Simultaneous visualization of G- and F-actin in endothelial cells

Depletion of nuclear actin is a key mediator of quiescence in epithelial cells

Functional differentiation is orchestrated by precise growth-regulatory controls conveyed by the tissue microenvironment. Cues from laminin 111 (LN1) lower transcription and suppress mammary epithelial cell growth in culture, but how LN1 induces quiescence is unknown. Recent literature points to involvement of nuclear β-actin in transcriptional regulation. Here, we show that quiescence induced by growth factor withdrawal, or LN1 addition, rapidly decreases nuclear β-actin. LN1, but not other extracellular matrix (ECM) molecules, decreases the levels of nuclear β-actin and destabilizes RNA polymerase (RNA Pol) II and III binding to transcription sites, leading to a dramatic drop in transcription and DNA synthesis. Constitutive overexpression of globular β-actin in the nucleus reverses the effect of LN1 on transcription and RNA Pol II association and prevents the cells from becoming quiescent in the presence of LN1. The physiological relevance of our findings was verified by identifying a clear spatial separation of LN1 and β-actin in developing mammary end buds. These data indicate a novel role for nuclear β-actin in growth arrest of epithelial cells and underscore the importance of the integrity of the basement membrane in homeostasis.

Reorganization of the actin cytoskeleton upon G-protein coupled receptor signaling

The actin cytoskeleton is involved in a multitude of cellular responses besides providing structural support. While the role of the actin cytoskeleton in cellular processes such as trafficking and motility has been extensively studied, reorganization of the actin cytoskeleton upon signaling by G-protein coupled receptors (GPCRs) represents a relatively unexplored area. The G-protein coupled receptor superfamily is an important protein family in mammals, involved in signal transduction across membranes. G-protein coupled receptors act as major signaling hubs and drug targets. The serotonin(1A) receptor is a representative member of the G-protein coupled receptor superfamily and plays a crucial role in the generation and modulation of various cognitive, developmental and behavioral functions. In order to monitor the changes in the actin cytoskeleton upon serotonin(1A) receptor signaling in a quantitative manner, we developed an approach based on high magnification imaging of F-actin in cells, followed by image reconstruction. Our results suggest that the actin cytoskeleton is reorganized in response to serotonin(1A) receptor signaling. In addition, we show that reorganization of the actin cytoskeleton is strongly dependent on adenosine 3',5'-cyclic monophosphate level, and is mediated by the activation of protein kinase A. Our results are consistent with the possibility of a feedback mechanism involving the actin cytoskeleton, adenosine 3',5'-cyclic monophosphate level and the serotonin(1A) receptor.

Early disruption of the actin cytoskeleton in cultured cerebellar granule neurons exposed to 3-morpholinosydnonimine-oxidative stress is linked to alterations of the cytosolic calcium concentration

Cytoskeleton damage is a frequent feature in neuronal cell death and one of the early events in oxidant-induced cell injury. This work addresses whether actin cytoskeleton reorganization is an early event of SIN-1-induced extracellular nitrosative/oxidative stress in cultured cerebellar granule neurons (CGN). The actin polymerization state, i.e. the relative levels of G-/F-actin, was quantitatively assessed by the ratio of the fluorescence intensities of microscopy images obtained from CGN double-labelled with Alexa594-DNase-I (for actin monomers) and Bodipy-FL-phallacidin (for actin filaments). Exposure of CGN to a flux of peroxynitrite as low as 0.5-1μM/min during 30min (achieved with 0.1mM SIN-1) was found to promote alterations of the actin cytoskeleton dynamics as it increases the G-actin/F-actin ratio. Because L-type voltage-operated Ca(2+) channels (L-VOCC) are primary targets in CGN exposed to SIN-1, the possible role of Ca(2+) dynamics on the perturbation of the actin cytoskeleton was also assessed from the cytosolic Ca(2+) concentration response to the L-VOCC's agonist FPL-64176 and to the L-VOCC's blocker nifedipine. The results showed that SIN-1 induced changes in the actin polymerization state correlated with its ability to decrease Ca(2+) influx through L-VOCC. Combined analysis of cytosolic Ca(2+) concentration and G-actin/F-actin ratio alterations by SIN-1, cytochalasin D, latrunculin B and jasplakinolide support that disruption of the actin cytoskeleton is linked to cytosolic calcium concentration changes.

Phosducin and monomeric β-actin have common epitope recognized by anti-phosducin antibodies

Phosducin family proteins are regulators of cytoplasmic processes. The main function ascribed to phosducin is the binding and sequestration of the β subunit of heterotrimeric G proteins. Phosducin-like protein 1, longer than phosducin by 37 amino-acids, is involved in chaperoning of newly synthesized proteins. β-Actin, a component of the cytoskeleton, participates in cell movement. There is no apparent evolutionary relationship between phosducin and β-actin nor structure similarity. Nevertheless we obtained the polyclonal antibodies named ap33, originally directed against a phosducin-derived peptide (SQSLEEDFEGQATHTGPK), that also recognized β-actin. The epitope on the β-actin molecule was characterized. It is a conformational epitope grouping some of the L-D-F-E-Q-A-T-K amino-acids found in the peptide originally used to obtain the antibodies. The main part of the epitope is localized on the actin-actin interface of polymerized actin, so it is accessible only on monomeric actin. The existence of a common epitope on the molecules of phosducin and β-actin may reflect a topological similarity of a small region of their surfaces.

Ostreocin-D impact on globular actin of intact cells

Ostreocin-D, discovered in the past decade, is a marine toxin produced by dinoflagellates. It shares structure with palytoxin, a toxic compound responsible for the seafood intoxication named clupeotoxism. At the cellular level, the action sites and pharmacological effects for ostreocin-D are still almost unknown. Previously, we demonstrated that these toxins change the filamentous actin cytoskeleton, which is essential for multiple cellular functions. However, nothing has yet been reported about what happens with the unpolymerized actin pool. Here (i) the effects induced by ostreocin-D on unpolymerized actin, (ii) the Ca2+ role in such a process, and (iii) the cytotoxic activity of ostreocin-D on the human neuroblastoma BE(2)-M17 cell line are shown for the first time. Fluorescently labeled DNase I was used for staining of monomeric actin prior to detection with both laser-scanning cytometry and confocal microscopy techniques. Cellular viability was tested through a microplate metabolic activity assay. Ostreocin-D elicited a rearrangement of monomeric actin toward the nuclear region. This event was not accompanied by changes in its content. In addition, the presence or absence of external Ca2+ did not change these results. This toxin was also found to cause a decrease in the viability of neuroblastoma cells, which was inhibited by the specific blocker of Na+/K+-ATPase, ouabain. All these responses were comparable to those obtained with palytoxin under identical conditions. The data suggest that ostreocin-D modulates the unassembled actin pool, activating signal transduction pathways not related to Ca2+ influx in the same way as palytoxin.

Manipulating actin dynamics affects human in vitro decidualization

The differentiation of uterine stromal fibroblasts into decidual cells is critical for establishing pregnancy. This process, called decidualization, requires the reorganization of the actin cytoskeleton, which mainly depends on actin dynamics and the phosphorylation status of the myosin light chain. We manipulated actin dynamics with jasplakinolide (100 nM) and latrunculin B (1 microM), both of which significantly inhibited the synthesis of decidualization markers induced by 6 days of treatment with embryo-mimicking stimulus interleukin 1beta (IL1B) and steroid hormones (SHs; 17beta-estradiol and medroxyprogesterone acetate) in the human uterine fibroblast (HuF) in vitro model. However, only jasplakinolide had long-lasting effects on the G-actin:F-actin ratio and prevented decidualization induced by the artificial stimulus cAMP (and SHs). Actin-binding protein cofilin mainly colocalized with G-actin in the nucleus as well as the cytoplasm. Only some spots of colocalization between cofilin and F-actin were detected in the cytoplasm. Brief extraction of cytosolic proteins from living cells revealed that in cells treated with IL1B or cAMP (and SHs) for 6 days, cofilin was mainly detected in the nucleus. The translocation of cofilin from cytosol to nucleus was also detected in HuFs treated for 12 days with SHs, IL1B and SHs, and cAMP and SHs. The same significant translocation was confirmed in primary baboon stromal uterine fibroblasts. We conclude that changes in actin dynamics, particularly the stabilization of F-actin, have a significant negative impact on decidualization, and the translocation of cofilin to the nucleus is a key feature of this process in the primate.

Monitoring membrane rafts in colorectal cancer cells by means of correlative fluorescence electron microscopy (CFEM)

Detergent-resistant membrane (DRM) rafts have been shown to play a pivotal role in regulating key cell biological processes, such as signal transduction, cellular transport and cell survival. The fine structure of membrane rafts are studied using various different imaging approaches and the outcomes are largely dependent on the detection methodology applied. All these microscopy techniques which employ light-, laser- and photon-optics, electrons as well as atomic force probing are characterized on their turn by their strengths and limitations for membrane raft identification. This explains in part the diversity of definitions available to describe these peculiar membrane structures. We present herewith an alternative and uncomplicated microscopy tool to study fluorescently labelled DRMs with information at the transmission electron microscopical level of the same cell, enabling us to obtain a snapshot of the morpho-functional relationships between the cell's interior and DRMs. The proposed approach of correlative fluorescence electron microscopy (CFEM) can therefore be considered as an additional alternative imaging approach to unravel DRM structure-function relationships from micro- to nanometre length scales, from the cell to the molecule.

Acute diabetes moderates trafficking of cardiac lipoprotein lipase through p38 mitogen-activated protein kinase-dependent actin cytoskeleton organization

OBJECTIVE

Heart disease is a leading cause of death in diabetes and could occur because of excessive use of fatty acid for energy generation. Our objective was to determine the mechanisms by which AMP-activated protein kinase (AMPK) augments cardiac lipoprotein lipase (LPL), the enzyme that provides the heart with the majority of its fatty acid.

RESEARCH DESIGN AND METHODS

We used diazoxide in rats to induce hyperglycemia or used 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and thrombin to directly stimulate AMPK and p38 mitogen-activated protein kinase (MAPK), respectively, in cardiomyocytes.

RESULTS

There was a substantial increase in LPL at the coronary lumen following 4 h of diazoxide. In these diabetic animals, phosphorylation of AMPK, p38 MAPK, and heat shock protein (Hsp)25 produced actin cytoskeleton rearrangement to facilitate LPL translocation to the myocyte surface and, eventually, the vascular lumen. AICAR activated AMPK, p38 MAPK, and Hsp25 in a pattern similar to that seen with diabetes. AICAR also appreciably enhanced LPL, an effect reduced by preincubation with the p38 MAPK inhibitor SB202190 or by cytochalasin D, which inhibits actin polymerization. Thrombin activated p38 MAPK in the absence of AMPK phosphorylation. Comparable with diabetes, activation of p38 MAPK and, subsequently, Hsp25 phosphorylation and F-actin polymerization corresponded with an enhanced LPL activity. SB202190 and silencing of p38 MAPK also prevented these effects induced by thrombin and AICAR, respectively.

CONCLUSIONS

We propose that AMPK recruitment of LPL to the cardiomyocyte surface (which embraces p38 MAPK activation and actin cytoskeleton polymerization) represents an immediate compensatory response by the heart to guarantee fatty acid supply when glucose utilization is compromised.

The benzamide derivative N-[1-(7-tert-Butyl-1H-indol-3-ylmethyl)-2-(4-cyclopropanecarbonyl-3-methyl-piperazin-1-yl)-2-oxo-ethyl]-4-nitro-benzamide (SP-10) reduces HIV-1 infectivity in vitro by modifying actin dynamics

Current treatments for patients infected with HIV are suboptimal. There is a need for new HIV therapies that act through different mechanisms than current treatments. We investigated the in vitro efficacy, safety and mechanism of action of the benzamide derivative N-[1-(7-tert-Butyl-1H-indol-3-ylmethyl)-2-(4-cyclopropanecarbonyl-3-methyl-piperazin-1-yl)-2-oxo-ethyl]-4-nitro-benzamide (SP-10), a potential new HIV treatment. When HIV-1-responsive engineered HeLa cells were pre-incubated for 48 h with either SP-10 or zidovudine (AZT), SP-10 was able to inhibit viral replication at much lower concentrations (IC50 = 0.036 nM) than AZT (IC50 = 27.4 nM). In contrast to AZT, SP-10 also inhibited replication of the multidrug-resistant HIV-1 strain MDR-769 in the HeLa cell model. In co-incubation experiments, SP-10 also inhibited the CCR5-sensitive HIV-1 BaL virus replication in human peripheral blood mononuclear cells. SP-10 displayed very low toxicity compared with current antiviral treatments. Confocal laser scanning microscopy and immunoprecipitation studies showed that SP-10 reduced the expression of CD4 and CCR5 on the surface of the host cell. SP-10 also reduced the level of gp120 binding to the cell surface. Confocal laser scanning microscopy studies showed that SP-10 blocked the formation of actin filaments (F-actin) and altered actin accumulation near the cell surface. These promising results suggest that SP-10 has a novel mechanism of action that enables effective inhibition of HIV-1 binding and cell entry. Further development of SP-10 as a new HIV treatment appears warranted.

Distribution of G-actin is related to root hair growth of wheat

BACKGROUND AND AIMS

Actin distribution in root hair tips is a controversial topic. Although the relationship between Ca2+ gradient and actin dynamics in plant tip-growth has been a focus of study, there is still little direct evidence on the exact relationship in root hair tip-growth.

METHODS

G-actin was labelled by fluorescein isothiocyanate-DNase I. F-actin was labelled by tetramethylrhodamine isothiocyanate-phalloidin. Actin in root hairs of Triticum aestivum (wheat) was investigated using confocal laser-scanning microscopy.

KEY RESULTS

Thick F-actin bundles did not extend into a region of approx. 5-10 microm from the tip of the growing root hairs, although they gave off branches of fine actin filaments in the hair tips. A tip-focused G-actin gradient was shown at the extreme apex of growing root hairs. In full-grown wheat root hairs, the tip-focused G-actin gradient disappeared while the thick F-actin bundles extended into the tips. BAPTA-AM, a Ca2+ disruption agent, also caused the tip-focused G-actin gradient to disappear and the diffuse F-actin bundles to appear in the tips of wheat root hairs.

CONCLUSIONS

These results suggest that the tip-focused gradient of intracellular G-actin concentration at the extreme apex may be essential for root hair growth, and that preserving the tip-focused gradient needs a high Ca2+ concentration in the root hair tips.

The effects of collapsing factors on F-actin content and microtubule distribution of Helisoma growth cones

Growth cone collapsing factors induce growth cone collapse or repulsive growth cone turning by interacting with membrane receptors that induce alterations in the growth cone cytoskeleton. A common change induced by collapsing factors in the cytoskeleton of the peripheral domain, the thin lamellopodial area of growth cones, is a decline in the number of radially aligned F-actin bundles that form the core of filopodia. The present study examined whether ML-7, a myosin light chain kinase inhibitor, serotonin, a neurotransmitter and TPA, an activator of protein kinase C, which induce growth cone collapse of Helisoma growth cones, depolymerized or debundled F-actin. We report that these collapsing factors had different effects. ML-7 induced F-actin reorganization consistent with debundling whereas serotonin and TPA predominately depolymerized and possibly debundled F-actin. Additionally, these collapsing factors induced the formation of a dense actin-ring around the central domain, the thicker proximal area of growth cones [Zhou and Cohan, 2001: J. Cell Biol. 153:1071-1083]. The formation of the actin-ring occurred subsequent to the loss of actin bundles. The ML-7-induced actin-ring was found to inhibit microtubule extension into the P-domain. Thus, ML-7, serotonin, and TPA induce growth cone collapse associated with a decline in radially aligned F-actin bundles through at least two mechanisms involving debundling of actin filaments and/or actin depolymerization.

Lysophosphatidic acid-mediated augmentation of cardiomyocyte lipoprotein lipase involves actin cytoskeleton reorganization

The lipoprotein lipase (LPL)-augmenting property of lysophosphatidylcholine requires the formation of lysophosphatidic acid (LPA) (J Mol Cell Cardiol 37: 931-938, 2004). Given that the actin cytoskeleton has been implicated in regulating cardiomyocyte LPL, we examined whether LPL secretion after LPA involves actin cytoskeleton reassembly. Incubation of myocytes with LPA (1-100 nM) increased basal and heparin-releasable LPL (HR-LPL), an effect that was independent of shifts in LPL mRNA. The influence of LPA on myocyte LPL was reflected at the coronary lumen, with substantial increases of the enzyme at this location. Incubation of myocytes with cytochalasin D not only blocked LPA-induced augmentation of HR-LPL but also abrogated filamentous actin formation. These effects of LPA were likely receptor mediated. Exposure of myocytes to LPA facilitated significant membrane translocation of RhoA and its downstream effector Rho kinase I (ROCK I), and blocking this effect with Y-27632 appreciably reduced basal and HR-LPL activity. Incubation of adipose tissue with LPA also significantly enhanced basal and HR-LPL activity, suggesting that sarcomeric actin likely has a limited role in influencing the LPL secretory function of LPA in the myocyte. Comparable to LPA, hyperglycemia also caused significant membrane translocation of RhoA and ROCK I in hearts isolated from diazoxide-treated animals, effects that were abrogated using insulin. Overall, our data suggest that comparable to hyperglycemia, LPA-induced increases in cardiac LPL occurred via posttranscriptional mechanisms and processes that likely required RhoA activation and actin polymerization. Whether this increase in LPL augments triglyceride deposition in the heart leading to eventual impairment in contractile function is currently unknown.

Flow cytometric screening of aldolase catalytic antibodies

High-throughput screening of cells expressing active catalytic antibody clones by flow cytometry is described. A fluorogenic retro-aldol retro-Michael substrate was designed and synthesized with incorporation of a chloromethyl moiety for intracellular retention. Hybridoma or transfected mammalian cells expressing catalytic antibody molecules could be rapidly isolated.

Decreased uptake of bodipy-labelled compounds in the presence of the nuclear stain, DRAQ5

We have found the nuclear stain DRAQ5 to decrease the cellular uptake of a series of boron dipyrromethane (bodipy)-labelled compounds. This phenomenon is consistent between Lysotracker Green DND 26, Lysotracker Red DND 99 and bodipy-labelled mycolactone. Although DRAQ5 uptake was not prevented, DRAQ5 was in significant excess in each case. As the effect is consistent among two cell types, RAW 264.7 monocyte/macrophages and Bend 3 endothelial cells, we hypothesize that it may be a result of the two dyes complexing in solution into a form that is not taken up by the cells. This hypothesis is confirmed by fluorescence resonance energy transfer analysis in solution.

Distribution and regulation of expression of serum- and glucocorticoid-induced kinase-1 in the rat kidney

The serum- and glucocorticoid-induced kinase-1 (sgk1) increases the activity of a number of epithelial ion channels and transporters. The present study examines the distribution and subcellular localization of sgk1 protein in the rat kidney and the regulation of levels of expression induced by steroids. The results indicate that the kidney expresses predominantly the sgk1 isoform with a distribution restricted to the thick ascending limb of Henle, distal convoluted, connecting and cortical collecting tubules. Within cells, sgk1 strongly associates with the microsomal fraction of homogenates and it colocalizes with the Na+,K+-ATPase to the basolateral membrane. Analysis of the levels of expression of sgk1 by Western blotting and immunohistochemistry indicates constitutive high expression under basal conditions. Approximately half of the basal level is maintained by glucocorticoids whereas physiological fluctuations of aldosterone produce minor changes in sgk1 abundance in adrenal-intact animals. These results do not support the notion that physiological changes of aldosterone concentration turn the expression of sgk1 'on and off' in the mammalian kidney. Additionally, localization of sgk1 to the basolateral membrane indicates that the effects mediated by sgk1 do not require a direct interaction with the ion channels and transporters whose activity is modulated, since most of these proteins are located in the apical membrane of renal epithelial cells.

Antimycin A-induced defenestration in rat hepatic sinusoidal endothelial cells

Liver sinusoidal endothelial cells (LSECs) possess fenestrae arranged in sieve plates. Hepatic endothelial fenestrae are open pores approximately 100 to 200 nm in diameter. Alterations in their number or diameter by hormones, xenobiotics, and diseases have important implications for hepatic microcirculation and function. Numerous reports of hepatotoxin-induced defenestration suggest that the cytoskeleton and the energy status of hepatic endothelial cells play a key role in the regulation of fenestrae. Therefore, we investigated the effect of antimycin A, an inhibitor of mitochondrial energy production, on the number of fenestrae in cultured LSECs using high-resolution microscopy and immunocytochemistry. Prolonged incubation (greater than 30 min) with antimycin A resulted in defenestrated cells and coincided with the appearance of F-actin dots, whereas the distribution of G-actin remained unchanged. Adenosine triphosphate (ATP) was depleted dramatically to less than 5% within 30 minutes within the LSECs. After treatment with antimycin A, unusual elevated fenestrated complexes were apparent, organized as a meshwork of anastomosing fenestrae at the center of and above the sieve plates. The position and appearance of these novel structures and their association with defenestration suggest that they are implicated in the process of defenestration. In conclusion, the results of experiments with antimycin A suggest that ATP is needed to maintain fenestrae and the underlying fenestrae-associated cytoskeleton rings that maintain fenestrae patency. Antimycin A-induced defenestration of LSECs is associated with the development of a structure in the sieve plate that appears to be intrinsically involved in defenestration.

The actin cytoskeleton and cytotoxic T lymphocytes: evidence for multiple roles that could affect granule exocytosis-dependent target cell killing

One important mechanism cytotoxic T lymphocytes (CTLs) use to kill virus-infected, transplanted or tumour targets is exocytosis of granules that contain cytotoxic agents such as perforin and granzymes. Granule exocytosis-dependent target cell killing is a complex process, involving initial T-cell receptor (TCR)-dependent signalling that includes Ca2+ influx and activation of protein kinase C, shape changes that serve to bind the CTL to the target and, finally, exocytosis of lytic granules at the site of contact with the target cell. Although there is reason to propose that multiple steps in the lytic process could involve the actin cytoskeleton of CTLs, few studies have examined this issue, and those that have do not allow the specific step(s) involved to be determined. We have used the potent membrane-permeant actin cytoskeleton-modifying drugs jasplakinolide and latrunculin A to investigate the actin dependence of defined processes that are expected to be important for granule exocytosis-dependent killing. Our results, obtained using TALL-104 human leukaemic CTLs as a model system, are consistent with the idea that a functional actin cytoskeleton is required for TCR/CD3-dependent signalling, for activation of store-dependent Ca2+ influx and for CTL shape changes. When cells were stimulated with solid-phase anti-CD3 antibodies, treatment with either jasplakinolide or latrunculin A abolished granule exocytosis. However, when cells were stimulated in a manner that bypasses TCR/CD3-dependent signalling, granule exocytosis was not significantly altered, suggesting that the actin cytoskeleton does not function as a barrier to exocytosis.

Use of fluorescently labelled deoxyribonuclease I to spatially measure G-actin levels in migrating and non-migrating cells

Lamellipodium protrusion is linked to actin filament disassembly in migrating fibroblasts [Cramer, 1999: Curr. Biol. 9:1095-1105]. To further study this relationship, we have identified a method to specifically and sensitively detect G-actin in distinct spatial locations in motile cells using deoxyribonuclease I (DNase I). Although DNase I can bind both G- and F-actin in vitro [Mannherz et al., 1980: Eur. J. Biochem. 95:377-385], when cells were fixed in formaldehyde and permeabilized in detergent, fluorescently-labelled DNase I specifically stained G-actin and not F-actin. 92-98% of actin molecules were stably retained in cells during fixation and permeabilization. Further, increasing or decreasing cellular G-actin concentration by treating live cells with latrunculin-A or jasplakinolide, respectively, caused a respective increase and decrease in DNase I cell-staining intensity as expected. These changes in DNase I fluorescence intensity accurately reflected increases and decreases in cellular G-actin concentration independently measured in lysates prepared from drug-treated live cells (regression coefficient = 0.98). This shows that DNase I cell-staining is very sensitive using this method. Applying this method, we found that the ratio of G-/F-actin is lower in both the lamellipodium and in a broad band immediately behind the lamellipodium in migrating compared to non-migrating fibroblasts. Thus, we predict that protrusion of the lamellipodium in migrating fibroblasts requires tight coupling to filament disassembly at least in part because G-actin is relatively limited within and behind the lamellipodium. This is the first report to directly demonstrate high sensitivity of cell-staining for any G-actin probe and this, together with the ready commercial accessibility of fluorescently-labelled DNase I, make it a simple, convenient, and sensitive tool for cell-staining of G-actin.

Threshold duration of ischemia for myogenic tone in middle cerebral arteries: effect on vascular smooth muscle actin

BACKGROUND AND PURPOSE

We investigated the effect of different periods of ischemia on the myogenic tone of middle cerebral arteries (MCAs) and tested the hypothesis that ischemia disrupts the actin cytoskeleton in vascular smooth muscle.

METHODS

The MCA occlusion model was used in male Wistar rats (n=27) to induce different periods of ischemia (15, 30, and 120 minutes) with 24 hours of reperfusion. Successful occlusion was determined by laser-Doppler flowmetry. MCAs were then studied in vitro with a specialized arteriograph system that allowed control of transmural pressure and measurement of lumen diameter. After equilibration for 1 hour at transmural pressure of 75 mm Hg, lumen diameter was measured, and the amount of spontaneous myogenic tone was determined. Arteries were then fixed with 10% formalin while still pressurized in the arteriograph bath and stained for filamentous (F-) actin with fluorescently labeled phalloidin, a specific probe for F-actin. The amount of F-actin was quantified by confocal microscopy.

RESULTS

The amount of tone was similar between control and 15 minutes of ischemia (27.0+/-2.0% and 25.3+/-1.7%, respectively; P>0.05) but was significantly diminished after 30 and 120 minutes (11.7+/-2.0% and 8.5+/-2.0%, respectively; P<0.01 versus control). F-actin content also decreased at the longer ischemic periods and correlated significantly with vascular tone (P=0.04) such that the lesser the tone, the lesser was the F-actin content. Fluorescence intensity for control and 15, 30, and 120 minutes of ischemia was (x10(7)) 3.21+/-0.25, 2.54+/-0.32 (P>0.05), 2.32+/-0.15 (P<0.01), and 2.22+/-0.16 (P<0.01), respectively.

CONCLUSIONS

These results demonstrate that ischemia disrupts the actin cytoskeleton in smooth muscle and diminishes vascular tone of MCAs in a threshold-dependent manner. This effect likely exacerbates brain tissue damage during stroke, including infarction and edema formation.

Use of green fluorescent protein-conjugated beta-actin as a novel molecular marker for in vitro tumor cell chemotaxis assay

To study the dynamics of actin cytoskeleton rearrangement in living cells, an eukaryotic expression vector expressing a beta-actin-GFP fusion protein was generated. The expression construct when transfected into NIH3T3 fibroblast, A2058 human melanoma and 293T human embryonic kidney carcinoma cell lines expressed beta-actin-GFP fusion protein, which colocalized with endogenous cellular actin as determined by histoimmunofluorescence staining. The beta-actin-GFP was also observed to be reorganized in response to treatments with the chemoattractant type IV collagen. Cells extended pseudopodial protrusions and altered the morphology of their cortical structure in response to type IV collagen stimulation. More importantly, beta-actin-GFP accumulated in areas undergoing these dynamic cytoskeleton changes, indicating that beta-actin-GFP could participate in actin polymerization. Although ectopic expression of beta-actin-GFP lead to minor side effects on cell proliferation, these studies suggest that this strategy provides an alternative to the invasive techniques currently used to study actin dynamics and permits real-time visualization of actin rearrangements in response to environmental cues.

Localization of actobindin, profilin I, profilin II, and phosphatidylinositol-4,5-bisphosphate (PIP2) in Acanthamoeba castellanii

Specific polyclonal antisera were raised against purified Acanthamoeba actobindin and synthetic peptides corresponding to regions of maximum charge differences in Acanthamoeba profilin I and profilin II. Immunofluorescence studies with these antibodies showed profilin I to be distributed throughout the Acanthamoeba cytoplasm, except for lamellipodia, with the highest fluorescence intensity in cortical regions in which monomeric actin also was present, as shown by labeling with fluorescent DNase. In contrast, profilin II appeared to be uniformly associated with the plasma membrane except at sites of pseudopod extension, where the concentration was frequently decreased, in addition to cortical regions. Immunofluorescence studies using a monoclonal antibody specific for phosphatidylinositol-4,5-bisphosphate (PIP2) suggested that its distribution is mostly limited to the plasma membrane. In contrast to the distribution of profilin II, PIP2 immunofluorescence was prominent at the leading edge of cells, including the plasma membrane of lamellipodia. Quantitative immunoelectron microscopy showed that profilin II was approximately 36 times more likely to localize to the plasma membrane than profilin I. Immunofluorescence and confocal microscopy localized actobindin to the base of lamellipodia. The differential localization of the three actin monomer-binding proteins suggests that they have different biologic functions in Acanthamoeba and is consistent with the hypotheses that (1) profilin I functions predominantly as an actin monomer-binding protein; (2) profilin II regulates, or is regulated by, PIP2; and (3) actobindin inhibits nucleation of new filaments and facilitates elongation of existing polarized filaments in actively motile regions.

Function of the cytoskeleton in human neutrophils and methods for evaluation

The cytoskeleton plays a critical role in the determination of cell shape and serves as a scaffold for critical cellular enzymes and adhesion molecules. It provides structural integrity for the cell and regulates the function of many biochemical events that are critical to cellular function. The microfilamentous cytoskeleton participates in force generation necessary for shape change and motion. In neutrophils and other motile cells, polymerization of actin likely drives extension of the lamellae and participates in force generation through interaction with myosin, by polymerization alone and by osmotic mechanisms. Here, we will focus on the microfilamentous cytoskeleton in the neutrophil and briefly review its function as well as some direct and indirect methods that have been used to asses its role in neutrophil function. The discussion will address general approaches and leaves the details of the methods to the references.

Actin polymerization enhances Pasteurella haemolytica leukotoxicity

Pasteurella haemolytica leukotoxin is cytotoxic to bovine leukocytes, causing increased cell membrane permeability, osmotic swelling, release of cytosolic proteins and cell lysis. These studies were designed to test if leukotoxin causes release of the cytoskeletal protein, actin, from bovine leukemia cells and if purified actin-influenced bacterial growth or leukotoxin production. Culture supernatants caused a 7-fold decrease in viability of bovine leukemia cells and increased cell permeability that was accompanied by release of beta-actin into the cell culture supernatant. Exposing P. haemolytica to purified actin solutions induced the conversion of monomeric G-actin to polymerized F-actin. This conversion was partially inhibited by bovine P. haemolytica immune, but not pre-immune, serum. Loss of streptomycin resistance following treatment of the organism with acridine orange ablated the polymerizing activity. Incubation of P. haemolytica in the presence of purified F-actin did not affect growth but resulted in culture supernatant that had 3.0-3.9-fold greater leukotoxicity compared to medium alone or medium containing G-actin, heat-denatured actin or albumin. The effect of actin on leukotoxicity was concentration-dependent and directly associated with increases in secreted leukotoxin. The interaction between P. haemolytica and actin is potentially detrimental to the host by inducing polymerization of actin into insoluble filaments and by enhancing leukotoxicity.

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.

Osteocyte shape is dependent on actin filaments and osteocyte processes are unique actin-rich projections

Osteocytes are derived from a select group of osteoblasts that have undergone a final differentiation. Due to their inaccessibility when embedded in the bone matrix, very little is known about the osteocyte cytoskeleton. This study provides an extensive analysis of the osteocyte cytoskeleton, based on the successful isolation of osteocytes from 16-day embryonic chick calvariae. We used OB7.3, a chicken osteocyte-specific monoclonal antibody, to confirm the osteocytic phenotype of the isolated cells and established culture conditions to promote growth of cells that most resemble osteocytes in vivo. Immunofluorescence staining with antitubulin, antivimentin, and antiactin showed the relative distribution of the microtubules, intermediate filaments, and actin filaments in both osteocyte cell body and processes. Field emission scanning electron microscopy revealed the three-dimensional relationships of the cytoskeletal elements and a unique organization of actin bundles that spanned the cell body and osteocyte processes. When combined with drug studies, these experiments demonstrate that actin filaments are crucial for the maintenance of osteocyte shape. Furthermore, we identified two actin-bundling proteins, alpha-actinin and fimbrin, in osteocyte processes. The prominence and unique distribution of fimbrin in osteocyte processes provides the possibility of its use as an intracellular marker to distinguish osteocytes from osteoblasts.

SCAR, a WASP-related protein, isolated as a suppressor of receptor defects in late Dictyostelium development

G protein-coupled receptors trigger the reorganization of the actin cytoskeleton in many cell types, but the steps in this signal transduction cascade are poorly understood. During Dictyostelium development, extracellular cAMP functions as a chemoattractant and morphogenetic signal that is transduced via a family of G protein-coupled receptors, the cARs. In a strain where the cAR2 receptor gene is disrupted by homologous recombination, the developmental program arrests before tip formation. In a genetic screen for suppressors of this phenotype, a gene encoding a protein related to the Wiskott-Aldrich Syndrome protein was discovered. Loss of this protein, which we call SCAR (suppressor of cAR), restores tip formation and most later development to cAR2(-) strains, and causes a multiple-tip phenotype in a cAR2(+) strain as well as leading to the production of extremely small cells in suspension culture. SCAR-cells have reduced levels of F-actin staining during vegetative growth, and abnormal cell morphology and actin distribution during chemotaxis. Uncharacterized homologues of SCAR have also been identified in humans, mouse, Caenorhabditis elegans, and Drosophila. These data suggest that SCAR may be a conserved negative regulator of G protein-coupled signaling, and that it plays an important role in regulating the actin cytoskeleton.

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.

N-methyl-D-aspartate evokes rapid net depolymerization of filamentous actin in cultured rat cerebellar granule cells

Filamentous actin (F-actin) was measured in cultured rat cerebellum granule neurons with the use of fluorescently labeled phallotoxin as a site-specific probe for F-actin, and fluorescence microscopy. The averaged apparent intensity of soma-associated F-actin-derived fluorescence (F(app)) was measured from fixed cells after incubation in either 1) normal Krebs solution containing 2 mM extracellular calcium ([Ca2+]ex) or 2) normal Krebs solution plus N-methyl-D-aspartate (NMDA) for 2 min immediately before fixation. NMDA (10, 50, and 100 microM) decreased F(app) to 63 +/- 5% (mean +/- SE), 53 +/- 4%, and 47 +/- 2%, respectively, of that measured from control cells. This effect was mimicked by treatment of cells with ionomycin. The ability of NMDA to reduce the F(app) in the presence of [Ca2+]ex was abolished when cells were maintained in [Ca2+]ex-free medium. Cells first treated with NMDA for 2 min and then left in normal medium for 30 min before fixation gave F(app) fluorescence similar to control values (91 +/- 12%). However, if the F-actin polymerization inhibitor cytochalasin D was added to cells immediately after NMDA was removed, the F(app) did not recover with time (36 +/- 3%). Cells treated for 30 min with cytochalasin D alone showed a small reduction in staining (approximately 20%). It is concluded that the actin polymerization state of rat cerebellar granule neurons is sensitive to changes in intracellular calcium, and that NMDA receptor activation evokes an initial rapid depolymerization of F-actin.

Texas Res-X and rhodamine Red-X, new derivatives of sulforhodamine 101 and lissamine rhodamine B with improved labeling and fluorescence properties

Texas Red sulfonyl chloride (TR-SC) and Lissamine rhodamine B sulfonyl chloride (LRB-SC) are popular dyes often used to prepare red fluorescent conjugates that are useful second labels in combination with fluorescein. Unfortunately, being sulfonyl chloride derivatives, both are unstable to moisture during storage and prone to hydrolysis in the conjugation reaction. Their instability causes the percentage of reactive dye to vary from lot to lot and requires use of low temperatures and a relatively high pH to optimize conjugation efficiency. Succinimidyl esters of the aminohexanoic acid sulfonamides of both dyes have been prepared, which are designated Texas Red-X succinimidyl ester (TR-X-SE) and Rhodamine Red-X succinimidyl ester, respectively. Their spectral properties are similar to those of their sulfonyl chloride analogs; moreover, incorporation of the succinimidyl ester at the end of the aliphatic chain spacer facilitates conjugation, decreases precipitation of proteins during conjugation and storage, and usually increases the fluorescence yield of the conjugate. Comparison of the rate of hydrolysis of TR-SC with that of TR-X-SE shows that, while the former was completely hydrolyzed within 5 min by exposure to water, TR-X-SE retains most of its reactivity for more than an hour. The reactivity of both new derivatives is high between pH 7.5 and 8.5, allowing conjugation of proteins that do not tolerate the high pH required for reaction with sulfonyl chlorides. In addition, Texas Red maleimides and haloacetamides containing spacer groups were prepared for labeling sulfhydryl groups. A Texas Red-X derivative of phalloidin has also been prepared, and its use for labeling F-actin has been characterized.

Cytoskeletal changes during neurogenesis in cultures of avain neural crest cells

Neural crest cells are motile and mitotic, whereas their neuronal derivatives are terminally post-mitotic and consist of stationary cell body from which processes grow. The present study documents changes in the cytoskeleton that occur during neurogenesis in cultures of avain neural crest cells. The undifferentiated neural crest cells contain dense bundles of actin filaments throughout their cytoplasm, and a splayed array of microtubules attached to the centrosome. In newly differentiating neurons, the actin bundles are disrupted and most of the remaining actin filaments are reorganized into a cortical layer underlying the plasma membrane of the cell body and processes. Microtubules are more abundant in newly-differentiating neurons than in the undifferentiated cells, and individual microtubules can be seen dissociated from the centrosome. Neuron-specific beta-III tubulin appears in some crest cells prior to cessation of motility and cell division, and expression increases with total microtubule levels during neurogenesis. To investigate how these early cytoskeletal changes might contribute to alterations in morphology during neurogenesis, we have disrupted the cytoskeleton with pharmacologic agents. Microfilament disruption by cytochalasin immediately arrests the movement of neural crest cells and causes them to round-up, but does not significantly change the morphology of the immature neurons. Microtubule depolymerization by nocodazole slows the movement of undifferentiated cells and causes retraction of processes extended by the immature neurons. These results suggest that changes in the actin and microtubule arrays within neural crest cells govern distinct aspects of their morphogenesis into neurons.

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.