Cytochalasin inhibits the rate of elongation of actin filament fragments

Comprehensive Cell Biological Investigation of Cytochalasin B Derivatives with Distinct Activities on the Actin Network

In search of a more comprehensive structure-activity relationship (SAR) regarding the inhibitory effect of cytochalasin B (2) on actin polymerization, a virtual docking of 2 onto monomeric actin was conducted. This led to the identification of potentially important functional groups of 2 (i.e., the NH group of the isoindolone core (N-2) and the hydroxy groups at C-7 and C-20) involved in interactions with the residual amino acids of the binding pocket of actin. Chemical modifications of 2 at positions C-7, N-2, and C-20 led to derivatives 3-6, which were analyzed for their bioactivities. Compounds 3-5 exhibited reduced or no cytotoxicity in murine L929 fibroblasts compared to that of 2. Moreover, short- and long-term treatments of human osteosarcoma cells (U-2OS) with 3-6 affected the actin network to a variable extent, partially accompanied by the induction of multinucleation. Derivatives displaying acetylation at C-20 and N-2 were subjected to slow intracellular conversion to highly cytotoxic 2. Together, this study highlights the importance of the hydroxy group at C-7 and the NH function at N-2 for the potency of 2 on the inhibition of actin polymerization.

Differential interference with actin-binding protein function by acute Cytochalasin B

Dynamic actin filament remodeling is crucial for a plethora of fundamental cell biological processes, ranging from cell division and migration to cell communication, intracellular trafficking or tissue development. Cytochalasin B and -D are fungal secondary metabolites frequently used for interference with such processes. Although generally assumed to block actin filament polymerization at their rapidly growing barbed ends and compete with regulators at these sites, our molecular understanding of their precise effects in dynamic actin structures is scarce. Here we combine live cell imaging and analysis of fluorescent actin-binding protein dynamics with acute treatment of lamellipodia in migrating cells with cytochalasin B. Our results show that in spite of an abrupt halt of lamellipodium protrusion, cytochalasin B affects various actin filament barbed end-binding proteins in a differential fashion. Cytochalasin B enhances instead of diminishes the accumulation of prominent barbed end-binding factors such as Ena/VASP family proteins and heterodimeric capping protein (CP) in the lamellipodium. Similar results were obtained with cytochalasin D. All these effects are highly specific, as cytochalasin-induced VASP accumulation requires the presence of CP, but not vice versa , and coincides with abrogation of both actin and VASP turnover. Cytochalasin B can also increase apparent barbed end interactions with the actin-binding β-tentacle of CP and partially mimic its Arp2/3 complex-promoting activity in the lamellipodium. In conclusion, our results reveal a new spectrum of cytochalasin activities on barbed end-binding factors, with important implications for the interpretation of their effects on dynamic actin structures.

Cytochalasins as Modulators of Stem Cell Differentiation

Regenerative medicine aims to identify new research strategies for the repair and restoration of tissues damaged by pathological or accidental events. Mesenchymal stem cells (MSCs) play a key role in regenerative medicine approaches due to their specific properties, such as the high rate of proliferation, the ability to differentiate into several cell lineages, the immunomodulatory potential, and their easy isolation with minimal ethical issues. One of the main goals of regenerative medicine is to modulate, both in vitro and in vivo, the differentiation potential of MSCs to improve their use in the repair of damaged tissues. Over the years, much evidence has been collected about the ability of cytochalasins, a large family of 60 metabolites isolated mainly from fungi, to modulate multiple properties of stem cells (SCs), such as proliferation, migration, and differentiation, by altering the organization of the cyto- and the nucleo-skeleton. In this review, we discussed the ability of two different cytochalasins, cytochalasins D and B, to influence specific SC differentiation programs modulated by several agents (chemical or physical) or intra- and extra-cellular factors, with particular attention to human MSCs (hMSCs).

Efficient clathrin-mediated entry of enteric adenoviruses in human duodenal cells

IMPORTANCE

Enteric adenoviruses have historically been difficult to grow in cell culture, which has resulted in lack of knowledge of host factors and pathways required for infection of these medically relevant viruses. Previous studies in non-intestinal cell lines showed slow infection kinetics and generated comparatively low virus yields compared to other adenovirus types. We suggest duodenum-derived HuTu80 cells as a superior cell line for studies to complement efforts using complex intestinal tissue models. We show that viral host cell factors required for virus entry differ between cell lines from distinct origins and demonstrate the importance of clathrin-mediated endocytosis.

Cytochalasans and Their Impact on Actin Filament Remodeling

The eukaryotic actin cytoskeleton comprises the protein itself in its monomeric and filamentous forms, G- and F-actin, as well as multiple interaction partners (actin-binding proteins, ABPs). This gives rise to a temporally and spatially controlled, dynamic network, eliciting a plethora of motility-associated processes. To interfere with the complex inter- and intracellular interactions the actin cytoskeleton confers, small molecular inhibitors have been used, foremost of all to study the relevance of actin filaments and their turnover for various cellular processes. The most prominent inhibitors act by, e.g., sequestering monomers or by interfering with the polymerization of new filaments and the elongation of existing filaments. Among these inhibitors used as tool compounds are the cytochalasans, fungal secondary metabolites known for decades and exploited for their F-actin polymerization inhibitory capabilities. In spite of their application as tool compounds for decades, comprehensive data are lacking that explain (i) how the structural deviances of the more than 400 cytochalasans described to date influence their bioactivity mechanistically and (ii) how the intricate network of ABPs reacts (or adapts) to cytochalasan binding. This review thus aims to summarize the information available concerning the structural features of cytochalasans and their influence on the described activities on cell morphology and actin cytoskeleton organization in eukaryotic cells.

Hypothesis: can transfer of primary neoplasm-derived extracellular vesicles and mitochondria contribute to the development of donor cell-derived hematologic neoplasms after allogeneic hematopoietic cell transplantation?

Allogeneic hematopoietic cell transplantation (allo-HCT) is an essential treatment option for various neoplastic and non-neoplastic hematologic diseases. Although its efficacy is modest, a significant proportion of patients experience relapse, graft-versus-host disease, infection or impaired hematopoiesis. Among these, the most frequent cause of post-transplant mortality is relapse, whereas the development of de novo hematologic neoplasms from donor cells after allo-HCT occurs on some occasion as a rare complication. The mechanisms involved in the pathogenesis of the de novo hematologic neoplasms from donor cells are complex, and a multifactorial process contributes to the development of this complication. Recently, extracellular vesicles, particularly exosomes, and mitochondria have been shown to play crucial roles in intercellular communication through the transfer of specific constituents, such as deoxyribonucleic acids, ribonucleic acids, lipids, metabolites and cytosolic and cell-surface proteins. Here, I discuss the potential causative roles of these subcellular components in the development of de novo hematologic neoplasms from donor cells after allo-HCT, in addition to other etiologies.

Zebrafish fin regeneration involves generic and regeneration-specific osteoblast injury responses

Successful regeneration requires the coordinated execution of multiple cellular responses to injury. In amputated zebrafish fins, mature osteoblasts dedifferentiate, migrate towards the injury and form proliferative osteogenic blastema cells. We show that osteoblast migration is preceded by cell elongation and alignment along the proximodistal axis, which require actomyosin, but not microtubule turnover. Surprisingly, osteoblast dedifferentiation and migration can be uncoupled. Using pharmacological and genetic interventions, we found that NF-ĸB and retinoic acid signalling regulate dedifferentiation without affecting migration, while the complement system and actomyosin dynamics affect migration but not dedifferentiation. Furthermore, by removing bone at two locations within a fin ray, we established an injury model containing two injury sites. We found that osteoblasts dedifferentiate at and migrate towards both sites, while accumulation of osteogenic progenitor cells and regenerative bone formation only occur at the distal-facing injury. Together, these data indicate that osteoblast dedifferentiation and migration represent generic injury responses that are differentially regulated and can occur independently of each other and of regenerative growth. We conclude that successful fin bone regeneration appears to involve the coordinated execution of generic and regeneration-specific responses of osteoblasts to injury.

Integrative transcriptome-wide analysis of atopic dermatitis for drug repositioning

Atopic dermatitis (AD) is one of the most common inflammatory skin diseases, which significantly impact the quality of life. Transcriptome-wide association study (TWAS) was conducted to estimate both transcriptomic and genomic features of AD and detected significant associations between 31 expression quantitative loci and 25 genes. Our results replicated well-known genetic markers for AD, as well as 4 novel associated genes. Next, transcriptome meta-analysis was conducted with 5 studies retrieved from public databases and identified 5 additional novel susceptibility genes for AD. Applying the connectivity map to the results from TWAS and meta-analysis, robustly enriched perturbations were identified and their chemical or functional properties were analyzed. Here, we report the first research on integrative approaches for an AD, combining TWAS and transcriptome meta-analysis. Together, our findings could provide a comprehensive understanding of the pathophysiologic mechanisms of AD and suggest potential drug candidates as alternative treatment options.

Integrative genomic and transcriptomic analyses on publicly available data-sets together with in silico drug repositioning identifies alternative therapeutic options to treat atopic dermatitis.

Size effect and mucus role on the intestinal toxicity of the E551 food additive and engineered silica nanoparticles

The E551 food additive is composed of synthetic amorphous silica particles. The current regulation does not mention any specifications regarding their size and granulometric distribution, thus allowing the presence of silica nanoparticles despite their potential toxicity. The digestion process could modify their physicochemical properties and then influence their toxicological profile. After physicochemical characterization, subacute toxicity of engineered silica nanoparticles from 20 to 200 nm, native and digested E551 additives were evaluated from in vitro models of the intestinal barrier. Single cultures and a co-culture of enterocytes and mucus-secreting cells were established to investigate the mucus role. Toxicological endpoints including cytotoxicity, ROS production, intestinal permeability increase, and actin filament disruption were addressed after a 7-day exposure. The results showed a size-dependent effect of silica nanoparticles on cytotoxicity and intestinal permeability. A time-dependent disruption of actin filaments was observed in Caco-2 cells. The mucus layer spread on the HT29-MTX single culture acted as an efficient protective barrier while in the co-culture, small nanoparticles were able to cross it to reach the cells. From a hydrodynamic diameter of 70 nm, nanoparticles were not internalized in the intestinal cells, even in mucus-free models. Digestion did not affect the physicochemical properties of the additive. Due to a mean hydrodynamic diameter close to 200 nm, both native and digested E551 additives did not induce any toxic effect in intestinal barrier models. This study emphasized a cutoff size of 70 nm from which the interactions of the E551 additive with intestinal cells would be limited.

Ion channels as key partners of cytoskeleton in cancer disease

Several processes occur during tumor development including changes in cell morphology, a reorganization of the expression and distribution of the cytoskeleton proteins as well as ion channels. If cytoskeleton proteins and ion channels have been widely investigated in understanding cancer mechanisms, the interaction between these two elements and the identification of the associated signaling pathways are only beginning to emerge. In this review, we summarize the work published over the past 15 years relating to the roles played by ion channels in these mechanisms of reorganization of the cellular morphology, essential to metastatic dissemination, both through the physical interactions with elements of the cytoskeleton and by intracellular signaling pathways involved.

Tension Sensor Based on Fluorescence Resonance Energy Transfer Reveals Fiber Diameter-Dependent Mechanical Factors During Myelination

Oligodendrocytes (OLs) form a myelin sheath around neuronal axons to increase conduction velocity of action potential. Although both large and small diameter axons are intermingled in the central nervous system (CNS), the number of myelin wrapping is related to the axon diameter, such that the ratio of the diameter of the axon to that of the entire myelinated-axon unit is optimal for each axon, which is required for exerting higher brain functions. This indicates there are unknown axon diameter-dependent factors that control myelination. We tried to investigate physical factors to clarify the mechanisms underlying axon diameter-dependent myelination. To visualize OL-generating forces during myelination, a tension sensor based on fluorescence resonance energy transfer (FRET) was used. Polystyrene nanofibers with varying diameters similar to neuronal axons were prepared to investigate biophysical factors regulating the OL-axon interactions. We found that higher tension was generated at OL processes contacting larger diameter fibers compared with smaller diameter fibers. Additionally, OLs formed longer focal adhesions (FAs) on larger diameter axons and shorter FAs on smaller diameter axons. These results suggest that OLs respond to the fiber diameter and activate mechanotransduction initiated at FAs, which controls their cytoskeletal organization and myelin formation. This study leads to the novel and interesting idea that physical factors are involved in myelin formation in response to axon diameter.

Comparison of Serotonin-Regulated Calcific Processes in Aortic and Mitral Valvular Interstitial Cells

Calcification is an important pathological process and a common complication of degenerative valvular heart diseases, with higher incidence in aortic versus mitral valves. Two phenotypes of valvular interstitial cells (VICs), activated VICs and osteoblastic VICs (obVICs), synergistically orchestrate this pathology. It has been demonstrated that serotonin is involved in early stages of myxomatous mitral degeneration, whereas the role of serotonin in calcific aortic valve disease is still unknown. To uncover the link between serotonin and osteogenesis in heart valves, osteogenesis of aortic and mitral VICs was induced in vitro. Actin polymerization and serotonin signaling were inhibited using cytochalasin D and serotonin inhibitors, respectively, to investigate the role of cell activation and serotonin signals in valvular cell osteogenesis. To evaluate calcification progress, calcium and collagen deposits along with the expression of protein markers, including the rate-limiting enzyme of serotonin synthesis [tryptophan hydroxylase 1 (TPH1)], were assessed. When exposed to osteogenic culture conditions and grown on soft surfaces, passage zero aortic VICs increased extracellular collagen deposits and obVIC phenotype markers. A more intense osteogenic process was observed in aortic VICs of higher passages, where cells were activated prior to osteogenic induction. For both, TPH1 expression was upregulated as osteogenesis advanced. However, these osteogenic changes were reversed upon serotonin inhibition. This discovery provides a better understanding of signaling pathways regulating VIC phenotype transformation and explains different manifestations of degenerative pathologies. In addition, the discovery of serotonin-based inhibition of valvular calcification will contribute to the development of potential novel therapies for calcific valvular diseases.

Vav2 catalysis-dependent pathways contribute to skeletal muscle growth and metabolic homeostasis

Skeletal muscle promotes metabolic balance by regulating glucose uptake and the stimulation of multiple interorgan crosstalk. We show here that the catalytic activity of Vav2, a Rho GTPase activator, modulates the signaling output of the IGF1- and insulin-stimulated phosphatidylinositol 3-kinase pathway in that tissue. Consistent with this, mice bearing a Vav2 protein with decreased catalytic activity exhibit reduced muscle mass, lack of proper insulin responsiveness and, at much later times, a metabolic syndrome-like condition. Conversely, mice expressing a catalytically hyperactive Vav2 develop muscle hypertrophy and increased insulin responsiveness. Of note, while hypoactive Vav2 predisposes to, hyperactive Vav2 protects against high fat diet-induced metabolic imbalance. These data unveil a regulatory layer affecting the signaling output of insulin family factors in muscle.

Skeletal muscle plays a key role in regulating systemic glucose and metabolic homeostasis. Here, the authors show that the catalytic activity of Vav2, an activator of Rho GTPases, modulates those processes by favoring the responsiveness of this tissue to insulin and related factors.

A cross-platform approach to characterize and screen potential neurovascular unit toxicants

Development of the neurovascular unit (NVU) is a complex, multistage process that requires orchestrated cell signaling mechanisms across several cell types and ultimately results in formation of the blood-brain barrier. Typical high-throughput screening (HTS) assays investigate single biochemical or single cell responses following chemical insult. As the NVU comprises multiple cell types interacting at various stages of development, a methodology combining high-throughput results across pertinent cell-based assays is needed to investigate potential chemical-induced disruption to the development of this complex cell system. To this end, we implemented a novel method for screening putative NVU disruptors across diverse assay platforms to predict chemical perturbation of the developing NVU. HTS assay results measuring chemical-induced perturbations to cellular key events across angiogenic and neurogenic outcomes in vitro were combined to create a cell-based prioritization of NVU hazard. Chemicals were grouped according to similar modes of action to train a logistic regression literature model on a training set of 38 chemicals. This model utilizes the chemical-specific pairwise mutual information score for PubMed MeSH annotations to represent a quantitative measure of previously published results. Taken together, this study presents a methodology to investigate NVU developmental hazard using cell-based HTS assays and literature evidence to prioritize screening of putative NVU disruptors towards a knowledge-driven characterization of neurovascular developmental toxicity. The results from these screening efforts demonstrate that chemicals representing a range of putative vascular disrupting compound (pVDC) scores can also produce effects on neurogenic outcomes and characterizes possible modes of action for disrupting the developing NVU.

Human Adventitial Fibroblast Phenotype Depends on the Progression of Changes in Substrate Stiffness

Adventitial fibroblasts (AFs) are major contributors to vascular remodeling and maladaptive cascades associated with arterial disease, where AFs both contribute to and respond to alterations in their surrounding matrix. The relationships between matrix modulus and human aortic AF (AoAF) function are investigated using poly(ethylene glycol)-based hydrogels designed with matrix metalloproteinase (MMP)-sensitive and integrin-binding peptides. Initial equilibrium shear storage moduli for the substrates examined are 0.33, 1.42, and 2.90 kPa; after 42 days of culture, all hydrogels exhibit similar storage moduli (0.3-0.7 kPa) regardless of initial modulus, with encapsulated AoAFs spreading and proliferating. In 10 and 7.5 wt% hydrogels, modulus decreases monotonically throughout culture; however, in 5 wt% hydrogels, modulus increases after an initial 7 days of culture, accompanied by an increase in myofibroblast transdifferentiation and expression of collagen I and III through day 28. Thereafter, significant reductions in both collagens occur, with increased MMP-9 and decreased tissue inhibitor of metalloproteinase-1/-2 production. Releasing cytoskeletal tension or inhibiting cellular protein secretion in 5 wt% hydrogels block the stiffening of the polymer matrix. Results indicate that encapsulated AoAFs initiate cell-mediated matrix remodeling and demonstrate the utility of dynamic 3D systems to elucidate the complex interactions between cell behavior and substrate properties.

Infectious Entry of Merkel Cell Polyomavirus

Merkel cell polyomavirus (MCPyV) is a small, nonenveloped tumor virus associated with an aggressive form of skin cancer, Merkel cell carcinoma (MCC). MCPyV infections are highly prevalent in the human population, with MCPyV virions being continuously shed from human skin. However, the precise host cell tropism(s) of MCPyV remains unclear: MCPyV is able to replicate within a subset of dermal fibroblasts, but MCPyV DNA has also been detected in a variety of other tissues. However, MCPyV appears different from other polyomaviruses, as it requires sulfated polysaccharides, such as heparan sulfates and/or chondroitin sulfates, for initial attachment. Like other polyomaviruses, MCPyV engages sialic acid as a (co)receptor. To explore the infectious entry process of MCPyV, we analyzed the cell biological determinants of MCPyV entry into A549 cells, a highly transducible lung carcinoma cell line, in comparison to well-studied simian virus 40 and a number of other viruses. Our results indicate that MCPyV enters cells via caveolar/lipid raft-mediated endocytosis but not macropinocytosis, clathrin-mediated endocytosis, or glycosphingolipid-enriched carriers. The viruses were internalized in small endocytic pits that led the virus to endosomes and from there to the endoplasmic reticulum (ER). Similar to other polyomaviruses, trafficking required microtubular transport, acidification of endosomes, and a functional redox environment. To our surprise, the virus was found to acquire a membrane envelope within endosomes, a phenomenon not reported for other viruses. Only minor amounts of viruses reached the ER, while the majority was retained in endosomal compartments, suggesting that endosome-to-ER trafficking is a bottleneck during infectious entry.IMPORTANCE MCPyV is the first polyomavirus directly implicated in the development of an aggressive human cancer, Merkel cell carcinoma (MCC). Although MCPyV is constantly shed from healthy skin, the MCC incidence increases among aging and immunocompromised individuals. To date, the events connecting initial MCPyV infection and subsequent transformation still remain elusive. MCPyV differs from other known polyomaviruses concerning its cell tropism, entry receptor requirements, and infection kinetics. In this study, we examined the cellular requirements for endocytic entry as well as the subcellular localization of incoming virus particles. A thorough understanding of the determinants of the infectious entry pathway and the specific biological niche will benefit prevention of virus-derived cancers such as MCC.

High-throughput cell mechanical phenotyping for label-free titration assays of cytoskeletal modifications

The mechanical fingerprint of cells is inherently linked to the structure of the cytoskeleton and can serve as a label‐free marker for cell homeostasis or pathologic states. How cytoskeletal composition affects the physical response of cells to external loads has been intensively studied with a spectrum of techniques, yet quantitative and statistically powerful investigations in the form of titration assays are hampered by the low throughput of most available methods. In this study, we employ real‐time deformability cytometry (RT‐DC), a novel microfluidic tool to examine the effects of biochemically modified F‐actin and microtubule stability and nuclear chromatin structure on cell deformation in a human leukemia cell line (HL60). The high throughput of our method facilitates extensive titration assays that allow for significance assessment of the observed effects and extraction of half‐maximal concentrations for most of the applied reagents. We quantitatively show that integrity of the F‐actin cortex and microtubule network dominate cell deformation on millisecond timescales probed with RT‐DC. Drug‐induced alterations in the nuclear chromatin structure were not found to consistently affect cell deformation. The sensitivity of the high‐throughput cell mechanical measurements to the cytoskeletal modifications we present in this study opens up new possibilities for label‐free dose‐response assays of cytoskeletal modifications.

Microtubule motors involved in nuclear movement during skeletal muscle differentiation

Nuclear mispositioning in muscle is often associated with muscular diseases, but little is known about the mechanisms governing nuclear motion in these cells. A screen is presented for molecular motors involved in moving nuclei during myofiber differentiation.

Nuclear positioning is a determining event in several cellular processes, such as fertilization, cell migration, and cell differentiation. The structure and function of muscle cells, which contain hundreds of nuclei, have been shown to rely in part on proper nuclear positioning. Remarkably, in the course of muscle differentiation, nuclear movements along the myotube axis might represent the event required for the even positioning of nuclei in the mature myofiber. Here we analyze nuclear behavior, time in motion, speed, and alignment during myotube differentiation and temporal interference of cytoskeletal microtubule-related motors. Using specific inhibitors, we find that nuclear movement and alignment are microtubule dependent, with 19 microtubule motor proteins implicated in at least one nuclear behavior. We further focus on Kif1c, Kif5b, kif9, kif21b, and Kif1a, which affect nuclear alignment. These results emphasize the different roles of molecular motors in particular mechanisms.

A possible aid in targeted insertion of large DNA elements by CRISPR/Cas in mouse zygotes

The CRISPR/Cas system has rapidly emerged recently as a new tool for genome engineering, and is expected to allow for controlled manipulation of specific genomic elements in a variety of species. A number of recent studies have reported the use of CRISPR/Cas for gene disruption (knockout) or targeted insertion of foreign DNA elements (knock-in). Despite the ease of simple gene knockout and small insertions or nucleotide substitutions in mouse zygotes by the CRISPR/Cas system, targeted insertion of large DNA elements remains an apparent challenge. Here the generation of knock-in mice with successful targeted insertion of large donor DNA elements ranged from 3.0 to 7.1 kb at the ROSA26 locus using the CRISPR/Cas system was achieved. Multiple independent knock-in founder mice were obtained by injection of hCas9 mRNA/sgRNA/donor vector mixtures into the cytoplasm of C57BL/6N zygotes when the injected zygotes were treated with an inhibitor of actin polymerization, cytochalasin. Successful germ line transmission of three of these knock-in alleles was also confirmed. The results suggested that treatment of zygotes with actin polymerization inhibitors following microinjection could be a viable method to facilitate targeted insertion of large DNA elements by the CRISPR/Cas system, enabling targeted knock-in readily attainable in zygotes.

Using cytochalasins to improve current chemotherapeutic approaches

Although the amount of progress cancer therapy has made in recent years is commendable, considerable limitations still remain. Most agents preferentially target rapidly proliferating cells, thereby destroying tumorigenic growths. Unfortunately, there are many labile cells in the patient that are also rapidly dividing, ultimately perpetuating significant side effects, including immunosuppression. Cytochalasins are microfilament-directed agents most commonly known for their use in basic research to understand cytoskeletal mechanisms. However, such agents also exhibit profound anticancer activity, as indicated by numerous in vitro and in vivo studies. Cytochalasins appear to preferentially damage malignant cells, as shown by their minimal effects on normal epithelial and immune cells. Further, cytochalasins influence the end stages of mitosis, suggesting that such agents could be combined with microtubule-directed agents to elicit a profound synergistic effect on malignant cells. Therefore, it is likely that cytochalasins could be used to supplement current chemotherapeutic measures to improve efficacy rates, as well as decrease the prevalence of drug resistance in the clinical setting.

Inability to sustain intraphagolysosomal killing of Staphylococcus aureus predisposes to bacterial persistence in macrophages

Macrophages are critical effectors of the early innate response to bacteria in tissues. Phagocytosis and killing of bacteria are interrelated functions essential for bacterial clearance but the rate-limiting step when macrophages are challenged with large numbers of the major medical pathogen Staphylococcus aureus is unknown. We show that macrophages have a finite capacity for intracellular killing and fail to match sustained phagocytosis with sustained microbial killing when exposed to large inocula of S. aureus (Newman, SH1000 and USA300 strains). S. aureus ingestion by macrophages is associated with a rapid decline in bacterial viability immediately after phagocytosis. However, not all bacteria are killed in the phagolysosome, and we demonstrate reduced acidification of the phagolysosome, associated with failure of phagolysosomal maturation and reduced activation of cathepsin D. This results in accumulation of viable intracellular bacteria in macrophages. We show macrophages fail to engage apoptosis-associated bacterial killing. Ultittop mately macrophages with viable bacteria undergo cell lysis, and viable bacteria are released and can be internalized by other macrophages. We show that cycles of lysis and reuptake maintain a pool of viable intracellular bacteria over time when killing is overwhelmed and demonstrate intracellular persistence in alveolar macrophages in the lungs in a murine model.

Chemotherapy with cytochalasin congeners in vitro and in vivo against murine models

Background Despite inherent differences between the cytoskeletal networks of malignant and normal cells, and the clinical antineoplastic activity of microtubule-directed agents, there has yet to be a microfilament-directed agent approved for clinical use. One of the most studied microfilament-directed agents has been cytochalasin B, a mycogenic toxin known to disrupt the formation of actin polymers. Therefore, this study sought to expand on our previous work with the microfilament-directed agent, along with other less studied cytochalasin congeners. Materials and Methods We determined whether cytochalasin B exerted significant cytotoxic effects in vitro on adherent M109 lung carcinoma and B16BL6 and B16F10 murine melanomas, or on suspension P388/ADR murine leukemia cells. We also examined whether cytochalasin B, its reduced congener 21, 22-dihydrocytochalasin B (DiHCB), or cytochalasin D could synergize with doxorubicin (ADR) against ADR-resistant P388/ADR leukemia cells, and produce significant cytotoxicity in vitro. For in vivo characterization, cytochalasins B and D were administered intraperitoneally (i.p.) to Balb/c mice challenged with drug sensitive P388-S or multidrug resistant P388/ADR leukemias. Results Cytochalasin B demonstrated higher cytotoxicity against adherent lung carcinoma and melanoma cells than against suspension P388/ADR leukemia cells, as assessed by comparative effects on cell growth, and IC₅₀ and IC₈₀ values. Isobolographic analysis indicated that both cytochalasin B and DiHCB demonstrate considerable drug synergy with ADR against ADR-resistant P388/ADR leukemia, while cytochalasin D exhibits only additivity with ADR against the same cell line. In vivo, cytochalasins B and D substantially increased the life expectancy of mice challenged with P388/S and P388/ADR leukemias, and in some cases, produced long-term survival. Conclusion Taken together, it appears that cytochalasins have unique antineoplastic activity that could potentiate a novel class of chemotherapeutic agents.

Pneumolysin activates macrophage lysosomal membrane permeabilization and executes apoptosis by distinct mechanisms without membrane pore formation

Intracellular killing of Streptococcus pneumoniae is complemented by induction of macrophage apoptosis. Here, we show that the toxin pneumolysin (PLY) contributes both to lysosomal/phagolysosomal membrane permeabilization (LMP), an upstream event programing susceptibility to apoptosis, and to apoptosis execution via a mitochondrial pathway, through distinct mechanisms. PLY is necessary but not sufficient for the maximal induction of LMP and apoptosis. PLY's ability to induce both LMP and apoptosis is independent of its ability to form cytolytic pores and requires only the first three domains of PLY. LMP involves TLR (Toll-like receptor) but not NLRP3/ASC (nucleotide-binding oligomerization domain [Nod]-like receptor family, pyrin domain-containing protein 3/apoptosis-associated speck-like protein containing a caspase recruitment domain) signaling and is part of a PLY-dependent but phagocytosis-independent host response that includes the production of cytokines, including interleukin-1 beta (IL-1β). LMP involves progressive and selective permeability to 40-kDa but not to 250-kDa fluorescein isothiocyanate (FITC)-labeled dextran, as PLY accumulates in the cytoplasm. In contrast, the PLY-dependent execution of apoptosis requires phagocytosis and is part of a host response to intracellular bacteria that also includes NO generation. In cells challenged with PLY-deficient bacteria, reconstitution of LMP using the lysomotrophic detergent LeuLeuOMe favored cell necrosis whereas PLY reconstituted apoptosis. The results suggest that PLY contributes to macrophage activation and cytokine production but also engages LMP. Following bacterial phagocytosis, PLY triggers apoptosis and prevents macrophage necrosis as a component of a broad-based antimicrobial strategy. This illustrates how a key virulence factor can become the focus of a multilayered and coordinated innate response by macrophages, optimizing pathogen clearance and limiting inflammation. Importance: Streptococcus pneumoniae, the commonest cause of bacterial pneumonia, expresses the toxin pneumolysin, which can make holes in cell surfaces, causing tissue damage. Macrophages, resident immune cells essential for responses to bacteria in tissues, activate a program of cell suicide called apoptosis, maximizing bacterial clearance and limiting harmful inflammation. We examined pneumolysin's role in activating this response. We demonstrate that pneumolysin did not directly form holes in cells to trigger apoptosis and show that pneumolysin has two distinct roles which require only part of the molecule. Pneumolysin and other bacterial factors released by bacteria that have not been eaten by macrophages activate macrophages to release inflammatory factors but also make the cell compartment containing ingested bacteria leaky. Once inside the cell, pneumolysin ensures that the bacteria activate macrophage apoptosis, rather than necrosis, enhancing bacterial killing and limiting inflammation. This dual response to pneumolysin is critical for an effective immune response to S. pneumoniae.

Actin dynamics regulate subcellular localization of the F-actin-binding protein PALLD in mouse Sertoli cells

Sertoli cells undergo terminal differentiation at puberty to support all phases of germ cell development, which occurs in the mouse beginning in the second week of life. By ∼18 days postpartum (dpp), nearly all Sertoli cells have ceased proliferation. This terminal differentiation is accompanied by the development of unique and regionally concentrated filamentous actin (F-actin) structures at the basal and apical aspects of the seminiferous epithelium, and this reorganization is likely to involve the action of actin-binding proteins. Palladin (PALLD) is a widely expressed F-actin-binding and bundling protein recently shown to regulate these structures, yet it is predominantly nuclear in Sertoli cells at puberty. We found that PALLD localized within nuclei of primary Sertoli cells grown in serum-free media but relocalized to the cytoplasm upon serum stimulation. We utilized this system with in vivo relevance to Sertoli cell development to investigate mechanisms regulating nuclear localization of this F-actin-binding protein. Our results indicate that PALLD can be shuttled from the nucleus to the cytoplasm, and that this relocalization occurred following depolymerization of the F-actin cytoskeleton in response to cAMP signaling. Nuclear localization was reduced in Hpg-mutant testes, suggesting the involvement of gonadotropin signaling. We found that PALLD nuclear localization was unaffected in testis tissues from LH receptor and androgen receptor-mutant mice. However, PALLD nuclear localization was reduced in the testes of FSH receptor-mutant mice, suggesting that FSH signaling during Sertoli cell maturation regulates this subcellular localization.

The anti-actin drugs latrunculin and cytochalasin affect the maturation of spruce somatic embryos in different ways

The role of the actin cytoskeleton in somatic embryo development was investigated using latrunculin B and cytochalasin D. Brief treatments (1h) with either drug at the start of maturation fragmented the actin in suspensor cells and/or depolymerized actin filaments in meristematic cells. The drugs targeted different cells: latB primarily affected the suspensor cells, but cchD damaged both suspensor and meristematic cells. Lethal damage to the meristematic and suspensor cells was observed when the drugs were applied throughout the maturation period, although the severity of this effect depended on their concentrations. The drugs' effects on the yield of mature somatic embryos were investigated by applying them to embryo cultures throughout the maturation period or for one week at three different points in the maturation process: immediately prior to the start of maturation, during the first week of maturation, and during the fourth week of maturation. The strongest effects were observed when the drugs were applied at the start of maturation. Under these conditions, latB destroyed the suspensors, eliminating the underdeveloped embryos that depend on them. This accelerated the development of embryos that were capable of separating from the suspensors. Thus, while the total number of embryos at the end of the maturation period was lower than in untreated control cultures, the surviving mature embryos were of high quality. cchD treatment at the start of maturation strongly inhibited embryo development. Drug treatment at the end of the maturation period did not significantly affect embryo development: latB caused no change in the yield of somatic embryos, but cchD treatment increased the number of malformed embryos compared to untreated controls.

Actin mediates the nanoscale membrane organization of the clustered membrane protein influenza hemagglutinin

The influenza viral membrane protein hemagglutinin (HA) is required at high concentrations on virion and host-cell membranes for infectivity. Because the role of actin in membrane organization is not completely understood, we quantified the relationship between HA and host-cell actin at the nanoscale. Results obtained using superresolution fluorescence photoactivation localization microscopy (FPALM) in nonpolarized cells show that HA clusters colocalize with actin-rich membrane regions (ARMRs). Individual molecular trajectories in live cells indicate restricted HA mobility in ARMRs, and actin disruption caused specific changes to HA clustering. Surprisingly, the actin-binding protein cofilin was excluded from some regions within several hundred nanometers of HA clusters, suggesting that HA clusters or adjacent proteins within the same clusters influence local actin structure. Thus, with the use of imaging, we demonstrate a dynamic relationship between glycoprotein membrane organization and the actin cytoskeleton at the nanoscale.

Fungal and bacterial metabolites in commercial poultry feed from Nigeria

Metabolites of toxigenic fungi and bacteria occur as natural contaminants (e.g. mycotoxins) in feedstuffs making them unsafe to animals. The multi-toxin profiles in 58 commercial poultry feed samples collected from 19 districts in 17 states of Nigeria were determined by LC/ESI-MS/MS with a single extraction step and no clean-up. Sixty-three (56 fungal and seven bacterial) metabolites were detected with concentrations ranging up to 10,200 µg kg⁻¹ in the case of aurofusarin. Fusarium toxins were the most prevalent group of fungal metabolites, whereas valinomycin occurred in more than 50% of the samples. Twelve non-regulatory fungal and seven bacterial metabolites detected and quantified in this study have never been reported previously in naturally contaminated stored grains or finished feed. Among the regulatory toxins in poultry feed, aflatoxin concentrations in 62% of samples were above 20 µg kg⁻¹, demonstrating high prevalence of unsafe levels of aflatoxins in Nigeria. Deoxynivalenol concentrations exceeded 1000 µg kg⁻¹ in 10.3% of samples. Actions are required to reduce the consequences from regulatory mycotoxins and understand the risks of the single or co-occurrence of non-regulatory metabolites for the benefit of the poultry industry.

Interfacial behavior of immortalized hypothalamic mouse neurons detected by acoustic wave propagation

The attachment of immortalized hypothalamic murine neurons onto the surface of an acoustic wave device yields both positive series resonant frequency (f(s)) and motional resistance (R(m)) shifts as opposed to commonly reported negative f(s) and positive R(m) shifts observed for other cell types. These unique shifts have been confirmed by a variety of experiments in order to verify the source and the validity of the signals. These studies involved monitoring responses to solution flow, the absence of serum proteins, the effect of reducing specific cell -surface interactions and the disruption of the neuronal cytoskeleton components. For the adhesion and deposition of neurons, f(s) and R(m) shifts are positively correlated to the amount of adhered neurons on the sensor surface, whereas non-adhered neurons do not produce any significant change in the monitored parameters. In the absence of serum proteins, initial cell adhesion is followed by subsequent cell death and removal from the sensor surface. The presence of the peptide, GRGDS is observed to significantly reduce cell-surface specific interactions compared to the control of SDGRG and this produces f(s) and R(m) responses that are opposite in direction to that observable for cell adhesion. Cytoskeletal studies, using the drugs nocodazole (10 μM), colchicine (1 μM), cytochalasin B (10 μM) and cytochalasin D (2 μM) all elicit neuronal responses that are validated by phalloidin actin-filament staining. These results indicate that the responses are associated with a wide range of cellular changes that can be monitored and studied using the acoustic wave method in real time, under optimal physiological conditions.

Functional and morphological preservation of adult ventricular myocytes in culture by sub-micromolar cytochalasin D supplement

In cardiac myocytes, cytochalasin D (CytoD) was reported to act as an actin disruptor and mechanical uncoupler. Using confocal and super-resolution STED microscopy, we show that CytoD preserves the actin filament architecture of adult rat ventricular myocytes in culture. Five hundred nanomolar CytoD was the optimal concentration to achieve both preservation of the T-tubular structure during culture periods of 3 days and conservation of major functional characteristics such as action potentials, calcium transients and, importantly, the contractile properties of single myocytes. Therefore, we conclude that the addition of CytoD to the culture of adult cardiac myocytes can indeed be used to generate a solid single-cell model that preserves both morphology and function of freshly isolated cells. Moreover, we reveal a putative link between cytoskeletal and T-tubular remodeling. In the absence of CytoD, we observed a loss of T-tubules that led to significant dyssynchronous Ca(2+)-induced Ca(2+) release (CICR), while in the presence of 0.5 μM CytoD, T-tubules and homogeneous CICR were majorly preserved. Such data suggested a possible link between the actin cytoskeleton, T-tubules and synchronous, reliable excitation-contraction-coupling. Thus, T-tubular re-organization in cell culture sheds some additional light onto similar processes found during many cardiac diseases and might link cytoskeletal alterations to changes in subcellular Ca(2+) signaling revealed under such pathophysiological conditions.

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

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

The dynamics of the actin cytoskeleton during sporogenesis in Psilotum nudum L

The actin cytoskeleton (microfilaments, MFs) accompanies the tubulin cytoskeleton (microtubules) during the meiotic division of the cell, but knowledge about the scope of their physiological competence and cooperation is insufficient. To cast more light on this issue, we analysed the F-actin distribution during the meiotic division of the Psilotum nudum sporocytes. Unfixed sporangia of P. nudum were stained with rhodamine-phalloidin and 4',6-diamidino-2-phenylindole dihydrochloride, and we monitored the changes in the actin cytoskeleton and nuclear chromatin throughout sporogenesis. We observed that the actin cytoskeleton in meiotically dividing cells is not only part of the kariokinetic spindle and phragmoplast but it also forms a well-developed network in the cytoplasm present in all phases of meiosis. Moreover, in telophase I F-actin filaments formed short-lived phragmoplast, which was adjacent to the plasma membrane, exactly at the site of future cell wall formation. Additionally, the meiocytes were pre-treated with cytochalasin-B at a concentration that causes damage to the MFs. This facilitated observation of the effect of selective MFs damage on the course of meiosis and sporogenesis of P. nudum. Changes were observed that occurred in the cytochalasin-treated cells: the daughter nuclei were located abnormally close to each other, there was no formation of the equatorial plate of organelles and, consequently, meiosis did not occur normally. It seems possible that, if the actin cytoskeleton only is damaged, regular cytokinesis will not occur and, hence, no viable spores will be produced.

Combined action of the type IV secretion effector proteins BepC and BepF promotes invasome formation of Bartonella henselae on endothelial and epithelial cells

Bartonella henselae (Bhe) can invade human endothelial cells (ECs) by two distinguishable entry routes: either individually by endocytosis or as large bacterial aggregates by invasome-mediated internalization. Only the latter process is dependent on a functional VirB/VirD4 type IV secretion system (T4SS) and the thereby translocated Bep effector proteins. Here, we introduce HeLa cells as a new cell system suitable to study invasome formation. We describe a novel route to trigger invasome formation by the combined action of the effectors BepC and BepF. Co-infections of either HUVEC or HeLa cells with the Bep-deficient ΔbepA-G mutant expressing either BepC or BepF restores invasome formation. Likewise, ectopic expression of a combination of BepC and BepF in HeLa cells enables invasome-mediated uptake of the Bhe ΔbepA-G mutant strain. Further, eGFP-BepC and eGFP-BepF fusion proteins localize to the cell membrane and, upon invasome formation, to the invasome. Furthermore, the combined action of BepC and BepF inhibits endocytic uptake of inert microspheres. Finally, we show that BepC and BepF-triggered invasome formation differs from BepG-triggered invasome formation in its requirement for cofilin1, while the Rac1/Scar1/WAVE/Arp2/3 and Cdc42/WASP/Arp2/3 signalling pathways are required in both cases.

Luteinizing hormone receptor-stimulated progesterone production by preovulatory granulosa cells requires protein kinase A-dependent activation/dephosphorylation of the actin dynamizing protein cofilin

Activation of the LH receptor (LHR) on preovulatory granulosa cells stimulates the cAMP/protein kinase A (PKA) pathway to regulate expression of genes required for ovulation and luteinization. LHR signaling also initiates rearrangement of the actin cytoskeleton. Because disruption of the actin cytoskeleton has been causally linked to steroidogenesis in various cell models, we sought to identify the cellular mechanisms that may modulate reorganization of the actin cytoskeleton and to determine whether cytoskeletal reorganization is required for steroidogenesis. Herein we report that LHR signaling in preovulatory granulosa cells promotes rapid dephosphorylation of the actin-depolymerizing factor cofilin at Ser3 that is dependent on PKA. The LHR-stimulated dephosphorylation of cofilin(Ser3) switches on cofilin activity to bind actin filaments and enhance their dynamics. Basal phosphorylation of cofilin(Ser3) is mediated by active/GTP-bound Rho and downstream protein kinases; LHR signaling promotes a decrease in active/GTP-bound Rho by a PKA-dependent mechanism. LHR-dependent Rho inactivation and subsequent activation of cofilin does not involve ERK, epidermal growth factor receptor, or phosphatidylinositol 3-kinase pathways downstream of PKA. To understand the biological significance of cofilin activation, preovulatory granulosa cells were transduced with a mutant cofilin adenoviral vector in which Ser3 was mutated to Glu (S-E cofilin). Inactive S-E cofilin abolished LHR-mediated reorganization of the actin cytoskeleton and caused a 70% decrease in LHR-stimulated progesterone that is obligatory for ovulation. Taken together, these results show that LHR signaling via PKA activates a cofilin-regulated rearrangement of the actin cytoskeleton and that active cofilin is required to initiate progesterone secretion by preovulatory granulosa cells.

The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1 (PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surface molecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocyte deformation under flow.

Jasplakinolide: an actin-specific reagent that promotes actin polymerization

Jasplakinolide, a cyclo-depsipeptide is a commonly used actin filament polymerizing and stabilizing drug. The substance has originally been isolated from a marine sponge, and can now be synthesized and has become commercially available. This, together with the benefit that jasplakinolide is membrane permeable has made it a commonly used tool in cell biology, when actin filament stabilization or polymerization has to be achieved. This may either be the case in studies on morphogenesis, motility, organelle movement, or when apoptosis has to be induced. Its use as a potent anticancer drug is discussed. The direct action on actin filaments may have further consequences in golgi body and membrane raft protein organization. In this chapter, the visualization of jasplaklinolide effects by different fluorescent and transmission electron microscopic methods is described. As competitive binding capacities of jasplakinolide and phalloidin make the detection of actin filaments by fluorescently labeled phalloidin problematic, alternatives are given here.

Actin microfilament involved in regulation of pacemaking activity in cultured interstitial cells of Cajal from murine intestine

The present study investigated the effect of actin microfilament structure on pacemaker currents and calcium oscillation in cultured murine intestinal interstitial cells of Cajal (ICCs) by whole-cell patch-clamp technique and calcium imaging technique. Cytochalasin B, a disruptor of actin microfilaments, decreased the amplitude and frequency of pacemaker currents from 491.32 +/- 160.33 pA and 11.73 +/- 0.79 cycles/min to 233.12 +/- 92.00 pA and 10.29 +/- 0.76 cycles/min. Cytochalasin B also decreased the amplitude and frequency of calcium oscillation from 0.32 +/- 0.08 (DeltaF/F0) and 2.75 +/- 0.17 cycles/min to 0.02 +/- 0.01 (DeltaF/F0) and 1.20 +/- 0.08 cycles/min. Phalloidin, a stabilizer of actin microfilaments, increased the amplitude and frequency of pacemaker currents from 751.79 +/- 282.82 pA and 13.93 +/- 1.00 cycles/min to 1234.34 +/- 607.83 pA and 14.68 +/- 1.00 cycles/min. Phalloidin also increased the amplitude and frequency of calcium oscillation from 0.26 +/- 0.01 (DeltaF/F0) and 2.27 +/- 0.18 cycles/min to 0.43 +/- 0.03 (DeltaF/F0) and 2.87 +/- 0.07 cycles/min. 2-Aminoethoxydiphenyl borane (2-APB), an IP(3) receptor blocker, suppressed both pacemaker currents and calcium oscillations. 2-APB also blocked the phalloidin-induced increase in pacemaker currents and calcium oscillation. Ryanodine, an inhibitor of calcium-induced calcium release, did not affect pacemaker current but suppressed calcium oscillations. Ryanodine had no effect on altering phalloidin-induced increases in pacemaker current and calcium oscillation. These results suggest that actin microfilaments regulate pacemaker activity via the IP(3)-induced calcium release signaling pathway.

Effect of competition on the production and activity of secondary metabolites in Aspergillus species

Secondary metabolites are of intense interest to humans due to their pharmaceutical and/or toxic properties. Also, these metabolites are clinically relevant because of their importance in fungal pathogenesis. Aspergillus species secrete secondary metabolites when grown individually and in the presence of other fungal species. However, it is not known whether secreted secondary metabolites provide a competitive advantage over other fungal species, or whether competition has any effect on the production of those metabolites. Here, we have performed co-cultivation competition assays among different species of Aspergillus to determine relative species fitness in culture, and to analyze the presence of possible antifungal activity of secondary metabolites in extracts. The results show that, for the most part, at 30 degrees C only one species is able to survive direct competition with a second species. In contrast, survival of both competitors was often observed at 37 degrees C. Consistent with these observations, antifungal activity of extracts from cultures grown at 30 degrees C was greater than that of extract from cultures at 37 degrees C. Interestingly, culture extracts from all species studied had some degree of antifungal activity, but in general, the extracts had greater antifungal activity when species were grown in the presence of a competitor. Using gas chromatography it was determined that the composition of extracts changed due to competition and a shift in temperature. These findings indicate that co-cultivation could be a very promising method for inducing and characterizing novel antifungal compounds produced by species of Aspergillus.

Porcine circovirus 2 infection of epithelial cells is clathrin-, caveolae- and dynamin-independent, actin and Rho-GTPase-mediated, and enhanced by cholesterol depletion

Epithelial cells are the major in vivo target cells for porcine circovirus type 2 (PCV2). Although these cells are used for most studies of PCV2 gene expression and, little is known on PCV2 entry, attachment and internalization, in epithelial cells. PCV2 attachment to epithelial cells occurred rapidly and in a time-dependent manner. In contrast to attachment, internalization was slow. Immunofluorescent stainings revealed that during internalization, PCV2 co-localized with clathrin, but not caveolin. Blocking clathrin-mediated endocytosis increased instead of decreased the number of PCV2-infected cells by threefold, suggesting that it does not represent the main internalization pathway leading to a full replication. Further analysis with different inhibitors revealed that also macropinocytosis, dynamin-dependent internalization and membrane cholesterol play no role in PCV2 entry that leads to infection. Inhibition of small GTPases with Clostridium difficile toxin B reduced the number of PCV2-infected PK-15, SK and STs to 63+/-25%, 47+/-21% and 14+/-6%, respectively. Finally, inhibiting actin polymerization also blocked PCV2 infection, showing the need for actin during PCV2 infection. Together, these data indicate that a dynamin- and cholesterol-independent, but actin- and small GTPase-dependent pathway, allows PCV2 internalization in epithelial cells that leads to infection and that clathrin-mediated PCV2 internalization in epithelial cells is not followed by a full replication.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

Control of the actin cytoskeleton in plant cell growth

Plant cells grow through increases in volume and cell wall surface area. The mature morphology of a plant cell is a product of the differential rates of expansion between neighboring zones of the cell wall during this process. Filamentous actin arrays are associated with plant cell growth, and the activity of actin-binding proteins is proving to be essential for proper cell morphogenesis. Actin-nucleating proteins participate in cell expansion and cell plate formation whereas the recycling of actin monomers is required to maintain actin dynamics and controlled growth. Coordination of actin-binding protein activity and other aspects of cytoskeletal behavior during cell development maintains cohesive cell expansion. Emerging plant signaling networks are proving to be powerful regulators of morphology-shaping cytoskeletal activity, and in this review we highlight current research in actin network regulation.

Binding and entry characteristics of porcine circovirus 2 in cells of the porcine monocytic line 3D4/31

Porcine circovirus 2 (PCV2) is associated with post-weaning multisystemic wasting syndrome and reproductive problems in pigs. Cells of the monocyte/macrophage lineage are important target cells in PCV2-infected pigs, but the method of binding and entry of PCV2 into these cells is unknown. Therefore, binding and entry of PCV2 to the porcine monocytic cell line 3D4/31 were studied by visualization of binding and internalization of PCV2 virus-like particles (VLPs) by confocal microscopy and chemical inhibition of endocytic pathways (clathrin- and caveolae-mediated endocytosis and macropinocytosis), followed by evaluation of the level of PCV2 infection. It was shown that PCV2 VLPs bound to all cells, with maximal binding starting from 30 min post-incubation. Bound PCV2 VLPs were internalized in 47+/-5.0 % of cells. Internalization was continuous, with 70.5+/-9.7 % of bound PCV2 VLPs internalized at 360 min post-incubation. Internalizing PCV2 VLPs co-localized with clathrin. PCV2 infection was decreased significantly by chemical inhibitors that specifically blocked (i) actin-dependent processes, including cytochalasin D (75.5+/-7.0 % reduction) and latrunculin B (71.0+/-3.0 % reduction), and (ii) clathrin-mediated endocytosis, including potassium depletion combined with hypotonic shock (50.2+/-6.3 % reduction), hypertonic medium (56.4+/-5.7 % reduction), cytosol acidification (59.1+/-7.1 % reduction) and amantadine (52.6+/-6.7 % reduction). Inhibiting macropinocytosis with amiloride and caveolae-dependent endocytosis with nystatin did not decrease PCV2 infection significantly. PCV2 infection was reduced by the lysosomotropic weak bases ammonium chloride (47.0+/-7.9 % reduction) and chloroquine diphosphate (49.0+/-5.6 % reduction). Together, these data demonstrate that PCV2 enters 3D4/31 cells predominantly via clathrin-mediated endocytosis and requires an acidic environment for infection.

Cytochalasin D can improve heterologous protein productivity in adherent Chinese hamster ovary cells

We generated a series of adherent gene-amplified CHO clones expressing human secreted alkaline phosphatase (SEAP) as a model for heterologous protein production. Clones demonstrate a 26- to 52-fold increase in productivity compared to controls after dhfr/methotrexate-mediated gene amplification and clone selection. SEAP is stably expressed in these clones over at least a 6-week period without significant productivity loss. Two-dimensional protein electrophoresis identified 21 proteins that exhibited altered expression in clones of increasing SEAP productivity. Based on MALDI TOF/TOF mass spectrometry of relevant protein spots, changes in translation, energy pathways, chaperones, regulatory proteins, and cytoskeletal proteins were observed, including a 4-fold expression increase in actin capping protein. We hypothesized that an alteration of the actin cytoskeleton using cytochalasin D as a mimic for actin-capping protein could have a beneficial effect on heterologous protein secretion. Treatment with 0.5 mug/mL cytochalasin D increased SEAP productivity 2- to 3-fold compared to an amplified control which resulted in an increase in productivity from 52- to 150-fold compared to a nonamplified parent.