Reduced microfilament organization in adenovirus type 5-infected rat embryo cells: a function of early region 1a

The C Terminus of Rotavirus VP4 Protein Contains an Actin Binding Domain Which Requires Cooperation with the Coiled-Coil Domain for Actin Remodeling

The interactions between viruses and actin cytoskeleton have been widely studied. We showed that rotaviruses remodel microfilaments in intestinal cells and demonstrated that this was due to the VP4 spike protein. Microfilaments mainly occur in the apical domain of infected polarized enterocytes and favor the polarized apical exit of viral progeny. The present work aims at the identification of molecular determinants of actin-VP4 interactions. We used various deletion mutants of VP4 that were transfected into Cos-7 cells and analyzed interactions by immunofluorescence confocal microscopy. It has been established that the C-terminal part of VP4 is embedded within viral particles when rotavirus assembles. The use of specific monoclonal antibodies demonstrated that VP4 is expressed in different forms in infected cells: classically as spike on the outer layer of virus particles, but also as free soluble protein in the cytosol. The C terminus of free VP4 was identified as interacting with actin microfilaments. The VP4 actin binding domain is unable to promote microfilament remodeling by itself; the coiled-coil domain is also required in this process. This actin-binding domain was shown to dominate a previously identified peroxisomal targeting signal, located in the three last amino acids of VP4. The newly identified actin-binding domain is highly conserved in rotavirus strains from species A, B, and C, suggesting that actin binding and remodeling is a general strategy for rotavirus exit. This provides a novel mechanism of protein-protein interactions, not involving cell signaling pathways, to facilitate rotavirus exit.IMPORTANCE Rotaviruses are causal agents of acute infantile viral diarrhea. In intestinal cells, in vitro as well as in vivo, virus assembly and exit do not imply cell lysis but rely on an active process in which the cytoskeleton plays a major role. We describe here a novel molecular mechanism by which the rotavirus spike protein VP4 drives actin remodeling. This relies on the fact that VP4 occurs in different forms. Besides its structural function within the virion, a large proportion of VP4 is expressed as free protein. Here, we show that free VP4 possesses a functional actin-binding domain. This domain, in coordination with a coiled-coil domain, promotes actin cytoskeleton remodeling, thereby providing the capacity to destabilize the cell membrane and allow efficient rotavirus exit.

Visualization of cytoskeletal dynamics in podocytes using adenoviral vectors

Glomerular visceral epithelial cells (podocytes) play a key role in maintaining selective protein filtration in the kidney. Podocytes have a complex cell shape characterized by the presence of numerous actin-rich processes, which cover the surface of glomerular capillaries and are connected by specialized cell-cell adhesion complexes (slit diaphragms). Human genetic studies and experiments in knockout mouse models show that actin filaments and actin-associated proteins are indispensable for the maintenance of podocyte shape, slit diaphragm integrity, and normal glomerular filtration. The ability to examine cytoskeletal protein organization and dynamics in podocytes and to test the effects of disease-associated mutations on protein localization provides valuable information for researchers aiming to dissect the molecular mechanisms of podocyte dysfunction. We describe how adenovirus-mediated transduction of cultured podocytes with DNA constructs can be used to reliably introduce fluorescently tagged cytoskeletal markers for live cell imaging with high efficiency and low toxicity. This technique can be used to study the dynamic reorganization of the podocyte cytoskeleton and to test the effects of novel mutations on podocyte cytoskeletal dynamics.

Cytoskeletal architecture differentially controls post-transcriptional processing of IL-6 and IL-8 mRNA in airway epithelial-like cells

Airway epithelial cells are critically dependent on an intact cytoskeleton for innate defense functions. There are various pathophysiological conditions that affect the cytoskeletal architecture. We studied the effect of cytoskeletal distortion in polarized airway epithelial-like NCI-H292 cells on inflammatory gene expression, exemplified by interleukin(IL)-6 and IL-8. Disruption of microtubule structure with vinblastin and of actin with cytochalasin D did not affect TNF-alpha-induced IL-6 and IL-8 gene transcription but stabilized IL-8 and IL-6 mRNA. In line with previous studies, IL-8 mRNA stabilization was paralleled by hyperresponsive IL-8 production, but surprisingly, IL-6 production was reduced despite IL-6 mRNA stabilization. Polysome profiling revealed that, in cells with a disrupted cytoskeleton, translational efficiency of IL-6 mRNA was reduced, whereas that of IL-8 mRNA remained unaffected. Our findings indicate that distortion of the cytoskeleton in airway epithelial cells differentially affects both degradation and translation of IL-6 and IL-8 mRNA, modifying inflammatory gene expression and thus their innate defense function.

Rotavirus spike protein VP4 binds to and remodels actin bundles of the epithelial brush border into actin bodies

We demonstrate here that VP4, a rotaviral protein, is able to specifically bind to bundled actin microfilaments that are subsequently profoundly remodeled into actin bodies. These cytoplasmic actin bodies do not localize within identified intracellular compartments. VP4-induced actin remodeling is similar to cytochalasin D effects with kinetics compatible with that of rotavirus infection. Actin bundles' remodeling occurs both in infected and in VP4-transfected cells and in various cell lines, indicating that this is a general property of the viral protein itself. Interestingly, in intestinal epithelial cells, which represent the natural target of rotavirus, VP4 is addressed to the apical membrane where it binds specifically to brush border actin bundles and elicits its remodeling, whereas cytochalasin D impaired all the filamentous actin. These observations indicate that these original properties of VP4 likely explain the previously described brush border alterations that follow rotavirus infection of enterocytes and may also participate to the mechanism of rotavirus final assembly.

Dystrophin deficiency markedly increases enterovirus-induced cardiomyopathy: a genetic predisposition to viral heart disease

Both enteroviral infection of the heart and mutations in the dystrophin gene can cause cardiomyopathy. Little is known, however, about the interaction between genetic and acquired forms of cardiomyopathy. We previously demonstrated that the enteroviral protease 2A cleaves dystrophin; therefore, we hypothesized that dystrophin deficiency would predispose to enterovirus-induced cardiomyopathy. We observed more severe cardiomyopathy, worsening over time, and greater viral replication in dystrophin-deficient mice infected with enterovirus than in infected wild-type mice. This difference appears to be a result of more efficient release of the virus from dystrophin-deficient myocytes. In addition, we found that expression of wild-type dystrophin in cultured cells decreased the cytopathic effect of enteroviral infection and the release of virus from the cell. We also found that expression of a cleavage-resistant mutant dystrophin further inhibited the virally mediated cytopathic effect and viral release. These results indicate that viral infection can influence the severity and penetrance of the cardiomyopathy that occurs in the hearts of dystrophin-deficient individuals.

Viral manipulations of the actin cytoskeleton

Viruses succeed as intracellular parasites because of their ability to invade cells and appropriate the cellular machinery required during their life cycle. The actin cytoskeleton of the host cell does not escape viral infection unscathed, but is often co-opted by the virus at many different stages of its life cycle to facilitate the infection process.

HgCl2-induced alteration of actin filaments in cultured primary rat proximal tubule epithelial cells labelled with fluorescein phalloidin

When proximal tubule epithelial cells are exposed to HgCl2, cytoplasmic blebs are formed. These represent an early, potentially reversible response to injury. These blebs are accompanied by reorganization of cytoskeletal proteins, and presumably by alternations in cytoskeletal-plasma membrane interactions. Ca(2+)-activated proteinases, such as calpain, are known to affect cytoskeletal proteins and to be involved in diverse cellular processes. However, the role of calpains in cytotoxicity due to HgCl2 is unknown. To determine the relationship between F-actin, calpain, and HgCl2 toxicity, cells were stained with fluorescein phalloidin before and after treatment with HgCl2. Cells were grown on coverslips and exposed to HgCl2 (10 or 25 microM) in the presence or absence of the calpain inhibitor, leupeptin. Untreated cells were flat, polygonal, and contained many fluorescent-stained cables of actin filaments. Generally, cells exposed to HgCl2 became pleomorphic and contracted as the blebs formed. These cells showed fewer actin cables and fluorescence was seen mostly as either compact areas of dense stain or as peripheral rings. In many cells, actin cables and filaments were completely absent. Disappearance of F-actin was initially seen by 2 min after exposure to HgCl2. Thus, disruption of the actin cytoskeleton and blebbing were found to be early events in HgCl2 toxicity. When leupeptin was used with HgCl2 treatment, the actin staining appeared similar to that of untreated cells.

Estramustine phosphate reversibly inhibits an early stage during adenovirus replication

Estramustine phosphate, an estradiol-mustard conjugate, was shown to reversibly inhibit a stage during the first hour of productive adenovirus 2 infection of HeLa cells. This drug, employed in the therapy of advanced prostatic cancer, specifically interacts with microtubule-associated proteins (MAPs) of the cytoskeleton. The results obtained under physiological conditions in vivo suggest a MAPs-interference with the microtubule-mediated vectorial migration of the virus inoculum to the nucleus. Virus attachment, uncoating kinetics and the appearance of established uncoating intermediates were not affected.

Relationship between organization of the actin cytoskeleton and the cell cycle in normal and adenovirus-infected rat cells

Flow cytometry and staining with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin were used to investigate organization of the actin cytoskeleton in rat embryo cells at different stages of normal and adenovirus E1A-induced cell cycles. In uninfected cells in G0-G1 and S phases, actin was predominantly in the form of stress fibers. In G2, this organization changed to peripheral rings of thin filaments, while during mitosis, actin had a diffuse distribution. Infection of quiescent rat cells by adenovirus caused them to enter the cell cycle and replicate DNA and also caused disruption of stress fibers. Rapid disappearance of stress fibers and the appearance of peripheral rings of actin filaments began from 13 h after infection and closely followed synthesis of the E1A proteins. Infected cells began S phase at about 24 h after infection, and cells in G2 and mitosis were seen from 30 to 50 h. Thus, disruption of the actin cytoskeleton is an early effect of E1A and not an indirect consequence of the entry of infected cells into the cell cycle.

Functions of the two adenovirus early E1A proteins and their conserved domains in cell cycle alteration, actin reorganization, and gene activation in rat cells

Rat embryo cells were infected with adenovirus type 5 mutants that code for only one of the two early E1A proteins, mutants with defects in one of the two conserved regions common to the two proteins, or mutants with defects in the 46-amino-acid region unique to the 289-amino-acid E1A protein. Cells were scored for altered cell cycle progression, disruption of actin stress fibers, and activation of E2A expression. Mutants lacking either E1A protein were able to cause all of these effects; but mutants lacking a 243-amino-acid protein had less effect, and mutants lacking a 289-amino-acid protein much less effect, than wild-type virus. A mutation in any of the three conserved regions caused a defect in each E1A effect. To investigate the reported function of conserved domain 2 in mitosis, we monitored by fluorescence-activated cell sorter the reduction in Hoechst 33342 fluorescence that occurs when cells divide after undergoing a round of DNA replication in 5-bromodeoxyuridine. A smaller percentage of adenovirus-infected cells than mock-infected cells divided within a given period after completing a round of DNA replication. Viruses with mutations in conserved domain 2 were defective for initiation of cellular DNA replication, as were all other E1A mutants we have examined, but had no specific defect in cell division compared with wild-type virus. Thus, although there may be some specialization of function between the two E1A proteins and between their conserved domains, it was not apparent in the aspects of E1A function and the mutants that we examined.

Expression of adenovirus E1B mutant phenotypes is dependent on the host cell and on synthesis of E1A proteins

Adenovirus mutants containing genetic alterations in the gene encoding the E1B 19,000-molecular-weight (19K) tumor antigen induce the degradation of host cell chromosomal DNA (deg phenotype) and enhanced cytopathic effect (cyt phenotype) after infection of HeLa and KB cells. The deg and cyt phenotypes are a consequence of viral early gene expression in the absence of the E1B 19K protein. The role of the E1A proteins in induction of the cyt and deg phenotypes was investigated by constructing E1A-E1B double mutant viruses. Viruses were constructed to express the individual E1A 13S, 12S, or 9S cDNA genes in the presence of a mutation in the gene encoding the E1B 19K tumor antigen. Expression of either the 13S or 12S E1A proteins in the absence of functional E1B 19K protein produced the deg and cyt phenotypes. In contrast, a virus which expressed exclusively the 9S E1A gene product in the absence of the E1B 19K gene product did not induce the deg and cyt phenotypes, even at high multiplicities of infection. Therefore, both the 13S and 12S E1A gene products could directly or indirectly cause the deg and cyt phenotypes during infection of HeLa cells with an E1B 19K gene mutant virus. Furthermore, the deg phenotype was found to be host cell type specific, occurring in HeLa and KB cells but not in growth-arrested human WI38 cells. These results indicate that expression of the E1A trans-activating and transforming proteins is necessary for the induction of the cyt and deg phenotypes and that host cell factors also play a role.

Progressive reorganization of the host cell cytoskeleton during adenovirus infection

Infection of cells with adenovirus lead to a characteristic reorganization of all cytoskeleton systems, starting with alterations at the microtubuli of the cells. During this progress, the flat, extended, and polar morphology of the cytoskeleton became nonpolar and rounder. These rearrangements were initiated before the appearance of adenovirus structural proteins hexon and fiber, as well as before the shutoff of host protein synthesis. We conclude that these alterations reflect a specific reorganization rather than an unorganized breakdown of the cell during adenovirus infection.

Adenovirus tumor-specific transplantation antigen is a function of the E1A early region

Viable recombinant adenoviruses that carry a portion of the type 12 E1A and E1B transcription units in a type 5 background were used to identify genes controlling expression of the adenovirus tumor-specific transplantation antigen (TSTA). The TSTA immunity is not crossreacting between the group A and group C adenovirus serotypes. Viruses carrying the E1A region (sub370-12E1A), or both E1A and E1B (sub370-12E1AB) regions of Ad12, induce a strong transplantation immunity against tumors induced by syngeneic cells transformed with adenovirus 12, but fail to induce any protection against syngeneic cells transformed with adenovirus 2. Immunization with the virus carrying only the E1B region (sub370-12E1B) of adenovirus 12 induces no immunity to adenovirus 12 transformed cell line, but confers a strong protection against cells transformed with adenovirus 2. These results provide strong evidence that the adenovirus tumor-specific transplantation antigen is a function of the E1A early region.