1p/cofilin/actin filament complex may lead to a more detailed understanding of the mechanisms by which Aip1p and cofilin collaborate to rapidly disassemble filaments. We further characterized the actin-Aip1p interface through a random mutagenic screen of ACT1, identifying a novel Aip1p interaction site on actin. This finding is consistent with our current ternary complex model and offers insights into how Aip1p may disturb intersubunit contacts within an actin filament. In addition, site-directed mutagenesis aimed at interfering with salt bridge interactions at the predicted Aip1p-cofilin interface revealed hyperactive alleles of cof1 and aip1 that support the ternary complex model and suggest that conformational changes in cofilin structure may be transmitted to actin filaments, causing increased destabilization. Furthermore, these data support an active role for Aip1p in promoting actin filament turnover.

Caspase-11 regulates cell migration by promoting Aip1-Cofilin-mediated actin depolymerization

Coordinated regulation of cell migration, cytokine maturation and apoptosis is critical in inflammatory responses. Caspases, a family of cysteine proteases, are known to regulate cytokine maturation and apoptosis. Here, we show that caspase-11, a mammalian pro-inflammatory caspase, regulates cell migration during inflammation. Caspase-11-deficient lymphocytes exhibit a cell-autonomous migration defect in vitro and in vivo. We demonstrate that caspase-11 interacts physically and functionally with actin interacting protein 1 (Aip1), an activator of cofilin-mediated actin depolymerization. The caspase-recruitment domain (CARD) of caspase-11 interacts with the carboxy-terminal WD40 propeller domain of Aip1 to promote cofilin-mediated actin depolymerization. Cells with Aip1 or caspase-11 deficiency exhibit defects in actin dynamics. Using in vitro actin depolymerization assays, we found that caspase-11 and Aip1 work cooperatively to promote cofilin-mediated actin depolymerization. These data demonstrate a novel cell autonomous caspase-mediated mechanism that regulates actin dynamics and mammalian cell migration distinct from the receptor mediated Rho-Rac-Cdc42 pathway.

Actin organization and root hair development are disrupted by ethanol-induced overexpression of Arabidopsis actin interacting protein 1 (AIP1)

* Actin organization and dynamics are essential for cell division, growth and cytoplasmic streaming. Here we analyse the effects of the overexpression of Actin Interacting Protein 1 (AIP1) on Arabidopsis development. * Arabidopsis plants were transformed with an ethanol-inducible AIP1 construct and the characteristics of these plants were analysed after induction. * When AIP1 was increased to approx. 90% above wild-type values, root hair development and actin organization in all cell types examined were disrupted. * Our data demonstrate that AIP1 is a key regulator of actin organization and that its regulation is essential for normal plant cell morphogenesis.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Rapid actin monomer-insensitive depolymerization of Listeria actin comet tails by cofilin, coronin, and Aip1

Actin filaments in cells depolymerize rapidly despite the presence of high concentrations of polymerizable G actin. Cofilin is recognized as a key regulator that promotes actin depolymerization. In this study, we show that although pure cofilin can disassemble Listeria monocytogenes actin comet tails, it cannot efficiently disassemble comet tails in the presence of polymerizable actin. Thymus extracts also rapidly disassemble comet tails, and this reaction is more efficient than pure cofilin when normalized to cofilin concentration. By biochemical fractionation, we identify Aip1 and coronin as two proteins present in thymus extract that facilitate the cofilin-mediated disassembly of Listeria comet tails. Together, coronin and Aip1 lower the amount of cofilin required to disassemble the comet tail and permit even low concentrations of cofilin to depolymerize actin in the presence of polymerizable G actin. The cooperative activities of cofilin, coronin, and Aip1 should provide a biochemical basis for understanding how actin filaments can grow in some places in the cell while shrinking in others.

Time-resolved responses to chemoattractant, characteristic of the front and tail of Dictyostelium cells

In a gradient of chemoattractant, Dictyostelium cells are orientated with their front directed toward the source and their tail pointing into the opposite direction. The front region is specified by the polymerization of actin and the tail by the recruitment of filamentous myosin-II. We have dissected these front and tail responses by exposing cells to an upshift of cyclic AMP. A sharp rise and fall of polymerized actin within 10s is accompanied by the recruitment of proteins involved in turning actin polymerization on or off. The cortical accumulation of myosin-II starts when the front response has declined, supporting the concept of divergent signal transmission and adaptation pathways.

Dissection of amoeboid movement into two mechanically distinct modes

The current dominant model of cell locomotion proposes that actin polymerization pushes against the membrane at the leading edge producing filopodia and lamellipodia that move the cell forward. Despite its success, this model does not fully explain the complex process of amoeboid motility, such as that occurring during embryogenesis and metastasis. Here, we show that Dictyostelium cells moving in a physiological milieu continuously produce `blebs' at their leading edges, and demonstrate that focal blebbing contributes greatly to their locomotion. Blebs are well-characterized spherical hyaline protrusions that occur when a patch of cell membrane detaches from its supporting cortex. Their formation requires the activity of myosin II, and their physiological contribution to cell motility has not been fully appreciated. We find that pseudopodia extension, cell body retraction and overall cell displacement are reduced under conditions that prevent blebbing, including high osmolarity and blebbistatin, and in myosin-II-null cells. We conclude that amoeboid motility comprises two mechanically different processes characterized by the production of two distinct cell-surface protrusions, blebs and filopodia-lamellipodia.

Mechanically induced actin-mediated rocketing of phagosomes

Actin polymerization can be induced in Dictyostelium by compressing the cells to bring phagosomes filled with large particles into contact with the plasma membrane. Asymmetric actin assembly results in rocketing movement of the phagosomes. We show that the compression-induced assembly of actin at the cytoplasmic face of the plasma membrane involves the Arp2/3 complex. We also identify two other proteins associated with the mechanically induced actin assembly. The class I myosin MyoB accumulates at the plasma membrane-phagosome interface early during the initiation of the response, and coronin is recruited as the actin filaments are disassembling. The forces generated by rocketing phagosomes are sufficient to push the entire microtubule apparatus forward and to dislocate the nucleus.

Transcriptional pulsing of a developmental gene

It has not been possible to view the transcriptional activity of a single gene within a living eukaryotic cell. It is therefore unclear how long and how frequently a gene is actively transcribed, how this is modulated during differentiation, and how transcriptional events are dynamically coordinated in cell populations. By means of an in vivo RNA detection technique , we have directly visualized transcription of an endogenous developmental gene. We found discrete "pulses" of gene activity that turn on and off at irregular intervals. Surprisingly, the length and height of these pulses were consistent throughout development. However, there was strong developmental variation in the proportion of cells recruited to the expressing pool. Cells were more likely to re-express than to initiate new expression, indicating that we directly observe a transcriptional memory. In addition, we used a clustering algorithm to reveal synchronous transcription initiation in neighboring cells. This study represents the first direct visualization of transcriptional pulsing in eukaryotes. Discontinuity of transcription may allow greater flexibility in the gene-expression decisions of a cell.

Phagocyte meets prey: uptake, internalization, and killing of bacteria by Dictyostelium amoebae

Dictyostelium cells are professional phagocytes that avidly consume bacteria, their natural prey. Fluorescent probes have allowed us to monitor the initial steps in this process in living cells. Using probes that bind to F-actin, we have visualized the assembly and disassembly of actin filaments responsible for extending the phagocytic cup to engulf a bacterium, and, after the phagosome has sealed, the assembly of new actin filaments to propel the phagosome away from the site of uptake. Using bacteria expressing fluorescent proteins that are susceptible to proteolysis, we have monitored the loss of that fluorescent signal and the staining of the bacterial contents with neutral red, indicating permeabilization of the bacterial cell wall and acidification of the cytoplasm. We find that acidification occurs during a period of microtubule-based transport that promotes fusion of the phagosome with microtubule-associated acidic endosomes. Actin-powered phagosome internalization, transport of the phagosome along microtubules, proteolysis and acidification of bacterial contents, all typically occur within the first six or seven minutes after formation of the phagosome. Thus, tracking individual phagosomes has revealed that early steps in phagosome maturation occur much more rapidly than had been inferred from previous population studies.

Aip1 and cofilin promote rapid turnover of yeast actin patches and cables: a coordinated mechanism for severing and capping filaments

Rapid turnover of actin structures is required for dynamic remodeling of the cytoskeleton and cell morphogenesis, but the mechanisms driving actin disassembly are poorly defined. Cofilin plays a central role in promoting actin turnover by severing/depolymerizing filaments. Here, we analyze the in vivo function of a ubiquitous actin-interacting protein, Aip1, suggested to work with cofilin. We provide the first demonstration that Aip1 promotes actin turnover in living cells. Further, we reveal an unanticipated role for Aip1 and cofilin in promoting rapid turnover of yeast actin cables, dynamic structures that are decorated and stabilized by tropomyosin. Through systematic mutagenesis of Aip1 surfaces, we identify two well-separated F-actin-binding sites, one of which contributes to actin filament binding and disassembly specifically in the presence of cofilin. We also observe a close correlation between mutations disrupting capping of severed filaments in vitro and reducing rates of actin turnover in vivo. We propose a model for balanced regulation of actin cable turnover, in which Aip1 and cofilin function together to "prune" tropomyosin-decorated cables along their lengths. Consistent with this model, deletion of AIP1 rescues the temperature-sensitive growth and loss of actin cable defects of tpm1Delta mutants.