Tumor promoter induces reorganization of actin filaments and calspectin (fodrin or nonerythroid spectrin) in 3T3 cells

Expanding Insights: Harnessing Expansion Microscopy for Super-Resolution Analysis of HIV-1-Cell Interactions

Expansion microscopy has recently emerged as an alternative technique for achieving high-resolution imaging of biological structures. Improvements in resolution are achieved by physically expanding samples through embedding in a swellable hydrogel before microscopy. However, expansion microscopy has been rarely used in the field of virology. Here, we evaluate and characterize the ultrastructure expansion microscopy (U-ExM) protocol, which facilitates approximately four-fold sample expansion, enabling the visualization of different post-entry stages of the HIV-1 life cycle, focusing on nuclear events. Our findings demonstrate that U-ExM provides robust sample expansion and preservation across different cell types, including cell-culture-adapted and primary CD4+ T-cells as well as monocyte-derived macrophages, which are known HIV-1 reservoirs. Notably, cellular targets such as nuclear bodies and the chromatin landscape remain well preserved after expansion, allowing for detailed investigation of HIV-1-cell interactions at high resolution. Our data indicate that morphologically distinct HIV-1 capsid assemblies can be differentiated within the nuclei of infected cells and that U-ExM enables detection of targets that are masked in commonly used immunofluorescence protocols. In conclusion, we advocate for U-ExM as a valuable new tool for studying virus-host interactions with enhanced spatial resolution.

Genome sequence and cell biological toolbox of the highly regenerative, coenocytic green feather alga Bryopsis

Green feather algae (Bryopsidales) undergo a unique life cycle in which a single cell repeatedly executes nuclear division without cytokinesis, resulting in the development of a thallus (>100 mm) with characteristic morphology called coenocyte. Bryopsis is a representative coenocytic alga that has exceptionally high regeneration ability: extruded cytoplasm aggregates rapidly in seawater, leading to the formation of protoplasts. However, the genetic basis of the unique cell biology of Bryopsis remains poorly understood. Here, we present a high-quality assembly and annotation of the nuclear genome of Bryopsis sp. (90.7 Mbp, 27 contigs, N50 = 6.7 Mbp, 14 034 protein-coding genes). Comparative genomic analyses indicate that the genes encoding BPL-1/Bryohealin, the aggregation-promoting lectin, are heavily duplicated in Bryopsis, whereas homologous genes are absent in other ulvophyceans, suggesting the basis of regeneration capability of Bryopsis. Bryopsis sp. possesses >30 kinesins but only a single myosin, which differs from other green algae that have multiple types of myosin genes. Consistent with this biased motor toolkit, we observed that the bidirectional motility of chloroplasts in the cytoplasm was dependent on microtubules but not actin in Bryopsis sp. Most genes required for cytokinesis in plants are present in Bryopsis, including those in the SNARE or kinesin superfamily. Nevertheless, a kinesin crucial for cytokinesis initiation in plants (NACK/Kinesin-7II) is hardly expressed in the coenocytic part of the thallus, possibly underlying the lack of cytokinesis in this portion. The present genome sequence lays the foundation for experimental biology in coenocytic macroalgae.

Culture Isolate of Rickettsia felis from a Tick

Although the cat flea, Ctenocephalides felis, has been identified as the primary vector of Rickettsia felis, additional flea, tick, mite, and louse species have also been associated with this bacterium by molecular means; however, the role of these arthropods in the transmission of R. felis has not been clarified. Here, we succeeded in culture isolation of R. felis from a host-seeking castor bean tick, Ixodes ricinus, the most common tick in Slovakia. The bacterial isolation was performed on XTC-2 cells at 28 °C using the shell-vial technique. An evaluation of the growth properties was performed for both the XTC-2 and Vero cell lines. We observed R. felis in the infected host cells microscopically by Gimenez staining and immunofluorescence assay. The R. felis isolate was purified by gradient ultracentrifugation and visualized by electron microscopy. Fragments of the genes gltA, ompA, ompB, htrA, rpoB, sca4, rffE, and rrs were amplified and compared with the corresponding sequences of the type strain URRWXCal2 and other R. felis culture -isolated strains. We did not detect any nucleotide polymorphisms; however, plasmid pRFδ, characteristic of the standard strain, was absent in our isolate. Herein, we describe the first successful isolation and characterization of a tick-derived R. felis strain “Danube”, obtained from an I. ricinus nymph.

Optimized fixation of actin filaments for improved indirect immunofluorescence staining of rickettsiae

Objective

The objective was to investigate fixative solutions: 3.7% formaldehyde, 4% paraformaldehyde, 4% paraformaldehyde in the cytoskeletal buffer and 4% paraformaldehyde in PHEM buffer (containing PIPES, HEPES, EGTA and MgCl₂), applicable for immunofluorescence assay.

Results

Herein we optimized this serological technique, testing four fixative solutions, for the sensitive detection of rickettsial antigens, and preservation of intracellular structures of the host cells, particularly filamentous actin. Rickettsial antigens were presented equally well both with formaldehyde and all paraformaldehyde-based fixations, but only protocol with 4% paraformaldehyde in PHEM buffer allowed accurate imaging of actin filaments, and simultaneously allows monitoring of rickettsiae using actin-based motility during infection inside the host cells.

Maximizing the immunological output of the cervicovaginal explant model

In the field of sexually transmitted infections (STI), the cervicovaginal explant (CVEx) model, not only provides the opportunity to study the different immunological arms present in these tissues under steady state conditions, but also their response against ex vivo infection with relevant pathogens. The methodology associated to the establishment of the HIV infection model in the cervicovaginal tissue was described in detail by Grivel et al. earlier (Grivel and Margolis, 2009). With this model as a foundation, we illustrate different approaches to obtain a large number of immunological readouts from a single piece of tissue, thus maximizing the immunological output obtained. Additionally, we discuss several ideas to study some of the immunological subsets present in this mucosal tissue by enriching them with the addition of distinct chemokines or specifically inducing their activation. Importantly, most of the methodology and concepts proposed here can be applied to study the immune subsets resident in other tissues. In the field of mucosal immunology, the possibility of studying resident immune subsets from tissue explants offers a great opportunity to understand the real players against invading pathogens and localized pathologies. Furthermore, this model allows for addressing the therapeutic benefit of modulating the activity of certain molecules and immune subsets against invading pathogens, which may infer their contribution to pathogen control and direct novel therapeutic interventions.

A novel immuno-gold labeling protocol for nanobody-based detection of HER2 in breast cancer cells using immuno-electron microscopy

Immuno-electron microscopy is commonly performed with the use of antibodies. In the last decade the antibody fragment indicated as nanobody (VHH or single domain antibody) has found its way to different applications previously done with conventional antibodies. Nanobodies can be selected to bind with high affinity and specificity to different antigens. They are small (molecular weight ca. 15kDa) and are usually easy to produce in microorganisms. Here we have evaluated the feasibility of a nanobody binding to HER2 for application in immuno-electron microscopy. To obtain highest labeling efficiency combined with optimal specificity, different labeling conditions were analysed, which included nanobody concentration, fixation and blocking conditions. The obtained optimal protocol was applied for post-embedment labeling of Tokuyasu cryosections and for pre-embedment labeling of HER2 for fluorescence microscopy and both transmission and scanning electron microscopy. We show that formaldehyde fixation after incubation with the anti-HER2 nanobody, improves labeling intensity. Among all tested blocking agents the best results were obtained with a mixture of cold water fish gelatine and acetylated bovine serum albumin, which prevented a-specific interactions causing background labeling while preserving specific interactions at the same time. In conclusion, we have developed a nanobody-based protocol for immuno-gold labeling of HER2 for Tokuyasu cryosections in TEM as well as for pre-embedment gold labeling of cells for both TEM and SEM.

Aortic carboxypeptidase-like protein (ACLP) enhances lung myofibroblast differentiation through transforming growth factor β receptor-dependent and -independent pathways

Idiopathic pulmonary fibrosis (IPF) is a chronic and fatal lung disease characterized by the overgrowth, hardening, and scarring of lung tissue. The exact mechanisms of how IPF develops and progresses are unknown. IPF is characterized by extracellular matrix remodeling and accumulation of active TGFβ, which promotes collagen expression and the differentiation of smooth muscle α-actin (SMA)-positive myofibroblasts. Aortic carboxypeptidase-like protein (ACLP) is an extracellular matrix protein secreted by fibroblasts and myofibroblasts and is expressed in fibrotic human lung tissue and in mice with bleomycin-induced fibrosis. Importantly, ACLP knockout mice are significantly protected from bleomycin-induced fibrosis. The goal of this study was to identify the mechanisms of ACLP action on fibroblast differentiation. As primary lung fibroblasts differentiated into myofibroblasts, ACLP expression preceded SMA and collagen expression. Recombinant ACLP induced SMA and collagen expression in mouse and human lung fibroblasts. Knockdown of ACLP slowed the fibroblast-to-myofibroblast transition and partially reverted differentiated myofibroblasts by reducing SMA expression. We hypothesized that ACLP stimulates myofibroblast formation partly through activating TGFβ signaling. Treatment of fibroblasts with recombinant ACLP induced phosphorylation and nuclear translocation of Smad3. This phosphorylation and induction of SMA was dependent on TGFβ receptor binding and kinase activity. ACLP-induced collagen expression was independent of interaction with the TGFβ receptor. These findings indicate that ACLP stimulates the fibroblast-to-myofibroblast transition by promoting SMA expression via TGFβ signaling and promoting collagen expression through a TGFβ receptor-independent pathway.

Protein kinase C mediated inhibition of endothelial L-arginine transport is mediated by MARCKS protein

The endothelium plays a vital role in the maintenance of vascular tone and structural vascular integrity, principally mediated via the actions of nitric oxide (NO). L-arginine is the immediate substrate for NO synthesis, and the availability of extracellular L-arginine is critical for the production of NO. Activation of protein kinase C (PKC) dependent signalling pathways are a feature of a number of cardiovascular disease states, and in this study we aimed to systematically evaluate the mechanism by which PKC regulates L-arginine transport in endothelial cells. In response to PKC activation (PMA 100 nM, 30 min), [(3)H]L-arginine uptake by bovine aortic endothelial cells (BAEC) was reduced to 45+4% of control (p<0.05). This resulted from a 53% reduction in the Vmax (p<0.05), with no change in the K(m) for L-arginine. Western blot analysis and confocal microscopy revealed no change in the expression or membrane distribution of CAT-1, the principal BAEC L-arginine transporter. Moreover in (32)P-labeling studies, PMA exposure did not result in CAT-1 phosphorylation. We therefore explored the possibility that PKC altered and interaction with MARCKS protein, a candidate membrane associated protein. By co-immunoprecipitation we show that CAT-1 interacts with, a membrane associated protein, that was significantly inhibited by PKC activation (p<0.05). Moreover antisense inhibition of MARCKS abolished the PMA effect on L-arginine transport. PKC dependent mechanisms regulate the transport of L-arginine, mediated via process involving MARCKS.

Changes in the Balance between Caldesmon Regulated by p21-activated Kinases and the Arp2/3 Complex Govern Podosome Formation

Podosomes are dynamic cell adhesion structures that degrade the extracellular matrix, permitting extracellular matrix remodeling. Accumulating evidence suggests that actin and its associated proteins play a crucial role in podosome dynamics. Caldesmon is localized to the podosomes, and its expression is down-regulated in transformed and cancer cells. Here we studied the regulatory mode of caldesmon in podosome formation in Rous sarcoma virus-transformed fibroblasts. Exogenous expression analyses revealed that caldesmon represses podosome formation triggered by the N-WASP-Arp2/3 pathway. Conversely, depletion of caldesmon by RNA interference induces numerous small-sized podosomes with high dynamics. Caldesmon competes with the Arp2/3 complex for actin binding and thereby inhibits podosome formation. p21-activated kinases (PAK)1 and 2 are also repressors of podosome formation via phosphorylation of caldesmon. Consequently, phosphorylation of caldesmon by PAK1/2 enhances this regulatory mode of caldesmon. Taken together, we conclude that in Rous sarcoma virus-transformed cells, changes in the balance between PAK1/2-regulated caldesmon and the Arp2/3 complex govern the formation of podosomes.

Cytoplasmic Ca2+ oscillation coordinates the formation of actin filaments in the sea urchin eggs activated with phorbol ester

Changes in the intracellular Ca2+ concentration ([Ca2+]i) and the formation of actin filaments were investigated in unfertilized eggs of the sea urchin Hemicentrotus pulcherrimus after activation with a phorbol ester, 12-O-tetradecanoyl phorbol13-acetate (TPA). Intracellular Ca2+ oscillation was observed using a fluorescent Ca2+ indicator dye, calcium green dextran. From about 20 to 80 min after the addition of TPA to 100 microM, there was a rise in [Ca2+]i, which was followed by Ca2+ oscillation. A change in [Ca2+]i in response to TPA was not observed in eggs that had been injected with heparin, an inositol 1,4,5-triphosphate (IP3) receptor antagonist. Therefore, long-term exposure to a high concentration of TPA seems to induce Ca2+ release via the IP3 pathway, as well as causing the release of diacylglycerol from membrane lipids. Moreover, the elongation of actin filaments occurred in the cytoplasm during the rise in [Ca2+]i. Actin filaments also formed when TPA-induced cytoplasmic alkalization was inhibited by exposure to Na(+)-free sea water. These results suggest that the observed cytoplasmic formation of actin filaments may be related to change in the cytoplasmic [Ca2(+)]i, and not intracellular pH, induced by TPA. These phenomena may be similar to the changes in actin construction that occur during cell cycle events.

Vasopressin-induced disruption of actin cytoskeletal organization and canalicular function in isolated rat hepatocyte couplets: possible involvement of protein kinase C

The effect of vasopressin (VP) on canalicular function and hepatocellular morphology, with particular regard to actin cytoskeletal organization and the concomitant plasma membrane bleb formation, was studied in isolated rat hepatocyte couplets. VP induced the concentration-dependent formation of multiple plasma membrane blebs as well as simultaneous impairment in both canalicular vacuolar accumulation (cVA) and retention (cVR) of the fluorescent bile acid, cholyl-lysyl-fluorescein (CLF), which evaluate couplet secretory function and tight-junction integrity, respectively. These effects were mimicked by the protein kinase C (PKC) activator, phorbol dibutyrate (PDB), but not by the protein kinase A (PKA) activator, dibutyryl-cAMP. VP-induced bleb formation and canalicular dysfunction were fully prevented by the protein kinase inhibitor, H-7, but not by the PKA inhibitor, KT5720, further suggesting a specific role of PKC. VP-induced alterations were also prevented by pretreatment with the Ca2+-buffering agent, BAPTA/AM, but not with the calmodulin-dependent protein kinase II antagonist, calmidazolium. Neither the Ca2+-activated neutral protease inhibitor, leupeptin, nor the antioxidants, alpha-tocopherol or deferoxamine, were able to prevent either VP-induced plasma membrane blebbing or canalicular dysfunction. The Ca2+-ionophore, A23187, mimicked the VP-induced alterations, but its harmful effects were completely prevented by H-7. Bleb formation induced by VP and PDB was accompanied by an extensive redistribution of filamentous actin from the pericanalicular area to the cell body, and this effect was fully prevented by H-7. These results suggest that VP-induced canalicular and cytoskeletal dysfunction is mediated by PKC and that classical (Ca2+-dependent) PKC appear to be involved because intracellular Ca2+ is required for VP to induce its harmful effects.

Angiotensin II activates RhoA in cardiac myocytes: a critical role of RhoA in angiotensin II-induced premyofibril formation

The organization of actin into striated fibers (myofibrils) is one of the major features of cardiac hypertrophy. However, its signal transduction mechanism is not well understood. Although Rho-family small G proteins have been implicated in actin organization in many cell types, it is not fully elucidated whether Rho mediates the organization of actin fibers by hypertrophic stimuli in cardiac myocytes. Therefore, we examined (1) whether Rho is activated by the hypertrophic stimulus, angiotensin II (Ang II), and (2) whether Rho mediates the Ang II-induced organization of actin fibers in cultured neonatal rat cardiac myocytes. Treatment of myocytes with Ang II caused a rapid formation of both striated (mature myofibrils) and nonstriated (premyofibrils) actin fibers within 30 minutes, as determined by phalloidin stainings of the polymerized actin and troponin T stainings. Immunoblot analyses and immunostainings have indicated that cardiac myocytes express RhoA, but RhoB is undetectable. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II, consistent with activation of RhoA by Ang II. Incubation of myocytes with exoenzyme C3 for 48 hours completely ADP-ribosylated Rho in vivo. The C3 treatment abolished formation of premyofibrils induced by Ang II, suggesting that Ang II causes premyofibril formation via a Rho-dependent mechanism. The Ang II-induced mature myofibril formation was only partly abolished by C3. Expression of constitutively active RhoA (V14RhoA) caused the formation of premyofibrils but not mature myofibrils. The C3 treatment inhibited Ang II-induced atrial natriuretic factor induction, whereas it had no effect on c-fos induction. These results indicate that RhoA is activated by Ang II and mediates the Ang II-induced formation of premyofibrils and induction of a subset of genes. Distinct signaling mechanisms seem to be responsible for striated mature myofibril formation by Ang II.

In vitro stimulation of articular chondrocyte mRNA and extracellular matrix synthesis by hydrostatic pressure

This study tested the effects of hydrostatic pressure (10 MPa) on adult articular chondrocyte mRNA and extracellular matrix synthesis in vitro. High density primary cultures of bovine chondrocytes were exposed to hydrostatic pressure applied intermittently at 1 Hz or constantly for 4 hours in serum-free medium or in medium containing 1% fetal bovine serum. mRNAs for aggrecan, types I and II collagen, and beta-actin were analyzed by Northern blots and quantified by slot blots. Proteoglycan synthesis was quantified by 35SO4 uptake into cetylpyridinium chloride-precipitable glycosaminoglycans, and cell-associated aggrecan and type-II collagen were detected by immunohistochemical techniques. In serum-free medium, intermittent pressure increased aggrecan mRNA signal by 14% and constant pressure decreased type-II collagen mRNA signal by 16% (p < 0.05). In the presence of 1% fetal bovine serum, intermittent pressure increased aggrecan and type-II collagen mRNA signals by 31% (p < 0.01) and 36% (p < 0.001), respectively, whereas constant pressure had no effect on either mRNA. Intermittent and constant pressure stimulated glycosaminoglycan synthesis 65% (p < 0.001) and 32% (p < 0.05), respectively. Immunohistochemical detection of cell-associated aggrecan and type-II collagen was increased in response to both intermittent and constant pressure. These data support the hypothesis that physiologic hydrostatic pressure directly influences the extracellular matrix metabolism of articular chondrocytes.

Detection of candidates for cancer cell motility inhibitory protein in the Dunning adenocarcinoma model

The more differentiated components of a primary tumor may produce substances that reduce the growth rate and metastatic potential of more aggressive components. In the Dunning R-3327 prostatic adenocarcinoma model, cancer cell motility is required for metastatic potential. Medium conditioned by the non-motile, non-metastatic G subline contains proteins of molecular weight 50-100 kDa that inhibited the motility of the highly motile, highly metastatic MAT-LyLu subline. G subline-conditioned medium was separated by DEAE-cellulose chromatography using a linear gradient of 0-0.5 M NaCl in 100 mM Tris at pH 8.3. The motility inhibitory activity of G-conditioned medium was localized to column fractions 51-70 that contained 18% of the applied protein and only 6.5% of the proteins secreted by the G cells. Analysis of pooled fractions 51-60 and 61-70 by two-dimensional gel electrophoresis identified five protein families, with a total of 12 charged proteins of molecular weights approximating 66, 54, 50, 41 and 34 kDa, that were not present or present in reduced quantities in column fractions that did not inhibit motility. Isolation and identification of motility inhibitory protein may prove it the first substance discovered that is produced by a more differentiated component of a neoplasm that directly inhibits a metastasis-associated property.

Reperfusion of rat heart after brief ischemia induces proteolysis of calspectin (nonerythroid spectrin or fodrin) by calpain

Rat myocardium expresses the 240- and 235-kD polypeptides antigenically related to alpha- and beta-subunits of brain calspectin (nonerythroid spectrin or fodrin), respectively. In the subcellular fractions of the myocardium, alpha-calspectin was found in the 600g, 10,000g, and 100,000g pellets, whereas beta-calspectin was localized to the 10,000g pellet. On the basis of the Na+,K(+)-ATPase activity and the contents of a gap junction protein, the sarcolemma was distributed to the 10,000g and 100,000g pellets, and the intercalated disks were enriched in the 10,000g pellet. Both alpha- and beta-calspectin were proteolyzed by calpain in vitro. The two subunits were also proteolyzed in vivo, when the rat hearts underwent 10 to 60 minutes of global ischemia followed by 30 minutes of reperfusion. The reperfusion following the ischemia induced the proteolysis of alpha-calspectin in the 10,000g and 100,000g pellets, producing the 150-kD fragment. A synthetic calpain inhibitor, calpain inhibitor-1, suppressed the degradation of calspectin in vivo, which indicates that calpain is responsible for the reperfusion-induced proteolysis of calspectin. The inhibitor also improved myocardial stunning. Immunohistochemical study revealed that the proteolysis of alpha-calspectin occurs at the intercalated disks and the sarcolemma after postischemic reperfusion, in accord with the biochemical data. These results suggest that degradation of calspectin partly accounts for the contractile failure of the myocardium after postischemic reperfusion by disrupting the membrane skeleton and the intercalated disks.

Phosphorylation of the tight junction protein cingulin and the effects of protein kinase inhibitors and activators in MDCK epithelial cells

In previous studies we have shown that protein kinase inhibitors and extracellular calcium can affect dramatically the assembly of tight junctions (TJ) and the localization of the TJ protein cingulin at sites of cell-cell contact in renal epithelial (MDCK) cells. To characterize in more detail the relationships between kinase activity and junction organization, we have studied the effects of the protein kinase C agonist phorbol myristate acetate (PMA) on the intracellular localization of cingulin, E-cadherin, desmoplakin and actin microfilaments in confluent MDCK monolayers. To study cingulin phosphorylation, MDCK cells were metabolically labelled with [32P]orthophosphate and immunoprecipitates were prepared with anti-cingulin antiserum. We show here that cingulin is phosphorylated in vivo on serine, and its specific phosphorylation is not significantly changed by treatment of confluent MDCK monolayers with PMA, with the protein kinase inhibitor H-7, or with the calcium chelator EGTA. Metabolic labeling with a pulse of [35S]methionine/cysteine showed that at normal extracellular calcium net cingulin biosynthesis was not affected by PMA or H-7. During junction assembly by calcium switch, H-7 did not change the specific phosphorylation of the immunoprecipitated cingulin, however, it prevented the increase in the amount of cingulin in the immunoprecipitates, suggesting that H-7 may block tight junction assembly by interfering with cellular processes that lead to the accumulation and stabilization of TJ proteins at sites of cell-cell contact.

The RNA-binding protein TIAR is translocated from the nucleus to the cytoplasm during Fas-mediated apoptotic cell death

We have determined the structure, intracellular localization, and tissue distribution of TIAR, a TIA-1-related RNA-binding protein. Two related isoforms of TIAR, migrating at 42 and 50 kDa, are expressed in primate cells. Unlike TIA-1, which is found in the granules of cytotoxic lymphocytes, TIAR is concentrated in the nucleus of hematopoietic and nonhematopoietic cells. Because TIAR can trigger DNA fragmentation in permeabilized thymocytes, it is a candidate effector of apoptotic cell death. Consistent with this possibility, we have found that the expression and intracellular localization of TIAR change dramatically during Fas-mediated apoptosis. TIAR moves from the nucleus to the cytoplasm within 30 min of Fas ligation. Redistribution of TIAR precedes the onset of DNA fragmentation and is not a nonspecific consequence of nuclear disintegration. Cytoplasmic redistribution of TIAR is not observed during cellular activation triggered by mitogens such as concanavalin A or phytohemagglutinin. Our results suggest that cytoplasmic redistribution of TIAR may be a general feature of the apoptotic program.

Beta-actin mRNA localization is regulated by signal transduction mechanisms

Beta-actin mRNA is localized in the leading lamellae of chicken embryo fibroblasts (CEFs) (Lawrence, J., and R. Singer. 1986. Cell. 45:407-415), close to where actin polymerization in the lamellipodia drives cellular motility. During serum starvation beta-actin mRNA becomes diffuse and non-localized. Addition of FCS induces a rapid (within 2-5 min) redistribution of beta-actin mRNA into the leading lamellae. A similar redistribution was seen with PDGF, a fibroblast chemotactic factor. PDGF-induced beta-actin mRNA redistribution was inhibited by the tyrosine kinase inhibitor herbimycin, indicating that this process requires intact tyrosine kinase activity, similar to actin filament polymerization and chemotaxis. Lysophosphatidic acid, which has been shown to rapidly induce actin stress fiber formation (Ridley, A., and A. Hall. 1992. Cell. 790:389-399), also increases peripheral beta-actin mRNA localization within minutes. This suggests that actin polymerization and mRNA localization may be regulated by similar signaling pathways. Additionally, activators or inhibitors of kinase A or C can also delocalize steady-state beta-actin mRNA in cells grown in serum, and can inhibit the serum induction of peripherally localized beta-actin mRNA in serum-starved CEFs. These data show that physiologically relevant extracellular factors operating through a signal transduction pathway can regulate spatial sites of actin protein synthesis, which may in turn affect cellular polarity and motility.

Quick-freeze, deep-etch visualization of exocytosis in anterior pituitary secretory cells: localization and possible roles of actin and annexin II

The exocytotic process in the anterior pituitary secretory cells was studied using quick-freeze deep-etch electron microscopy, fluorescein-isothiocyanate-phalloidin staining, heavy meromyosin decoration, and immuno-electron microscopy. The subcortical actin filaments are distributed unevenly in the peripheral cytoplasm. Few secretory granules are seen beneath the plasma membrane in the region where the peripheral cytoplasm is occupied by numerous subcortical actin filaments. On the contrary, in the region free of the subcortical actin filaments, many secretory granules lie in contact with the plasma membrane. Thus, the subcortical actin filaments may control the approach of the secretory granules to the plasma membrane in these cells. The granule and plasma membranes that lie in close proximity are linked by intervening strands. Unfused portions of both membranes remain linked by these strands during membrane fusion and opening. These strands may be involved in membrane contact, fusion and opening during exocytosis. Annexin II (calpactin I) has been demonstrated immunocytochemically to be localized at the contact sites between the granule and plasma membranes, and is therefore a possible component of the intervening strands. Membrane fusion starts within focal regions of both membranes less than 50 nm in diameter. The plasma membrane shows inward depressions toward the underlying granules immediately before fusion. The disappearance of intramembranous particles from the exocytotic site of the membrane has not been observed.

Signal transduction pathways regulating Rho-mediated stress fibre formation: requirement for a tyrosine kinase

Lysophosphatidic acid (LPA) and bombesin rapidly stimulate the formation of focal adhesions and actin stress fibres in serum-starved Swiss 3T3 fibroblasts, a process regulated by the small GTP binding protein Rho. To investigate further the signalling pathways leading to these responses, we have tested the roles of three intracellular signals known to be induced by LPA: activation of protein kinase C (PK-C), Ca2+ mobilization and decreased cAMP levels. Neither PK-C activation nor increased [Ca2+]i, alone or in combination, induced stress fibre formation, and in fact activators of PK-C inhibited this response to LPA and bombesin. The G(i)-mediated decrease in cAMP was not required for the response to LPA, and increased cAMP levels did not prevent stress fibre formation. In contrast, the tyrosine kinase inhibitor genistein inhibited the formation of stress fibres induced by both extracellular factors and microinjected Rho protein. Genistein also inhibited the Rho-dependent clustering of phosphotyrosine-containing proteins at focal adhesions, and the increased tyrosine phosphorylation of several proteins including pp125FAK, induced by LPA and bombesin. This suggests a model where Rho-induced activation of a tyrosine kinase is required for the formation of stress fibres.

Membrane ruffling and signal transduction

One of the earliest structural changes observed in cells in response to many extracellular factors is membrane ruffling: the formation of motile cell surface protrusions containing a meshwork of newly polymerized actin filaments. It is becoming clear that actin reorganization is an integral part of early signal transduction pathways, and that many signalling molecules interact with the actin cytoskeleton. The small GTP-binding protein Rac is a key regulator of membrane ruffling, and proteins that can regulate Rac activity, such as Bcr, are likely to act on this signalling pathway. In addition, several previously characterized signal transducing molecules are implicated in the membrane-ruffling response, including Ras, the adaptor protein Grb2, phosphatidyl inositol 3-kinase, phospholipase A2 and phorbol ester-responsive proteins. Changes in polyphosphoinositide metabolism and intracellular Ca2+ levels may also play a role. A number of actin-binding and organizing proteins localize to membrane ruffles and are potential targets for these signal transducing molecules.

Phosphorylation of the growth arrest-specific protein Gas2 is coupled to actin rearrangements during Go-->G1 transition in NIH 3T3 cells

Growth arrest-specific (Gas2) protein has been shown to be a component of the microfilament system, that is highly expressed in growth arrested mouse and human fibroblasts and is hyperphosphorylated upon serum stimulation of quiescent cells. (Brancolini, C., S. Bottega, and C. Schneider. 1992. J. Cell Biol. 117:1251-1261). In this study we demonstrate that the kinetics of Gas2 phosphorylation, during Go-->G1 transition, as induced by addition of 20% FCS to serum starved NIH 3T3 cells, is temporally coupled to the reorganization of actin cytoskeleton. To better dissect the relationship between Gas2 phosphorylation and the modification of the microfilament architecture we used specific stimuli for both membrane ruffling (PDGF and PMA) and stress fiber formation (L-alpha-lysophosphatidic acid LPA) (Ridley, A. J., and A. Hall. 1992. Cell. 70:389-399). All of them, similarly to 20% FCS, are able to downregulate Gas2 biosynthesis. PDGF and PMA induce Gas2 hyperphosphorylation that is temporally coupled with the appearance of membrane ruffling where Gas2 localizes. On the other hand LPA, a specific stimulus for stress fiber formation, fails to induce a detectable Gas2 hyperphosphorylation. Thus, Gas2 hyperphosphorylation is specifically correlated with the formation of membrane ruffling possibly implying a role of Gas2 in this process.

Immunolocalization of PKC zeta in rat photoreceptor inner segments

We have utilized several peptide specific antisera directed against the C-terminals (Wetsel et al, 1992) of several protein kinase C (PKC) isozymes (alpha, beta 1, beta 11, gamma, delta, epsilon, zeta) to delineate the cellular localization of these PKC isozymes in rat retina. Antisera against PKC beta 1, beta 11, gamma, delta and epsilon were non-reactive in frozen rat retina sections, whereas, anti PKC alpha was strongly reactive with the outer plexiform, inner plexiform and nerve fiber cell layers. The most specific localization of immunoreactivity was observed with PKC zeta, which reacted strongly and exclusively with photoreceptor inner segments, but not outer segments. Immunoblot analysis of whole rat retina homogenate showed that anti-PKC alpha recognized an antigen of approximately 80kD and anti-PKC zeta recognized a approximately 72kD protein. Immunolocalization of PKC zeta to photoreceptor inner segments and possible functional significance are discussed.

Transition metal chelator TPEN counteracts phorbol ester-induced actin cytoskeletal disruption in C6 rat glioma cells without inhibiting activation or translocation of protein kinase C

Phorbol ester-induced reorganization of the actin cytoskeleton was investigated in C6 rat glioma cells. Observations by fluorescence microscopy and photoelectron microscopy indicated that pretreatment with the transition metal chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) for 1-2 h at 50 microM reduced the sensitivity of the actin cytoskeleton to disruption by the subsequent addition of 200 nM phorbol myristate acetate (PMA). The protective effect of TPEN was eliminated by adding back Zn2+ prior to PMA addition, implicating chelation of metal ions as the mechanism of action of TPEN. C6 cells exposed to PMA experience potent activation of protein kinase C (PKC) and substantial redistribution of the kinase from a soluble to a particulate cellular fraction (translocation). TPEN pretreatment did not block PKC translocation in PMA-exposed cells. By two-dimensional gel analysis, TPEN also did not reduce, but rather slightly increased, the PMA-stimulated phosphorylation of the acidic 80 kDa endogenous PKC substrate, as well as two other proteins at 18 kDa and 50 kDa. In contrast, TPEN significantly suppressed phosphorylation of a 20 kDa protein, both in cells treated with TPEN only and in TPEN-pretreated PMA-exposed cells. The results indicate that the ability of TPEN to protect against PKC-mediated actin cytoskeletal disruption is not due to either a block of PKC translocation or to general inhibition of PKC activity. Rather, the action of TPEN is more selective and probably involves chelation of Zn2+ at a critical Zn(2+)-dependent phosphorylation step downstream from the initial tumor promoter-induced effects on PKC.

Rous sarcoma virus-transformed cells develop peculiar adhesive structures along the cell periphery

Alteration of the cell/substratum adhesive structures of rat fibroblasts (3Y1 cells) upon transformation by Rous sarcoma virus (RSV) was investigated by immunofluorescence microscopy. In serum-containing culture medium, 3Y1 cells developed focal adhesions as their main adhesive structures, while BY1 cells expressed peculiar close contacts along the cell periphery with the vitronectin receptor integrin, in addition to podosomes. These peripheral close contacts are referred to as the peripheral adhesions. The peripheral adhesions were observed as a darker region than podosomes by interference reflection microscopy. They were more easily destroyed by incubating the cells with RGD-containing peptide than were the focal adhesions. In contrast to focal adhesions and podosomes, actin bundles were not detected within the peripheral adhesions, where pp60v-src and tyrosine-phosphorylated proteins accumulated. Expression of the integrin was determined by the substratum composition when BY1 cells were cultured in serum-free culture medium. Under such conditions, BY1 cells expressed the peripheral adhesions within 3 hours on adhesion molecule-coated glass. On the other hand, in serum-containing medium, they first developed focal adhesions transiently at their early stage of adhesion, and then the peripheral adhesions were predominantly expressed within 12 hours. Podosomes were formed in a time course similar to that of the peripheral adhesions. These findings suggest that the peripheral adhesion is a class of stable adhesive structure distinct from the focal adhesion or podosome of BY1 cells. Similar close contact-type peripheral adhesions with the integrin were also observed in a variety of cultured cells such as normal fibroblasts at their logarithmic growth phase, phorbol ester-treated fibroblasts, and several malignant tumor cells, with poorly organized focal adhesions and stress fibers. These findings further suggest that the peripheral adhesions may be widely involved in the adhesion of cells that inadequately develop stress fibers and focal adhesions.

Morphological and biochemical analyses of contractile proteins (actin, myosin, caldesmon and tropomyosin) in normal and transformed cells

The expression and intracellular distribution of four contractile proteins (actin, myosin, caldesmon and tropomyosin) in normal fibroblasts and their transformed counterparts by Rous or avian sarcoma virus were compared. By analyzing the isoformal expression of actin, caldesmon and tropomyosin using two-dimensional gel electrophoresis, only tropomyosin showed significant alteration in its isoformal expression accompanied by transformation. Morphological study revealed that in normal cells, myosin, caldesmon and tropomyosin were distributed periodically along stress fibers, but were excluded from focal adhesions (adhesion plaques), at which stress fibers terminate. By contrast, the contractile proteins were concentrated within the protrusions of the ventral cell surface of transformed cells, which are cell-adhesive structures with high motility (podosomes). Regional analysis indicated that the contractile proteins do not show diffuse distribution within podosomes. Myosin, some caldesmon and tropomyosin in association with F-actin were localized in the region surrounding the core domains of podosomes. A major part of the caldesmon was, however, located in the core domain with short F-actin bundles. In order to compare the stability and the molecular organization of stress fibers with that of the short F-actin bundles within podosomes, the dorsal plasma membranes of the cells were removed by lysis and squirting. Then, the ruptured cells were treated with various buffers containing high salt, ATP or Ca2+/calmodulin. Myosin, caldesmon and tropomyosin were strongly associated with stress fibers of the ruptured normal fibroblasts even in a buffer containing high salt or Ca2+/calmodulin. On the other hand, myosin and tropomyosin within podosomes were easily extracted by lysis and squirting. And, the remaining caldesmon in podosomes was separated from the short F-actin bundles with high salt or Ca2+/calmodulin buffer. The present findings suggest that the high motility of podosomes from transformed cells is based on the actomyosin system, and that the stable adherence of focal adhesions of normal cells is due to a lack of this system. The accumulation of contractile proteins and their dynamic association within podosomes might be the cause of the short half-life of the structure. In relation to its localization in the core domain of podosomes without myosin and tropomyosin, the function of caldesmon has been discussed.

Different organizational states of fodrin in cultured MDCK cells are induced by treatment with low pH, calmodulin antagonist TFP, and tumor promoter PMA

We have investigated the molecular mechanisms underlying dynamic organization of the fodrin network by treating the epithelial MDCK cells with various agents affecting intracellular pH, intracellular calcium ion concentration, intracellular calmodulin, and protein kinase C (PKC) activity. Elevation of intracellular calcium level by A23187 or treatment with trifluoperazine (TFP), a calmodulin inhibitor, did not have any drastic effect on the fodrin distribution as judged by immunofluorescence microscopy. A long-term incubation with phorbol-12-myristate-13-acetate (PMA), a protein kinase C activator, in contrast, released fodrin from the lateral walls of the MDCK cells, leading to a diffuse cytoplasmic distribution. TFP, along with PMA, accelerated destabilization of the fodrin skeleton. Treatment with TFP alone rapidly released the cells from the substratum, which, however, could be prevented by PMA. We have previously shown that lowering of intracellular pH (< 6.5) leads to a removal of fodrin from its basolateral residence (Eskelinen et al., 1992) and that this translocation is reversed upon returning normal pH. We now show that the rebuilding of the membrane skeleton can be prevented if TFP is added to the acidified cells. Moreover, in TFP-treated acidified cells, fodrin shows a clusterlike organization similar to that observed in resting lymphocytes. We also noticed that interconversions between these different organizational states of fodrin are independent of the intracellular calcium concentration. Thus manipulation of the intracellular pH and treatment with TFP and PMA reveals different organizational states of the fodrin skeleton. This suggests that fodrin may participate in PMA-, TFP- and pH-sensitive signal transduction pathways.

Phorbol ester-induced actin cytoskeletal reorganization requires a heavy metal ion

The cell-permeant heavy metal chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine(TPEN) was found to counteract phorbol ester-induced actin reorganization in PTK2 and Swiss 3T3 cells. By using fluorescence and the higher resolution technique of photoelectron microscopy to monitor actin patterns, 15-min pretreatment with 25-50 microM TPEN was found to dramatically reduce actin alterations resulting from subsequent phorbol ester treatment in PTK2 cells. Similar results were obtained with Swiss 3T3 cells using 50 microM TPEN for 1.5 h. Phorbol ester-induced actin alterations are thought to depend on activation of protein kinase C (PKC). In contrast to the phorbol ester effect, the PKC-independent actin cytoskeletal disruption caused by staurosporine and cytochalasin B was unaffected by TPEN pretreatment. TPEN did not block phorbol ester-induced activation of PKC in Swiss 3T3 cells, as observed by the phosphorylation of the 80K PKC substrate protein (MARCKS protein). TPEN also did not inhibit partially purified PKC from Swiss 3T3 cells in an in vitro PKC-specific commercial assay. To establish that the effect of TPEN is the removal of metal ions and not some other nonspecific effect of TPEN, a series of transition metal ions was added at the end of the TPEN pretreatment. The results indicate that the transient but dramatic phorbol ester-induced reorganization of the actin cytoskeleton in cultured cells depends on an interaction of PKC with a heavy metal, probably zinc.

Differential localization of protein kinase C isozymes in retinal neurons

We report the immunohistochemical localization of protein kinase C isozymes (types I, II, and III) in the rabbit retina using the monospecific monoclonal antibodies MC-1a, MC-2a, and MC-3a. Using immunoblot analysis of partially purified protein kinase C preparations of rabbit retina, types II and III isozymes alone were detected. The activity of type III was the stronger. By light microscopic immunohistochemical analysis, retinal neurons were negative for type I and positive for type II and type III isozymes. Type II was more diffusely distributed through the retinal layers, but was distinctive in ganglion cells, bipolar cells, and outer segments. The immunoreactivity was stronger for type III isozyme, and it was observed in mop (rod) bipolar cells and amacrine cells. By using immunoelectron microscopy, the cytoplasm of the cell body, the axon, and dendrites of the mop bipolar cells were strongly immunoreactive for type III. The so-called rod bipolar cells were for the first time seen to form synapses with rod photoreceptor cells. These differential localizations of respective isozymes in retinal neurons suggest that each isozyme has a different site of function in each neuron.

Phorbol myristate acetate inhibits HeLa 229 invasion by Bordetella pertussis and other invasive bacterial pathogens

The microfilament inhibitors cytochalasins B and D have been traditionally used to indirectly evaluate the requirement for actin in the uptake of invasive bacterial pathogens by nonprofessional phagocytes. Through their effects on microfilaments, both cytochalasins also impart profound alterations in cellular morphology and surface topology, which likely interfere with adherence. Alterations affecting adherence would complicate interpretation of the effect of cytochalasins on entry alone. As an alternative to cytochalasins, the effect of the tumor promoter phorbol myristate acetate (PMA) was examined for its effects on uptake of several invasive bacterial pathogens by HeLa 229 cells. In this communication, PMA was shown to induce a similar change in HeLa cell actin distribution, but, in contrast to cytochalasins B and D, PMA had no significant effect on gross cell morphology. The modified actin distribution was shown to reduce internalization of Bordetella pertussis, Yersinia pseudotuberculosis, Shigella flexneri, and Salmonella hadar in a dose-dependent manner at concentrations ranging from 1 to 1,000 ng/ml. The magnitude of reduction at a PMA concentration of 1,000 ng/ml was greater than the reduction elicited by cytochalasin B at 2.5 micrograms/ml but was less than that elicited by cytochalasin D at 2.5 micrograms/ml. Mezerein, a functional analog of PMA, caused a similar dose-dependent reduction in uptake of B. pertussis, whereas an inactive analog of PMA, alpha-4-phorbol-12,13-didecanoate was without effect on invasion. Binding studies further reveal that pretreatment of HeLa cells with PMA or mezerein did not significantly impair the ability of B. pertussis to adhere, in contrast to cytochalasins B and D, which caused a marked reduction in adherence.

Identification of a ribosome receptor in the rough endoplasmic reticulum

Attachment of ribosomes to the membrane of the endoplasmic reticulum is one of the crucial first steps in the transport and secretion of intracellular proteins in mammalian cells. The process is mediated by an integral membrane protein of relative molecular mass 180,000 (Mr 180K), having a large (at least 160K) cytosolic domain that, when proteolytically detached from the membrane, can competitively inhibit the binding of ribosomes to intact membranes. Isolation of this domain has led to the identification, purification and characterization of the intact ribosome receptor, as well as its functional reconstitution into lipid vesicles.