Fate of microfilaments in vero cells infected with measles virus and herpes simplex virus type 1

Adherent and suspension baby hamster kidney cells have a different cytoskeleton and surface receptor repertoire

Animal cell culture, with single cells growing in suspension, ideally in a chemically defined environment, is a mainstay of biopharmaceutical production. The synthetic environment lacks exogenous growth factors and usually requires a time-consuming adaptation process to select cell clones that proliferate in suspension to high cell numbers. The molecular mechanisms that facilitate the adaptation and that take place inside the cell are largely unknown. Especially for cell lines that are used for virus antigen production such as baby hamster kidney (BHK) cells, the restriction of virus growth through the evolution of undesired cell characteristics is highly unwanted. The comparison between adherently growing BHK cells and suspension cells with different susceptibility to foot-and-mouth disease virus revealed differences in the expression of cellular receptors such as integrins and heparan sulfates, and in the organization of the actin cytoskeleton. Transcriptome analyses and growth kinetics demonstrated the diversity of BHK cell lines and confirmed the importance of well-characterized parental cell clones and mindful screening to make sure that essential cellular features do not get lost during adaptation.

Inhibition of Rho-associated coiled-coil-forming kinase increases efficacy of measles virotherapy

RhoA and its downstream effector Rho-associated coiled-coil-forming kinase (ROCK) are known regulators of the formation of actin cytoskeleton in cells. Actin cytoskeleton is involved in paramyxovirus infection; we, therefore, examined the effect of ROCK inhibition on measles virus (MV) cytopathic effect and replication. Treatment with the ROCK inhibitor, Y27632, significantly increased syncytia size in tumor cell lines following MV infection, associated with cytoskeleton disruption as demonstrated by actin staining. Treatment of prostate cancer, breast cancer and glioblastoma tumor cell lines with Y27632 following MV infection resulted in increased cytopathic effect, as assessed by trypan blue exclusion assays. In addition, there was a significant increase in viral proliferation by at least one log or more as tested in one-step viral growth curves. Increased viral replication was also observed in athymic nude mice bearing MDA-MB-231 xenografts following combination treatment with MV and Y27632. In summary, inhibition of the ROCK kinase by Y27632 enhanced the oncolytic effect of MV and viral proliferation; this approach merits further translational investigation.

Role of tumor invasiveness, the extracellular matrix, and chromatin sequestration in the susceptibility of uveal melanoma to herpes simplex virus type 1

To better understand determinants of susceptibility/resistance of uveal melanomas to herpes simplex virus type 1 (HSV-1) oncolytic therapy, uveal melanoma cell lines of low (OCM1a) and of high (M619, MUM2B) invasive potential were infected with HSV-1 either in the presence or absence of a laminin-rich extracellular matrix (Matrigel). OCM1a cultures were destroyed faster by HSV-1 than M619 and MUM2B cultures. In the presence of Matrigel, all melanoma cultures demonstrated delayed destruction by HSV-1 relative to Matrigel-free cultures. As sequestration of chromatin is a characteristic feature of highly invasive uveal melanomas that is further increased by exposure to laminin, we explored whether chromatin sequestration could be reversed by HSV-1 infection. HSV-1 infection induced a global reversal of chromatin sequestration in highly invasive uveal melanoma cells. However, this viral effect was first observed only 2h following virus infection and required novel protein synthesis from input viral DNA. These findings suggest that tumor invasiveness, the spatial relationship of tumor cells to laminin and chromatin sequestration are determinants of susceptibility/resistance of melanomas to HSV-1 oncolytic therapy. Furthermore, these findings indicate for the first time that HSV-1 infection is associated with global exposure of normally highly sequestered cellular DNA in malignant cells.

[Viral fusion mechanisms]

The majority of viral fusion proteins can be divided into two classes. The influenza hemagglutinin (HA) belongs to the class I fusion proteins and undergoes a series of conformational changes at acidic pH, leading to membrane fusion. The crystal structures of the prefusion and the postfusion forms of HA have been revealed in 1981 and 1994, respectively. On the basis of these structures, a model for the mechanism of membrane fusion mediated by the conformational changes of HA has been proposed. The flavivirus E and alphavirus E1 proteins belong to the class II fusion proteins and mediate membrane fusion at acidic pH. Their prefusion structures are distinct from that of HA. Last year, however, it has become evident that the postfusion structures of these class I and class II fusion proteins are similar. The paramyxovirus F protein belongs to the class I fusion proteins. In contrast to HA, an interaction between F and its homologous attachment protein is required for F to undergo the conformational changes. Since F mediates fusion at neutral pH, the infected cells can fuse with neighboring uninfected cells. The crystal structures of F and the attachment protein HN have recently been clarified, which will facilitate studies of the molecular mechanism of F-mediated membrane fusion.

Filamentous particle formation by human parainfluenza virus type 2

Some paramyxoviruses form long filamentous virus particles: however, the determinants of filament formation and the role of such particles in virus transmission and pathogenicity are not clearly defined. By using conventional immunofluorescence microscopy, we found that human parainfluenza virus type 2 (HPIV2) forms filamentous particles ranging from 5 to 15 microm in length in virus-infected, polarized epithelial cells. The formation of filamentous particles was found to be virus type-specific and was not observed when the same cell types were infected with parainfluenza virus type 3 or Sendai virus, suggesting that different paramyxovirus genera exhibit distinct morphological properties. HPIV2 filamentous particle formation was found to be inhibited by cytochalasin D (CD) or jasplakinolide treatment in a dose-dependent manner. In the presence of 4 microg/ml CD or 1 microM jasplakinolide, the formation of filamentous particles was completely abolished, although similar haemagglutination and p.f.u. titres of virus were found to be released into the culture medium at 24 h post-infection. These observations indicate that host cell components, including the actin microfilament network, are important determinants of the morphology of parainfluenza viruses. The predominance of filamentous particles in polarized epithelial cells may reflect specific pathogenic roles of these particles in infection of human epithelial tissues.

Herpes simplex virus induces intracellular redistribution of E2F4 and accumulation of E2F pocket protein complexes

Accumulation of E2F-p107 and E2F-pRB DNA binding complexes occurred after herpes simplex virus infection of U2-OS cells. Accumulation of E2F-p107 also occurred by 4 h p.i. in C33 cells. This corresponded to a time when host DNA synthesis was reduced by 50%, and lagged by >/=1 h, the onset of viral DNA synthesis. To determine the basis for increased nuclear E2F complexes, we investigated the effects of virus infection on the intracellular distribution of the E2F-dependent DNA binding complexes and their protein constituents. Western blot analyses of whole cell extracts revealed that amounts of E2F4, E2F1, DP1, and p107 remained unchanged after infection of C33 cells. Analysis of cytoplasmic and nuclear fractions, however, revealed that cytoplasmic E2F4 decreased and nuclear E2F4 increased. This correlated with a loss of cytoplasmic E2F DNA-binding activity and a corresponding increase in nuclear DNA-binding activity. Concomitant with its redistribution, the apparent molecular weight of total and p107-associated E2F4 increased, at least partially as a result of protein phosphorylation. Increased nuclear E2F-pRB in U2-OS cells was accompanied by the conversion of pRB from a hyper- to a hypophosphorylated state. Infection of U2-OS cells with viral mutants indicated that viral protein IE ICP4 was necessary for the decrease in cytoplasmic E2F-p107, and that viral protein DE ICP8 was required for nuclear accumulation of p107-E2F. In contrast, ICP8 was not required for accumulation of E2F-pRB. These results indicate that the increase in E2F-p107 may be explained by the redistribution and modification of E2F4 and the increase in E2F-pRB by modification of pRB.

Mouse hepatitis virus type 3 infection provokes a decrease in the number of sinusoidal endothelial cell fenestrae both in vivo and in vitro

Fenestrations of hepatic endothelial cells play an active role as a sieving barrier allowing extensive exchange between the blood and liver parenchyma. Alteration of these structures may be induced in the course of various pathological events and provoke important perturbations of liver function. We demonstrate here that sinusoidal endothelial cells are permissive for mouse hepatitis virus 3 (MHV3) in vivo and in vitro and that this infection leads to a striking decrease in the number of fenestrae. The disappearance of these structures observed under scanning electron microscopy or in cryofracture preparations in vivo and in vitro cannot be reversed by the action of cytochalasin B on the microfilament network. The decrease in the porosity seems to be related directly to the productive infection of the endothelial cells, because it was not observed in A/J mice resistant to the virus and in susceptible BALB/c mice immunized with a thermosensitive mutant in which no viral replication occurs. In conclusion, a viral infection of liver endothelial cells may cause extensive loss of the fenestrations and thus lead to important functional pertubations.

Mouse hepatitis virus type 3 infection provokes a decrease in the number of sinusoidal endothelial cell fenestrae both in vivo and in vitro

Fenestrations of hepatic endothelial cells play an active role as a sieving barrier allowing extensive exchange between the blood and liver parenchyma. Alteration of these structures may be induced in the course of various pathological events and provoke important perturbations of liver function. We demonstrate here that sinusoidal endothelial cells are permissive for mouse hepatitis virus 3 (MHV3) in vivo and in vitro and that this infection leads to a striking decrease in the number of fenestrae. The disappearance of these structures observed under scanning electron microscopy or in cryofracture preparations in vivo and in vitro cannot be reversed by the action of cytochalasin B on the microfilament network. The decrease in the porosity seems to be related directly to the productive infection of the endothelial cells, because it was not observed in A/J mice resistant to the virus and in susceptible BALB/c mice immunized with a thermosensitive mutant in which no viral replication occurs. In conclusion, a viral infection of liver endothelial cells may cause extensive loss of the fenestrations and thus lead to important functional pertubations.

Actin cytoskeletal network in aging and cancer

The cytoskeleton is being recognized as an important modulator of metabolic functions of the cell. The actin cytoskeletal network, in particular, is involved in events regulating cell proliferation and differentiation. The state of actin in a variety of cell types is regulated by signals arising from the cell surface through a wide spectrum of interactions. In this review, we explore the role of actin cytoskeletal network in a series of events which are known to influence cell proliferation and differentiation. These include interaction of actin network with extracellular matrix proteins, cell surface membranes, second messengers, cytoplasmic enzymes and the nucleus. Because of the involvement of the actin network in such diverse interactions, we propose that alterations in the actin cytoskeletal function may be an important aspect of generalized decrease in cellular functions associated with aging. Preliminary data indicate that alterations in the cytoskeletal network do occur in cells obtained from older individuals. Alterations in actin state are also reported during malignant transformation of cells in culture, and in naturally occurring tumors. Taken together, the existing data seem to suggest that changes in the actin cytoskeletal network may be a part of the aging process as well as malignant transformation. Therefore, the study of the actin cytoskeletal network and its regulation has the potential to yield important information regarding cellular senescence and neoplastic transformation.

Host cell-dependent lateral mobility of viral glycoproteins

The lateral mobility of viral envelope proteins on the plasma membranes of infected cells is an important factor in both virus assembly and pathogenesis. The envelope glycoproteins of measles and human parainfluenza virus are mobile on the surfaces of infected HeLa cells and undergo lateral redistribution in the presence of specific antibody, forming unipolar caps. In contrast, no such redistribution was observed with influenza virus hemagglutinin (HA) or vesicular stomatitis virus (VSV) G glycoproteins on infected HeLa cell surfaces. However, the HA and G glycoproteins were both found to be mobile in the plasma membrane of CV-1 cells, or human or murine peritoneal macrophages. These results indicate that host cell-dependent as well as virus-specific factors are involved in determining viral glycoprotein mobility. No significant differences in the patterns of synthesis of influenza or VSV viral proteins were found in the various cell types examined. The HA and G proteins, when expressed from vaccinia virus recombinants, were each found to be immobile in HeLa cells and mobile in CV-1 cells, thus indicating that the host cell-dependent differences in mobility are an intrinsic property of each viral glycoprotein molecule and not the result of interaction with other viral components. It is suggested that the association of viral glycoproteins with either the cytoskeleton or membrane-associated cellular proteins may be related to the observed differences in lateral mobility.

Involvement of actin-containing microfilaments in HSV-induced cytopathology and the influence of inhibitors of glycosylation

Two and a half hours after infection with a high dose of different strains of HSV-1 which induce rounding of cells, breakdown of actin containing microfilaments can be observed. At the periphery of the cell, actin containing knob-like protuberances were visible. Later on, actin seems to be located exclusively on the surface of cells. Observations were done by immunofluorescence microscopy, scanning electron-microscopy and immunoperoxidase staining of ultrathin sections. The envelope of HSV appears to be stained by anti-actin. Strain IES produces rounding of cells at a high dose of infection before fusion proceeds at 37 degrees C. Similar alterations were not observed with the fusing strains MP and HFEM. Incubation of infected cells at 39 degrees C revealed strain dependent differences of the fusion activity. At 41 degrees C no "fusion from within" of cells but only rounding was detectable. Application of tunicamycin resulted in complete inhibition of fusion by all strains. The fusion activity of some strains of HSV-1 (ANG, HFEM, and MP) was not inhibited by addition of 2-deoxy-D-glucose and 2-fluoro-deoxy-D-glucose. A variant from strain MP could be isolated, which is sensitive to the effects of 2-deoxy-D-glucose. Inhibitors of processing of glycoproteins did not affect fusion of cells.

Cytochalasin D accelerates the release of Newcastle disease virus from infected cells

The role of the cellular cytoskeleton in Newcastle disease virus (NDV) infection was explored in two ways. First, the extent of the association of viral proteins with the cytoskeletal fraction of chicken embryo cells was determined. NDV-infected cells, pulse-labelled with [35S]methionine with or without a subsequent chase, were fractionated into Triton X-100-soluble and cytoskeletal fractions. All NDV proteins become associated with the cytoskeletal fraction of cells subsequent to their synthesis. Mixing experiments provided evidence against nonspecific sticking of proteins with this cell fraction. Second, the functional significance of the cytoskeletal association was explored using the inhibitor cytochalasin D. In the presence of this inhibitor, the rate of release of radioactively labelled virions was accelerated 2.5-fold. Colchicine did not significantly alter the rate of virion release. Virus particles released from cytochalasin D-treated cells had the same density as virions released from untreated cells, but were slightly less infectious and contained less actin. These results suggest that functional microfilaments do not play an obligatory role in viral morphogenesis but rather function to slow virus particle release.

The influence of contact-inhibited growth and of agents which alter cell morphology on the levels of G- and F-actin in cultured cells

The amounts of G-actin and F-actin were measured in cultured cells grown under various conditions. The percent of total actin as F-actin in monolayer cultures of asynchronous cells was 72.4% in Chinese hamster ovary (CHO) cells, 57.7% in HeLa cells, 69.8% in V79 cells, and 79.5% in 1080 cells. Actin comprises 2.4-3.1% of the total protein in these cell lines. Treatment of cells with 20 microM cytochalasin B (CB) caused different cytological effects from treatment with 10 microM colchicine, but the effects characteristic of each drug were observed throughout the range of cell lines used. Of the five cell lines treated with CB only the V79 and CHO cells showed a decrease (5-8%) in the level of F-actin. Colchicine treatment of HeLa cells resulted in a 13% increase in the percent F-actin, but similar treatment of CHO cells caused no significant change in F-actin. Therefore, a change in the steady state level of F-actin is not necessary for the observed cell shape change. The F-actin levels in CHO cells treated with 7 mM procaine decreased from 72 to 65% over the first 15 min of exposure, a time during which the cells rounded. After continuous exposure of the cells to procaine for 1 h, the F-actin percentage returned to control levels and the cells, though abnormal in appearance, flattened on the culture dish. The relationship between the level of F-actin and cell density was studied on the culture dish. The relationship between the level of F-actin and cell density was studied in the 10T 1/2 cells, a cell line which demonstrates density-dependent growth regulation. While contact inhibition was accompanied by a decrease in the F-actin percentage (from greater than 95% to about 60%) in one strain of 10T1/2 cells, two other strains of the same cells progressed from log phase growth (highly motile) to late plateau phase (non-motile, contact-inhibited) with a constant level of F-actin (about 60%). A spontaneous transformant of this cell line, which no longer demonstrated contact-inhibited cell growth, also maintained the same constant F-actin (60%). Thus, the maintenance of contact-inhibited growth control does not appear to depend upon the net distribution of actin between the globular and filamentous forms.

Lectin-induced actin polymerization in human lymphocytes: a possible signal for mitogenesis

Employing the DNase I inhibition assay, a decrease in G-actin is demonstrated in human mononuclear cells following stimulation with mitogenic lectins concanavalin A (Con A) and phytohemagglutinin (PHA), as well as a nonmitogenic lectin, wheat germ agglutinin (WGA). The decrease in G-actin can be prevented by pretreatment of cells with cytochalasin E, indicating that the decrease is likely due to conversion to F-actin. Thus, the receptor-mediated actin polymerization is common to both the mitogenic as well as the nonmitogenic lectins. The maximal decrease in G-actin with Con A and PHA occurs at the same concentrations of the lectins that give optimal mitogenic responses. It is a distinct possibility that actin polymerization could be one of the signals necessary for the initiation of mitogenesis. The difference between a mitogenic and a nonmitogenic lectin may lie in the fact that a second signal (or signals), derived from macrophages, may not be generated by a nonmitogenic lectin such as WGA.