Localization and dynamics of nonfilamentous actin in cultured cells

Changes of G-actin localisation in the mitotic spindle region or nucleus during mitosis and after heat shock: a histochemical study of G-actin in various cell lines with fluorescent labelled vitamin D-binding protein

The presence and localisation of G-actin in various cell lines was studied using the highly G-actin specific, fluorescence-labelled vitamin D-binding protein. In various cell-types, pig kidney-derived cells (LLC-PK1), Chinese hamster ovary (CHO) cells, SV-40 transformed African green monkey kidney (COS) cells and human hepatoma (HepG2) cells, G-actin was only visible in the cytoplasm of interphase cells. However, in mitotic cells, depending on the mitotic phase, intense G-actin staining was found associated with the mitotic spindle (early mitosis) or overlapping the DNA-staining pattern (late mitosis). Also after heat shock (60-180 min at 43 degrees C), an intense nuclear staining of G-actin was observed. In LLC-PK1 cells, the increase of nuclear G-actin staining disappeared again after 24 h at 37 degrees C, but in COS, CHO and HepG2 cells, it was still present in the nucleus after 24 h at 37 degrees C, indicating that the process was not rapidly reversible in these cells; the increased nuclear G-actin was not associated with cell division. Comparison of the amount of G-actin present in the nucleus and in the cytosol before and after heat shock using Western blotting demonstrated that the total amount of G-actin in both nucleus and cytosol was unchanged after heat shock. This indicates that the increased G-actin staining is not a result of import of G-actin into the nucleus. These observations suggest a rearrangement of G-actin in the nucleus during both mitosis and heat shock, which may be due to changes in interaction of G-actin with chromosomes.

Interplay between Rac and Rho in the control of substrate contact dynamics

BACKGROUND

Substrate anchorage and cell locomotion entail the initiation and development of different classes of contact sites, which are associated with the different compartments of the actin cytoskeleton. The Rho-family GTPases are implicated in the signalling pathways that dictate contact initiation, maturation and turnover, but their individual roles in these processes remain to be defined.

RESULTS

We monitored the dynamics of peripheral, Rac-induced focal complexes in living cells in response to perturbations of Rac and Rho activity and myosin contractility. We show that focal complexes formed in response to Rac differentiated into focal contacts upon upregulation of Rho. Focal complexes were dissociated by inhibitors of myosin-II-dependent contractility but not by an inhibitor of Rho-kinase. The downregulation of Rac promoted the enlargement of focal contacts, whereas a block in the Rho pathway not only caused a dissolution of focal contacts but also stimulated membrane ruffling and formation of new focal complexes, which were associated with the advance of the cell front.

CONCLUSIONS

Rac functions to signal the creation of new substrate contacts at the cell front, which are associated with the induction of ruffling lamellipodia, whereas Rho serves in the maturation of existing contacts, with both contact types requiring contractility for their formation. The transition from a focal complex to a focal contact is associated with a switch to Rho-kinase dependence. Rac and Rho also influence the development of focal contacts and focal complexes, respectively, through mutually antagonistic pathways.

Arp2/3 complex and actin depolymerizing factor/cofilin in dendritic organization and treadmilling of actin filament array in lamellipodia

The leading edge (approximately 1 microgram) of lamellipodia in Xenopus laevis keratocytes and fibroblasts was shown to have an extensively branched organization of actin filaments, which we term the dendritic brush. Pointed ends of individual filaments were located at Y-junctions, where the Arp2/3 complex was also localized, suggesting a role of the Arp2/3 complex in branch formation. Differential depolymerization experiments suggested that the Arp2/3 complex also provided protection of pointed ends from depolymerization. Actin depolymerizing factor (ADF)/cofilin was excluded from the distal 0.4 micrometer++ of the lamellipodial network of keratocytes and in fibroblasts it was located within the depolymerization-resistant zone. These results suggest that ADF/cofilin, per se, is not sufficient for actin brush depolymerization and a regulatory step is required. Our evidence supports a dendritic nucleation model (Mullins, R.D., J.A. Heuser, and T.D. Pollard. 1998. Proc. Natl. Acad. Sci. USA. 95:6181-6186) for lamellipodial protrusion, which involves treadmilling of a branched actin array instead of treadmilling of individual filaments. In this model, Arp2/3 complex and ADF/cofilin have antagonistic activities. Arp2/3 complex is responsible for integration of nascent actin filaments into the actin network at the cell front and stabilizing pointed ends from depolymerization, while ADF/cofilin promotes filament disassembly at the rear of the brush, presumably by pointed end depolymerization after dissociation of the Arp2/3 complex.

Induction of apoptosis and changes in nuclear G-actin are mediated by different pathways: the effect of inhibitors of protein and RNA synthesis in isolated rat hepatocytes

Stressor-induced changes in the cytoskeleton, of which actin is a major component, may lead to apoptosis. The role of drug-induced changes in nuclear G-actin and apoptosis was studied in freshly isolated hepatocytes. Several protein synthesis inhibitors, cycloheximide, puromycin, and emetine, induced 10 to 15% apoptosis in hepatocytes after 4 h, as was determined by changes in nuclear morphology and flow cytometric analysis of Annexin V-positive cells. Apoptosis induced by protein synthesis inhibition could be prevented by the caspase inhibitors Z-Val-Ala-DL-Asp fluormethylketone (zVAD-fmk) and Ac-Asp-Glu-Val-Asp-aldehyde (DEVD-cho). Several (chemical) stressors cause a rapid increase in nuclear G-actin staining in hepatocytes or cell lines (Meijerman et al., Biochem. Biophys. Res. Commun. 240, 697-700, 1997). The protein synthesis inhibitors also increased G-actin staining in nuclei after 2 h; this could not be inhibited by zVAD-fmk or DEVD-cho. Changes in the cytosolic F-actin pattern did not occur until nuclear G-actin staining had already increased. The mRNA synthesis inhibitor actinomycin D, also increased nuclear G-actin staining, but did not induce apoptosis within the studied time frame. The results suggest that the induction of apoptosis and the increased nuclear staining of G-actin by protein synthesis inhibition are differently controlled.

Surface-induced polymerization of actin

Living cells contain a very large amount of membrane surface area, which potentially influences the direction, the kinetics, and the localization of biochemical reactions. This paper quantitatively evaluates the possibility that a lipid monolayer can adsorb actin from a nonpolymerizing solution, induce its polymerization, and form a 2D network of individual actin filaments, in conditions that forbid bulk polymerization. G- and F-actin solutions were studied beneath saturated Langmuir monolayers containing phosphatidylcholine (PC, neutral) and stearylamine (SA, a positively charged surfactant) at PC:SA = 3:1 molar ratio. Ellipsometry, tensiometry, shear elastic measurements, electron microscopy, and dark-field light microscopy were used to characterize the adsorption kinetics and the interfacial polymerization of actin. In all cases studied, actin follows a monoexponential reaction-limited adsorption with similar time constants (approximately 10(3) s). At a longer time scale the shear elasticity of the monomeric actin adsorbate increases only in the presence of lipids, to a 2D shear elastic modulus of mu approximately 30 mN/m, indicating the formation of a structure coupled to the monolayer. Electron microscopy shows the formation of a 2D network of actin filaments at the PC:SA surface, and several arguments strongly suggest that this network is indeed causing the observed elasticity. Adsorption of F-actin to PC:SA leads more quickly to a slightly more rigid interface with a modulus of mu approximately 50 mN/m.

Polyamine depletion alters the relationship of F-actin, G-actin, and thymosin beta4 in migrating IEC-6 cells

The cause of reduced migration ability in polyamine-deficient cells is not known, but their actin cytoskeleton is clearly abnormal. We depleted polyamines with alpha-difluoromethylornithine (DFMO) in migrating cells with or without stimulation by epidermal growth factor (EGF) and investigated filamentous (F-) actin, monomeric (G-) actin, and thymosin beta4 (Tbeta4), using immunofluorescent confocal microscopy, DNase assay, and immunoblot analysis. DFMO reduced F-actin in the cell interior, increased it in the cell cortex, redistributed G-actin, and increased nuclear staining of Tbeta4. However, DFMO did not affect the amount of Tbeta4 mRNA. EGF caused a rapid increase in the staining of F-actin in control cells, but DFMO prevented this response to EGF. Despite the visible changes shown by immunocytochemistry, statistically significant changes in the amount of either actin isoform or of total actin did not occur. We propose that DFMO reduces migration by interfering with the sequestration of G-actin by Tbeta4 and the association of F-actin with activated EGF receptors.

Actin dynamics in living mammalian cells

The actin cytoskeleton maintains the cellular architecture and mediates cell movements. To explore actin cytoskeletal dynamics, the enhanced green fluorescent protein (EGFP) was fused to human β-actin. The fusion protein was incorporated into actin fibers which became depolymerized upon cytochalasin B treatment. This functional EGFP-actin construct enabled observation of the actin cytoskeleton in living cells by time lapse fluorescence microscopy. Stable expression of the construct was obtained in mammalian cell lines of different tissue origins. In stationary cells, actin rich, ring-like structured ‘actin clouds’ were observed in addition to stress fibers. These ruffle-like structures were found to be involved in the reorganization of the actin cytoskeleton. In migratory cells, EGFP-actin was found in the advancing lamellipodium. Immobile actin spots developed in the lamellipodium and thin actin fibers formed parallel to the leading edge. Thus EGFP-actin expressed in living cells unveiled structures involved in the dynamics of the actin cytoskeleton.

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

Sorting of beta-actin mRNA and protein to neurites and growth cones in culture

The transport of mRNAs into developing dendrites and axons may be a basic mechanism to localize cytoskeletal proteins to growth cones and influence microfilament organization. Using isoform-specific antibodies and probes for in situ hybridization, we observed distinct localization patterns for beta- and gamma-actin within cultured cerebrocortical neurons. beta-Actin protein was highly enriched within growth cones and filopodia, in contrast to gamma-actin protein, which was distributed uniformly throughout the cell. beta-Actin protein also was shown to be peripherally localized after transfection of beta-actin cDNA bearing an epitope tag. beta-Actin mRNAs were localized more frequently to neuronal processes and growth cones, unlike gamma-actin mRNAs, which were restricted to the cell body. The rapid localization of beta-actin mRNA, but not gamma-actin mRNA, into processes and growth cones could be induced by dibutyryl cAMP treatment. Using high-resolution in situ hybridization and image-processing methods, we showed that the distribution of beta-actin mRNA within growth cones was statistically nonrandom and demonstrated an association with microtubules. beta-Actin mRNAs were detected within minor neurites, axonal processes, and growth cones in the form of spatially distinct granules that colocalized with translational components. Ultrastructural analysis revealed polyribosomes within growth cones that colocalized with cytoskeletal filaments. The transport of beta-actin mRNA into developing neurites may be a sequence-specific mechanism to synthesize cytoskeletal proteins directly within processes and growth cones and would provide an additional means to deliver cytoskeletal proteins over long distances.

Sorting of beta-actin mRNA and protein to neurites and growth cones in culture

The transport of mRNAs into developing dendrites and axons may be a basic mechanism to localize cytoskeletal proteins to growth cones and influence microfilament organization. Using isoform-specific antibodies and probes for in situ hybridization, we observed distinct localization patterns for beta- and gamma-actin within cultured cerebrocortical neurons. beta-Actin protein was highly enriched within growth cones and filopodia, in contrast to gamma-actin protein, which was distributed uniformly throughout the cell. beta-Actin protein also was shown to be peripherally localized after transfection of beta-actin cDNA bearing an epitope tag. beta-Actin mRNAs were localized more frequently to neuronal processes and growth cones, unlike gamma-actin mRNAs, which were restricted to the cell body. The rapid localization of beta-actin mRNA, but not gamma-actin mRNA, into processes and growth cones could be induced by dibutyryl cAMP treatment. Using high-resolution in situ hybridization and image-processing methods, we showed that the distribution of beta-actin mRNA within growth cones was statistically nonrandom and demonstrated an association with microtubules. beta-Actin mRNAs were detected within minor neurites, axonal processes, and growth cones in the form of spatially distinct granules that colocalized with translational components. Ultrastructural analysis revealed polyribosomes within growth cones that colocalized with cytoskeletal filaments. The transport of beta-actin mRNA into developing neurites may be a sequence-specific mechanism to synthesize cytoskeletal proteins directly within processes and growth cones and would provide an additional means to deliver cytoskeletal proteins over long distances.

Nuclear accumulation of G-actin in isolated rat hepatocytes by adenine nucleotides

Extracellular ATP induces bleb formation in isolated rat hepatocytes. We examined the effect of extracellular ATP on the actin cytoskeleton of these hepatocytes. Exposure to 100 microM ATP caused pronounced nuclear accumulation of G-actin. ADP, AMP, adenosine, and dibutyryl-cAMP induced the same effect. Adenosine deaminase could inhibit both ATP- and adenosine-induced nuclear accumulation. The P2-receptor agonists, UTP and 2' & 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate, did not induce this redistribution of G-actin. Phalloidin, which prevents depolymerisation of F-actin filaments to G-actin monomers, inhibited adenosine-induced nuclear accumulation of G-actin. These observations suggest that nuclear accumulation of G-actin is mediated by adenosine receptors.

Microfilament dynamics during cell movement and chemotaxis monitored using a GFP-actin fusion protein

BACKGROUND

The microfilament system in the cortex of highly motile cells, such as neutrophils and cells of the eukaryotic microorganism Dictyostelium discoideum, is subject to rapid re-organization, both spontaneously and in response to external signals. In particular, actin polymerization induced by a gradient of chemoattractant leads to local accumulation of filamentous actin and protrusion of a 'leading edge' of the cell in the direction of the gradient. In order to study the dynamics of actin in these processes, actin was tagged at its amino terminus with green fluorescent protein (GFP) and observed with fluorescence microscopy in living cells of D. discoideum.

RESULTS

Purified GFP-actin was capable of copolymerizing with actin. In the transfected cells of D. discoideum studied, GFP-actin made up 10-20% of the total actin. Microfilaments containing GFP-actin were capable of generating force with myosin in an in vitro assay. Observations of single living cells using fluorescence microscopy showed that the fusion protein was enriched in cell projections, including filopodia and leading edges, and that the fusion protein reflected the dynamics of the microfilament system in cells that were freely moving, being chemotactically stimulated, or aggregated. When confocal sections of fixed cells containing GFP-actin were labeled with fluorescent phalloidin, which binds only to filamentous actin, there was a correlation between the areas of GFP-actin and phalloidin fluorescence, but there were distinct sites in which GFP-actin was more prominent.

CONCLUSIONS

Double labeling with GFP-actin and other probes provides an indication of the various states of actin in motile cells. A major portion of the actin assemblies visualized using GFP-actin are networks or bundles of filamentous actin. Other clusters of GFP-actin might represent stores of monomeric actin in the form of complexes with actin-sequestering proteins.

Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Centractin (Arp1), an actin-related protein, is a component of the dynactin complex. To investigate potential functions of the protein, we used transient transfections to overexpress centractin in mammalian cells. We observed that the overexpressed polypeptide formed filamentous structures that were significantly longer and more variable in length than those observed in the native dynactin complex. The centractin filaments were distinct from conventional actin in subunit composition and pharmacology as demonstrated by the absence of immunoreactivity of these filaments with an actin-specific antibody, by resistance to treatment with the drug cytochalasin D, and by the inability to bind phalloidin. We examined the transfected cells for evidence of specific associations of the novel centractin filaments with cellular organelles or cytoskeletal proteins. Using immunocytochemistry we observed the colocalization of Golgi marker proteins with the centractin polymers. Additional immunocytochemical analysis using antibodies to non-erythroid spectrin (fodrin) and Golgi-spectrin (beta I sigma *) revealed that spectrin colocalized with the centractin filaments in transfected cells. Biochemical assays demonstrated that spectrin was present in dynactin-enriched cellular fractions, was coimmunoprecipitated from rat brain cytosol using antibodies to dynactin subunits, and was coeluted with dynactin using affinity chromatography. Immunoprecipitations and affinity chromatography also revealed that actin is not a bona fide component of dynactin. Our results indicate that spectrin is associated with the dynactin complex. We suggest a model in which dynactin associates with the Golgi through an interaction between the centractin filament of the dynactin complex and a spectrin-linked cytoskeletal network.

Microinjection of ADP-ribosylated actin inhibits actin synthesis in hepatocyte-hepatoma hybrid cells

Treatment of hepatocyte-hepatoma hybrid cells with Clostridium botulinum C2 toxin led to a 167% increase in monomeric globular actin (G-actin) and to a 57% decrease in filamentous actin (F-actin) within 2 h. Simultaneously, the level of actin mRNA was specifically decreased to 49% and actin synthesis was significantly diminished. In contrast, treatment of hybrid cells with phalloidin led to a decrease in G-actin to 55% and to a reciprocal increase in actin mRNA to 244% and an increase in actin synthesis. These alterations of actin synthesis depending on the G-actin/F-actin ratio corresponded to the autoregulation of actin synthesis observed in primary cultures of rat hepatocytes. Microinjection of C2 toxin or of phalloidin into hepatocyte-hepatoma hybrid cells had the same effects on actin synthesis as incubation with either toxin in the culture medium. Microinjection of nonpolymerizable ADP-ribosylated G-actin into hepatocyte-hepatoma hybrid cells specifically decreased the incorporation of [35S]methionine into newly synthesized actin within 1 h. This decrease continued for at least 19 h. Microinjection of ADP-ribosylated actin led to rounding of cells and obvious disaggregation of actin filaments, which might be due to capping of actin filaments by the ADP-ribosylated actin. Because stabilization of actin filaments by phalloidin before microinjection of ADP-ribosylated actin also resulted in decreased actin synthesis, the concentration of monomeric G-actin seems to be responsible for the regulation of actin synthesis in hepatocyte-hepatoma hybrid cells, which can be regarded as immortalized hepatocytes.

Isolation and characterization of two distinct low-density, Triton-insoluble, complexes from porcine lung membranes

The Triton-insoluble complex from porcine lung membranes has been separated into two distinct subfractions visible as discrete light-scattering bands following buoyant density-gradient centrifugation in sucrose. Both of these detergent-insoluble complexes were enriched in the glycosyl-phosphatidylinositol (GPI)-anchored ectoenzymes alkaline phosphatase, aminopeptidase P and 5'-nucleotidase, and both complexes excluded the polypeptide-anchored ectoenzymes angiotensin-converting enzyme, dipeptidyl peptidase IV and aminopeptidases A and N. The GPI-anchored proteins in both complexes were susceptible to release by phosphatidylinositol-specific phospholipase C. Both complexes were also enriched in cholesterol and glycosphingolipids, and in caveolin/VIP21, although only the higher-density fraction was enriched in the plasmalemmal caveolar marker proteins Ca(2+)-ATPase and the inositol 1,4,5-trisphosphate receptor. Among the annexin family of proteins, annexins I and IV were absent from the two detergent-insoluble complexes, annexin V was present in both, and annexins II and VI were only enriched in the higher-density fraction. When the mental chelator EGTA was present in the isolation buffers, annexins II and VI dissociated from the higher-density detergent-insoluble complex and only a single light-scattering band was observed on the sucrose gradient, at the same position as for the lower-density complex. In contrast, in the presence of excess calcium only a single detergent-insoluble complex was isolated from the sucrose gradients, at an intermediate density. Thus the detergent-insoluble membrane complex can be subfractionated on the basis of what appears to be calcium-dependent, annexin-mediated, vesicle aggregation into two distinct populations, only one of which is enriched in plasmalemmal caveolar marker proteins.

The cortical actin cytoskeleton of unactivated zebrafish eggs: spatial organization and distribution of filamentous actin, nonfilamentous actin, and myosin-II

Actin and nonmuscle myosin heavy chain (myosin-II) have been identified and localized in the cortex of unfertilized zebrafish eggs using techniques of SDS-polyacrylamide gel electrophoresis, immunoblotting, and fluorescence microscopy. Whole egg mounts, egg fragments, cryosections, and cortical membrane patches probed with rhodamine phalloidin, fluorescent DNase-I, or anti-actin antibody showed the cortical cytoskeleton to contain two domains of actin: filamentous and nonfilamentous. Filamentous actin was restricted to microplicae and the cytoplasmic face of the plasma membrane where it was organized as an extensive meshwork of interconnecting filaments. The cortical cytoplasm deep to the plasma membrane contained cortical granules and sequestered actin in nonfilamentous form. The cytoplasmic surface (membrane?) of cortical granules displayed an enrichment of nonfilamentous actin. An antibody against human platelet myosin was used to detect myosin-II in whole mounts and egg fragments. Myosin-II colocalized with both filamentous and nonfilamentous actin domains of the cortical cytoskeleton. It was not determined if egg myosin was organized into filaments. Similar to nonfilamentous actin, myosin-II appeared to be concentrated over the surface of cortical granules where staining was in the form of patches and punctate foci. The identification of organized and interconnected domains of filamentous actin, nonfilamentous actin, and myosin-II provides insight into possible functions of these proteins before and after fertilization.

Site-directed mutagenesis enabled preparation of a functional fluorescent analog of profilin: biochemical characterization and localization in living cells

The preparation of fluorescent profilin analogs for binding and spectroscopic studies, in vitro and in vivo, has been hampered by the poor chemical reactivity of this protein in its native form. We have addressed this problem by labeling a mutant, chemically reactive form of profilin. Site-directed mutagenesis was first used to replace a serine residue in a non-essential domain with a reactive cysteine residue. The mutant protein was expressed in Escherichia coli and reacted with tetramethylrhodamine iodoacetamide. In vitro assays indicated that the fluorescent profilin maintained its ability to bind actin, polyproline, and PIP2, to inhibit actin polymerization, and to stimulate actin nucleotide exchange. Fluorescence spectroscopy showed that neither the excitation nor the emission of the analog was sensitive to the interaction with actin or polyproline. However, binding of PIP2 caused a 75% quenching of the fluorescent signal, suggesting a dramatic change in the immediate environment of the probe. When the fluorescent profilin was microinjected into living NRK cells, it became localized at cell-cell junctions and discrete sites near the anterior end, where it colocalized with aggregates of unpolymerized actin. Different engineered forms of profilin with fluorophores located at defined sites should greatly facilitate the study of its interactions with various ligands and cellular structures.

Beta cap73: a novel beta actin-specific binding protein

Whereas actin-binding proteins (ABPs) regulate network formation during the cell cycle, it is not known whether ABPs also function to sequester or target isoactins to specific subcellular compartments. Recently, we have shown that ezrin indirectly associates with beta, but not alpha actin filaments in a calcium- and cytochalasin-sensitive manner [Shuster and Herman, 1995: J. Cell Biol. 128:837-848]. To identify the beta actin-specific binding protein that fosters ezrin-beta actin interactions, we developed an isoactin affinity fractionation and F-isoactin overlay/Western blotting technique. Results reveal that a 73 kd polypeptide that co-precipitates with ezrin and beta actin [Shuster and Herman, 1995: J. Cell Biol. 128:837-848] can also binds directly to filaments of beta, but not alpha actin by isoactin overlay. In an effort to establish whether p73 plays a role in regulating beta actin dynamics in cells, we produced monoclonal antibodies by immunizing BALB/c mice with p73-containing lamellar lysates or high salt elutions from beta actin affinity columns. Two monoclonal antibodies were cloned that react with p73 present in fractions released from beta actin Sepharose-4B or purified to homogeneity by DEAE chromatography. Anti-p73 Western blots reveal that there is a 16-fold difference in p73 binding to beta actin vs. alpha actin affinity columns when experiments are performed in physiological salts. To characterize p73-beta actin binding in vitro and establish whether p73 binds along the lengths or at the barbed end of the beta actin filament, we asked whether cytochalasin D (CD) could displace p73 pre-bound to beta actin-Sepharose 4B. Anti-p73 Western blotting reveals that nanomolar concentrations of CD are capable of selectively eluting p73 and ezrin from beta actin Sepharose 4B, indicating that p73 binds beta actin via the barbed end. Simultaneous double antibody localization studies using anti-beta actin IgG and anti-p73 IgM reveal that p73 and beta actin are co-localized in the forward aspects of motile cytoplasmic domains, in close proximity to the plasma membrane. Because of its isoform-specific interactions with the barbed end of beta actin filaments, we have named this molecule beta cap73. These results indicate that isoform-specific actin-binding proteins can be identified from cortical cytoplasm, and suggest that beta cap73 may not only act to spatially regulate the intracellular distribution of isoactins, but may also facilitate forward protrusion formation through the regulated release of free filament ends during cell motility.

Dynamic localization of RNase MRP RNA in the nucleolus observed by fluorescent RNA cytochemistry in living cells

The dynamic intra-nuclear localization of MRP RNA, the RNA component of the ribonucleoprotein enzyme RNase MRP, was examined in living cells by the method of fluorescent RNA cytochemistry (Wang, J., L.-G. Cao, Y.-L. Wang, and T. Pederson. 1991. Proc. Natl. Acad. Sci. USA. 88:7391-7395). MRP RNA very rapidly accumulated in nucleoli after nuclear microinjection of normal rat kidney (NRK) epithelial cells. Localization was specifically in the dense fibrillar component of the nucleolus, as revealed by immunocytochemistry with a monoclonal antibody against fibrillarin, a known dense fibrillar component protein, as well as by digital optical sectioning microscopy and 3-D stereo reconstruction. When MRP RNA was injected into the cytoplasm it was not imported into the nucleus. Nuclear microinjection of mutant MRP RNAs revealed that nucleolar localization requires a sequence element (nucleotides 23-62) previously implicated as a binding site for a nucleolar protein, the To antigen. These results demonstrate the dynamic localization of MRP RNA in the nucleus and provide important insights into the nucleolar targeting of MRP RNA.

c-Src regulates the simultaneous rearrangement of actin cytoskeleton, p190RhoGAP, and p120RasGAP following epidermal growth factor stimulation

Analysis of C3H10T1/2 murine fibroblasts overexpressing wild type and dominant negative variants of c-Src has demonstrated a requirement for c-Src in EGF-induced mitogenesis. Correlating with the ability of c-Src variants to potentiate or inhibit EGF-dependent DNA synthesis is the phosphotyrosine content of multiple cellular proteins, including p190-RhoGAP, a protein thought to regulate growth factor-induced actin cytoskeleton remodeling by modulating the activity of the small GTP binding protein, Rho. Because the in vivo phosphotyrosine content of p190 varies with the level of active c-Src and not with EGF treatment, p190 is considered to be a preferred substrate of c-Src. To determine whether tyrosyl phosphorylation of p190 (by c-Src) could influence EGF-dependent actin remodeling, we used conventional and confocal immunofluorescence microscopy to examine the intracellular distribution of p190, actin, and p120RasGAP in EGF-stimulated or unstimulated 10T1/2 Neo control cells and cells that stably overexpress wild-type (K+) or kinase-defective (K-) c-Src. We found that in all cell lines, EGF induced a rapid and transient condensation of p190 and RasGAP into cytoplasmic, arclike structures. However, in K+ cells the rate of appearance and number of cells exhibiting arcs increased when compared with control cells. Conversely, K- cells exhibited delayed arc formation and a reduction in number of cells forming arcs. EGF-induced actin stress fiber disassembly and reassembly occurred with the same kinetics and frequency as did p190 and RasGAP rearrangements in all three cell lines. These results, together with the documented Rho-GAP activity intrinsic to p190 and the ability of Rho to modulate actin stress fiber formation, suggest that c-Src regulates EGF-dependent actin cytoskeleton reorganization through phosphorylation of p190.

Rapid TGF beta 1 effects on actin cytoskeleton of astrocytes: comparison with other factors and implications for cell motility

We have previously shown that long-term treatment of primary cultured astrocytes with TGF beta 1 induces morphological changes accompanied by increases in actin and GFAP synthesis, and a profound rearrangement of the cytoskeleton. The present report describes the short-term reorganization of actin filaments induced by TGF beta 1 in rat cerebellum cultured astrocytes and in an astrocytic cell line. TGF beta 1 caused the appearance of new actin and vinculin organizations, without protein synthesis. This cytoskeletal rearrangement was followed by altered cell-cell interactions. All these changes induced by TGF beta 1 were different and slower than those induced by serum, PDGF, and endothelin. TGF beta 1 induced the appearance of lamellipodia, organelles found at the cell front of motile cells in low-density cultures of immortalized astrocytes. These results indicate that the changes in the astrocyte cytoskeleton induced by TGF beta 1 are probably associated with cell movement. The events promoted by TGF beta 1 might help to clarify its action in the brain during embryogenesis and in tissue repair.

Cytoplasmic chaperonin complexes enter neurites developing in vitro and differ in subunit composition within single cells

Chaperonins containing t-complex polypeptide-1 (CCT) are cytosolic molecular chaperone particles implicated especially in the biogenesis of cytoskeletal proteins by promoting the correct folding of the major ubiquitous cytoskeletal components, tubulin and actin. We have purified cytosolic chaperonins from the ND7/23 cell line, determined their subunit composition and examined changes in the intracellular locations of their components during differentiation of ND7/23 cells to a neuronal phenotype by using immunocytochemistry and immunoblots. Chaperonins containing the CCT alpha (TCP1) subunit enter neuritic processes and are particularly noticeable at the leading edge of growth cone-like structures where they co-localise with actin. Chaperonins containing three other components (CCT beta, epsilon and gamma), however, remain predominantly restricted to perikaryal cytoplasm. These findings suggest a heterogeneous population of chaperonin particles within single differentiated ND7/23 cells and this may reflect specialisation of chaperonin function in different cytoplasmic compartments of a neurone. Further, since ribosomes do not enter neurites while CCT alpha-containing chaperonins do, the latter may play roles, subsequent to translation, which influence cytoskeletal elaboration during neuritogenesis.

Actin monomer binding proteins

Small actin monomer binding proteins are essential components of the actin polymerization machinery. Originally thought of as passive buffers that prevent polymerization of actin monomers, recent discoveries elucidate how some actin monomer binding proteins can promote as well as inhibit polymerization, and how they cooperate to regulate actin assembly.

Measurement and manipulation of cytoskeletal dynamics in living cells

A new ear in cell biology is at hand with the development of tools for imaging molecular functions in living cells and tissues. Specific chemical and molecular events can now be measured and manipulated in cells in order to explore the mechanisms of cell functions. In particular, cytoskeletal processes are being dissected temporally and spatially in single cells from lower eukaryotes, plants, and animals using light-based reagents and electronic light microscopy.

Serum-induced signal transduction determines the peripheral location of beta-actin mRNA within the cell

Cell motility is dependent upon the reorganization of the cellular cytoskeleton. Actin filaments form the major component of the cytoskeleton and respond rapidly to serum growth factors. We have previously shown that myoblasts sort the two cytoskeletal beta- and gamma-actin isoform mRNAs to different intracellular regions and that only beta-actin mRNA was associated with peripheral regions of cell motility (Hill, M.A. and P. Gunning. 1993. J. Cell Biol. 122: 825-832). We now show by in situ hybridization that 3T3 fibroblasts similarly sort actin isoform mRNAs and that peripheral beta-actin mRNA is regulated by serum. In the absence of serum, we could not detect beta-actin mRNA at the periphery. Addition of serum rapidly redistributed beta-actin mRNA to the periphery. gamma-actin mRNA distribution was not altered by serum addition at any time. Both proteins, as identified by immunochemistry with isoform-specific antibodies, were found in similar cellular structures. Serum-stimulated cell motility is mediated through the GTPase signal transduction pathway. We find that an RNA-binding protein, p62, that is part of this pathway, displays a localization pattern similar to beta-actin mRNA. Our results suggest a new biological mechanism which integrates signal transduction with the supply of an architectural component required for membrane remodeling. We propose that active transport of beta-actin mRNA to regions of cell motility is one possible objective of these signal transduction pathways.

Actin filament organization in activated mast cells is regulated by heterotrimeric and small GTP-binding proteins

Rat peritoneal mast cells, both intact and permeabilized, have been used widely as model secretory cells. GTP-binding proteins and calcium play a major role in controlling their secretory response. Here we have examined changes in the organization of actin filaments in intact mast cells after activation by compound 48/80, and in permeabilized cells after direct activation of GTP-binding proteins by GTP-gamma-S. In both cases, a centripetal redistribution of cellular F-actin was observed: the content of F-actin was reduced in the cortical region and increased in the cell interior. The overall F-actin content was increased. Using permeabilized cells, we show that AIF4-, an activator of heterotrimeric G proteins, induces the disassembly of F-actin at the cortex, while the appearance of actin filaments in the interior of the cell is dependent on two small GTPases, rho and rac. Rho was found to be responsible for de novo actin polymerization, presumably from a membrane-bound monomeric pool, while rac was required for an entrapment of the released cortical filaments. Thus, a heterotrimeric G-protein and the small GTPases, rho and rac, participate in affecting the changes in the actin cytoskeleton observed after activation of mast cells.

Characterization of caveolin-rich membrane domains isolated from an endothelial-rich source: implications for human disease

Caveolae are 50-100-nm membrane microdomains that represent a subcompartment of the plasma membrane. Previous morphological studies have implicated caveolae in (a) the transcytosis of macromolecules (including LDL and modified LDLs) across capillary endothelial cells, (b) the uptake of small molecules via a process termed potocytosis involving GPI-linked receptor molecules and an unknown anion transport protein, (c) interactions with the actin-based cytoskeleton, and (d) the compartmentalization of certain signaling molecules, including G-protein coupled receptors. Caveolin, a 22-kD integral membrane protein, is an important structural component of caveolae that was first identified as a major v-Src substrate in Rous sarcoma virus transformed cells. This finding initially suggested a relationship between caveolin, transmembrane signaling, and cellular transformation. We have recently developed a procedure for isolating caveolin-rich membrane domains from cultured cells. To facilitate biochemical manipulations, we have applied this procedure to lung tissue--an endothelial and caveolin-rich source-allowing large scale preparation of these complexes. These membrane domains retain approximately 85% of caveolin and approximately 55% of a GPI-linked marker protein, while they exclude > or = 98% of integral plasma membrane protein markers and > or = 99.6% of other organelle-specific membrane markers tested. Characterization of these complexes by micro-sequencing and immuno-blotting reveals known receptors for modified forms of LDL (scavenger receptors: CD 36 and RAGE), multiple GPI-linked proteins, an anion transporter (plasma membrane porin), cytoskeletal elements, and cytoplasmic signaling molecules--including Src-like kinases, hetero-trimeric G-proteins, and three members of the Rap family of small GTPases (Rap 1--the Ras tumor suppressor protein, Rap 2, and TC21). At least a fraction of the actin in these complexes appeared monomeric (G-actin), suggesting that these domains could represent membrane bound sites for microfilament nucleation/assembly during signaling. Given that the majority of these proteins are known molecules, our current studies provide a systematic basis for evaluating these interactions in vivo.

Subtraction scanning acoustic microscopy reveals motility domains in cells in vitro

Scanning acoustic microscopy (SAM) observes all mechanical properties of living cells. Subtraction of the SAM images (SubSAM) of live cells was developed as a method for investigating minimal changes in cellular topography and elasticity. The image formation in the SubSAM takes into account the motion of cell mass as well as the changes of tension. High spatial and temporal resolution of the SubSAM revealed the structure of motile processes that develops at increasing time intervals, thus allowing the arising complexity of motion to be registered and investigated. Independent spots of activity emerge on a quiescent background as motility domains; they may change position, divide, merge, or disappear after a long time interval. In addition, zones of quiescence were identified over central parts of cytoplasmic lamellae. Nonmalignant (Ep: tadpole epidermal cells, XTH2: endothelial cells from tadpole hearts, 3T3 cells) and neoplastic cells (K2 cells of rat fibrosarcoma, A870N cells selected from K2) were investigated with the SubSAM. Three types of domains of subcellular cytoplasmic motility were identified in time series of two-dimensional SubSAM iamges in normal and neoplastic cells. Of them only the wave-like domain is self-evident, being derived from ruffling and protruding activity at the cell margin. Two other domains wait for detailed analysis. The oscillating domain is a visualization of tension within the cell(s), and the nucleating domain indicates intracellular processes possibly preceding locomotion. Differences in motile domains were found between low K2 and high A870N metastatic cells. The dynamics of motility domains of the A870N cells resembled that of the highly motile Ep cells. Cell morphotype and motile activity of the A870N cells are significantly influenced by the pH of the medium. It became evident that identification of the otherwise invisible motile domains in living cells by SubSAM opens a new approach to a characterization of cell motility in vitro and to an understanding of early cellular reactions to various stimuli.