Electron microscopic observations of vesicular stomatitis virus particles penetration in human fibroblasts

The penetration of vesicular stomatitis virus (VSV) into the target cells is known to depend on the acid triggered receptor-mediated endocytosis pathway (RME). However, the possible contribution of phagocytosis and macropinocytosis both at early and late stages of infection and of plaque-forming process is unknown. Electron microscopic images of fibroblasts obtained one hour after VSV adsorption at 37 degrees C showed RME vesicles as well as clusters of particles in cytoplasmic inlets and phagocytic vacuoles which are morphologically similar with macropinosomes. Therefore, the possibility that VSV can use, in addition to RME, phagocytosis and macropinocytosis is discussed with respect to the wide range of host cell VSV infectivity and binding as well as to pseudotype formation.

[The mechanisms of persistent measles virus infection in HEp-2 cells]

Persistent infection with Edmonston strain of measles virus was established in a highly susceptible clone of HEp-2 cells. The morphological and cytogenic aspects of persistent infection are presented. Carriage of measles virus in cells persistently infected is marked by important differences between standard Edmonston strain and virus stocks isolated from time to time from the carrier state. It is especially significant that cell fusion capacity decreased in amount concordant with the drop in the yield of infectious virus. Addition of UV inactivated Sendai virus, trypsin treatment or incubation at suboptimal temperatures precipitate the cytocidal effect of measles virus. Some clinical implications of reactivation of the lytic cycle in the carrier state are suggested.

Cell-membrane phospholipase C is involved in inducing the antiviral effect of interferon

A monospecific inhibitory antibody directed to phospholipase C (phosphoinositidase C) blocked the antiviral effect of human interferons alpha and beta when tested on human quiescent fibroblasts challenged with the vesicular stomatitis virus. This action was due to specific inhibition of polyphosphoinositide hydrolysis because (a) the F(ab')2 fragment of the antibody molecule was also inhibitory; (b) excess antibodies directed to phospholipase A2 and to a phosphatidylcholine-preferring phospholipase C did not have any inhibitory effect, and (c) the combination of 12-O-tetradecanoyl-phorbol-acetate and calcium ionophore A23187 had an interferon-like antiviral effect which was not influenced by the inhibitory anti-phospholipase C antibodies. To avoid an interferon-like effect due to induction of interferon by second messengers, Vero cells, which lack interferon biosynthesis, were also used. Liposomes containing inositol 1,4,5-triphosphate and 1-oleoyl-2-acetyl-rac-glycerol protected Vero cells against the infection with the vesicular stomatitis virus. These results taken together show that phosphoinositide-derived second messengers are involved in triggering the antiviral effect of interferons alpha and beta.

Protein kinase C and the antiviral effect of human interferon

Protein kinase C (PKC) inhibitors: Hidaka's compounds H-7 (10 microM) and H-8 (20 microM), palmitoyl-carnitine (10 microM) and phloretin (50 microM), did not modify the antiviral effect of human natural or recombinant interferon alpha and of natural interferon beta. The tumor promoter 12-o-tetradecanoyl-phorbol-13-acetate (TPA) (200 nM), known as activator of PKC induced an antiviral state when tested on human embryo fibroblasts challenged with the vesicular stomatitis virus. The battery of PKC inhibitors used inhibited the antiviral effect induced by TPA. Palmitoyl-carnitine (10 microM) exerted a toxic effect that was reversed by interferon treatment (2,000 IU/ml interferon alpha). These results suggest that PKC, possibly activated by interferon-receptor interaction, is not essential for inducing the antiviral effect of interferon, but, probably, mediates the antiviral effect of TPA.

Interferon: signal molecules involved in its antiviral effect

A major problem concerning interferon (IFN)-cell interaction is the second messenger system that transduces the IFN signal. We discuss the evidences existing in literature and our arguments which suggest that the antiviral effect of IFNs alpha and beta are mediated by a membrane mechanism including a phospholipase C dependent hydrolysis of phosphoinositides. The resulting two second messengers: diacylglycerol and inositol triphosphate and subsequent, separate but interacting, signal pathways: activation of protein kinase C and ionic events are tested in respect with the antiviral effect of IFN.

Cyclic GMP and protein kinase G inhibit the quantal transmitter release induced by protein kinase C

Protein kinase G inhibits the spontaneous release of acetylcholine quanta at the frog neuromuscular junction as shown by the effects of H-8, a G kinase blocking agent. Moreover, the permeant dibutyryl cGMP blocked the frequency increase obtained in the presence of protein kinase C activators (diacylglycerol and phorbol ester) while the cAMP activated protein kinase A did show only an additive effect.

Interferon-induced antiviral state is inhibited by neomycin and mimicked by diacylglycerols

The antiviral effect of human interferons alpha and beta was inhibited in dose-dependent manner by submillimolar concentrations of neomycin, known to block phosphoinositide hydrolysis and therefore the diacylglycerol formation. On the contrary, the synthetic permeant diacylglycerols (1-oleoyl-2-acetyl-sn or rac-glycerol) were able to induce an interferon-like antiviral state when tested against the vesicular stomatitis virus and herpes simplex type I virus. Hidaka's compound H-8 (1.2 microM), expected to inhibit cAMP- and cGMP-dependent protein kinases, did not modify the antiviral effect of interferon. Our data suggest that the phosphoinositide pathway is involved in transducing the interferon antiviral signal, but, since the exogenous phospholipase C (0.1-1 U/ml) failed to induce an antiviral state, this pathway, although implicated, seems not the only one.

The antiviral effect of human interferon alpha is dependent on phosphoinositide-derived messengers

Neomycin the putative blocker of membrane polyphosphoinositide hydrolysis, inhibited the antiviral activity of human interferon alpha, when tested on human quiescent fibroblasts challenged with vesicular stomatitis virus. The anti-interferon effect of neomycin could be correlated in terms of dose dependence for both neomycin (0.05-1 mM) and interferon (100-5,000 IU/ml). The results suggest that the antiviral activity of interferon alpha depends on diacylglycerol formation. Indeed, the synthetic diacylglycerol (50 microM) was as effective as 100 IU/ml interferon in inducing the antiviral state.

Anti-phospholipase C antibodies inhibit the lectin-induced proliferation of human lymphocytes

A novel approach was used to assess the role of phosphoinositide hydrolysis in the mitogenic action of phytohemagglutinin (PHA) or concanavalin A (ConA). The treatment of human peripheral blood leukocytes (PBL) with monospecific antibodies against phospholipase C (PLC) produced a dose-dependent inhibition (up to 100%) of PHA (10 micrograms/ml) or ConA (25 micrograms/ml) proliferative effects. Thus, the activation of membrane-bound PLC is a sine-qua-non condition for lectin-induced proliferation of T lymphocytes. The key-role of PLC versus protein kinase C (PKC) is stressed by the fact that the inhibition of PKC with Hidaka's compound H-7 (40 microM) produced only a partial blockade (about 25%) of lectin mitogenic effect.

Antibodies against phospholipase C inhibit smooth muscle contraction induced by acetylcholine and histamine

The rationale of this study was to obtain a highly specific inhibitor of phospholipase C by raising rabbit antibodies against the purified bacterial phospholipase C. The antibodies inhibited the enzyme activity in vitro and, as shown by immunofluorescence, cross-reacted with the membrane-bound phospholipase C of isolated guinea-pig smooth muscle cells. Incubation (0-4 h) of guinea-pig taenia coli and ileum with antibodies resulted in a progressive inhibition (up to 85%) of the contractile response evoked by 2 microM acetylcholine or 2 microM histamine but did not inhibit significantly the contraction produced by prostaglandin F2 alpha (0.1 microM). These inhibitory antibodies presumably represent the 'missing tool' needed to establish unequivocally if a given agonist acts via stimulation of the membrane-bound phospholipase C, and implicitly phosphoinositide hydrolysis.

Phospholipase C contracts visceral smooth muscle

Phospholipase C added into the muscle bath (0.2-1.5 units/ml) contracted the guinea-pig taenia coli in a concentration-dependent manner. The fully developed contraction appeared within second, like the contractile effect evoked by 5 microM acetylcholine. Pharmacochemical dissection (using phospholipase D, indomethacin, neomycin, Li+, phorbol ester and oleoyl-acetyl-glycerol) showed that the contractions induced by exogenous phospholipase C are mediated by inositol trisphosphate, not by diacylglycerol. Thus, direct evidence is provided for the key role of the phospholipase C/inositol-trisphosphate system in smooth muscle contraction.

Oxytocin contracts the human uterus at term by inhibiting the myometrial Ca2+-extrusion pump

The sarcolemma of smooth-muscle cells from human pregnant myometrium possesses a high-affinity Ca2+-ATPase, which has the characteristics of an active Ca2+-extrusion pump. This pump enzyme, either membrane-bound or solubilized, was strongly inhibited by oxytocin (half-maximal inhibition at about 4 microU/ml or 10 pM). However, under similar conditions, oxytocin did not inhibit Ca2+-extrusion ATPase of the erythrocyte membrane. The inhibitory concentrations of oxytocin correspond to hormone plasma levels which initiate spontaneous labor.

Calmodulin-dependent Ca2+-pump ATPase of human smooth muscle sarcolemma

An enzymatically active Ca2+-stimulated ATPase has been isolated from the sarcolemmal sheets of human smooth muscle (myometrium). Ca2+-ATPase activity was quantitated in an assay medium which simulated the characteristic free ionic concentrations of the cytosol. New computer programs for calculating the composition of solutions containing metals (Ca, Mg, Na, K) and ligands (EGTA, ATP), based on the updated stability constants, were used. In detergent-soluble form the enzyme has a high Ca2+-affinity expressed by an apparent Km (Ca2+) of 0.25 +/- 0.04 microM. The maximum specific activity (about 20 nmol of Pi/mg protein/min) was found in the micromolar domain of free-Ca2+ concentrations, the same levels required for normal maximal contractions in smooth muscle. The variation of free-Ca2+ concentration in the assay medium over 4 orders of magnitude (pCa 9 to pCa 5) resulted in a sigmoidal dependence of enzymatic activity, with a Hill coefficient of 1.4, which suggested the regulation of Ca2+-ATPase by allosteric effectors. The presence and the activator role of endogenous calmodulin in smooth muscle sarcolemma was proved by calmodulin-depletion experiments and by using suitable anticalmodulinic concentrations of trifluoperazine. The addition of exogenous calmodulin restored the enzyme activity. Apparently, the concentration of calmodulin in isolated smooth muscle sarcolemma is about 0.1% of sarcolemmal proteins, as deduced from the comparison of calmodulin-depletion and calmodulin-readdition experiments. Calmodulin increased significantly the enzyme Ca2+-affinity and Vmax (by a factor of about 10). At variance with the sarcoplasmic reticulum Ca2+-ATPase, the sarcolemmal Ca2+-ATPase is extremely sensitive to orthovanadate, half-maximal inhibition being observed at 0.8 microM vanadate. In conclusion, the Ca2+-ATPase isolated from smooth muscle sarcolemma appears very similar to the well-known Ca2+-pump ATPases of erythrocyte membrane, heart sarcolemma or axolemma. We suggest that this high-affinity Ca2+-ATPase represents the calmodulin-regulated Ca2+-extrusion pump of the smooth muscle sarcolemma.

Intracellular distribution of calcium in smooth muscle: facts and artifacts. A correlation of cytochemical, biochemical and X-ray microanalytical findings

Cytochemical methods, based on the use of oxalate and pyroantimonate as Ca markers, show Ca accumulation in the sarcoplasmic reticulum (SR), mitochondria and nuclei of smooth muscle cells from the guinea-pig taenia coli. A good correlation was found between the Ca concentrations measured in subcellular components in situ by X-ray microanalysis and the endogenous Ca concentrations estimated in subcellular fractions by atomic absorption spectrophotometry. Our findings suggest that the in situ Ca concentrations (mmol/kg dry weight) are: about 100 in the SR and mitochondria, and about 60 inside the nucleus, significantly higher than in cytoplasm.

Calcium in smooth muscle sarcoplasmic reticulum in situ. Conventional and X-ray analytical electron microscopy

Numerous electron-opaque deposits appear in the SR of the relaxed smooth muscle cells of the guinea pig Taenia coli that had been treated, before fixation, with a depolarizing medium containing oxalate to precipitate calcium ions in situ. X-ray spectra obtained by spot and line-scanning analyses of these deposits in situ show characteristic calcium signals, thus providing direct evidence for calcium accumulation inside the sarcoplasmic reticulum of smooth muscle.