Glutamate-induced protein phosphorylation in cerebellar granule cells: role of protein kinase C

Effects of extremely low frequency (50 Hz) magnetic field on morphological and biochemical properties of human keratinocytes

We investigated the effects on human keratinocytes (HaCaT) of exposure to a sinusoidal magnetic field of 2 mT (50 Hz). These cells are a good model for studying interaction of nonionising radiation, because they are not shielded from fields in vivo and also because they are resistant to both mechanical and thermal stimuli. We performed scanning microscopy which showed modification in shape and morphology in exposed cells. This modification is related to differential actin distribution as revealed by phalloidin fluorescence analysis. Moreover, the exposed cells show increased clonogenic capacity, as well as increased cellular growth as showed by clonogenicity assays and growth curves. Indirect immunofluorescence analysis using a fluorescent antibody against involucrin and beta4 integrin, which are respectively differentiation and adhesion markers, revealed an increase of involucrin expression and segregation of beta4 integrin in the cell membrane in cells exposed to 50 Hz; a higher percentage of the exposed cells shows a modified pattern of adhesion and differentiation markers. We also present evidence that exposure of HaCaT cells can interfere with protein kinase activity. Our observations confirm the hypothesis that electromagnetic fields at 50 Hz may modify cell membrane morphology and interfere with initiation of the signal cascade pathway and cellular adhesion.

Isolation and characterization of VGF peptides in rat brain. Role of PC1/3 and PC2 in the maturation of VGF precursor

The neurotrophin responsive gene vgf is widely expressed in central and peripheral neurones, and in certain neuroendocrine cell populations. Its encoded VGF precursor protein (proVGF1: 617 amino acids in rat, 615 in man, > 85% homology) gives rise to several low molecular weight species. We studied a range of neuroendocrine and neuronal cells, in which VGF-processing products were prominent with an apparent molecular weight of 20 and 10 kDa (VGF20 and VGF10, respectively). Such peptides were recognized by antibodies specific for the C-terminal rat VGF nonapeptide, thus indicating that they included the C-terminus of proVGF. Ectopic expression of the neuroendocrine-specific prohormone convertases PC1/3 or PC2 in GH3 cells showed that both could generate VGF20, while VGF10 was preferentially produced by PC1/3. Site-directed mutagenesis was used to identify the KRKRKK(488) motif as the target within VGF sequence which leads to the production of VGF20. Molecular characterization of rat VGF10, on the other hand, revealed that this peptide is produced by cleavage at the RPR(555) site. By the combined use of high-resolution separation techniques, matrix-assisted laser desorption/ionization time of flight (MALDI-ToF) mass spectrometry and manual Edman degradation we identified in rat brain a VGF fragment analogous to bovine peptide V and two novel peptides also derived from the C-terminal region of proVGF.

Glutamate enhances phosphorylation of neurofilaments in cerebellar granule cell culture

In amyotrophic lateral sclerosis (ALS), an abnormal increase of glutamate in the central nervous system indicates that it may play a key role in motor neuron death. The neuronal accumulation of phosphorylated neurofilaments (NFs) suggests an alteration of phosphorylation of NFs is also involved. Rat cerebellar granule cells (CGCs) are sensitive to glutamate neurotoxicity and provide a suitable model system for clarifying its mechanisms. Using cultured CGCs, we investigated the relationship between glutamate neurotoxicity and the phosphorylation of NFs. Because glutamate showed a dose-dependent neurotoxicity for CGCs, we adopted a 10 microM glutamate treatment, which produced no acute neurotoxicity during the experiments. The number of phosphorylated heavy subunits of neurofilaments (NF-Hs) increased to approximately twice that of the control after 72 h, although the total number of NF-Hs remained constant throughout the experiment. The phosphorylation of NF-Hs was significantly suppressed by the AMPA-receptor antagonist CNQX, but not by the NMDA-receptor antagonist MK-801. Our findings therefore suggest that exposure to a low concentration of glutamate enhances the phosphorylation of NF-Hs, mainly via the AMPA receptor.

Influence of endogenous GM1 ganglioside on TrkB activity, in cultured neurons

We verified the hypothesis that changes in the endogenous GM1 ganglioside density in the environment of TrkB, receptor of brain-derived neurotrophic factor, can affect receptor activity, and focused on rat cerebellar granule cells expressing both GM1 and TrkB. Changes of the amount of GM1 associated to immunoprecipitated TrkB and of receptor tyrosine phosphorylation were evaluated after treatment with phorbol-12-myristate-13-acetate (1 microM, 7 min), reported to affect the plasma membrane distribution of endogenous gangliosides in the same cells. After treatment, the amount of GM1 associated to receptor and TrkB phosphorylation decreased by about 40%. The amount of associated GM1 decreased by about 33% also after concomitant treatment with phorbol ester and brain-derived neurotrophic factor, but in this case the neurotrophin was unable to enhance receptor tyrosine phosphorylation. These results for the first time suggest that changes in the amount of endogenous GM1 in the environment of TrkB can modulate receptor activity, and offer new clues for a better understanding of physiological and pathological events of the nervous system.

Effect of extremely low frequency (ELF) magnetic field exposure on morphological and biophysical properties of human lymphoid cell line (Raji)

Human B lymphoid cells (Raji) were exposed for 72 h to a 50 Hz sinusoidal magnetic field at a density of 2 milliTesla (rms). The results of exposure showed a decrease in membrane fluidity as detected by Laurdan emission spectroscopy and DPH fluorescence polarization. Field exposure also resulted in a reorganization of cytoskeletal components. Scanning electron microscopy (SEM) revealed a loss of microvilli in the exposed cells. This change in plasma membrane morphology was accompanied by a different actin distribution, as detected by phalloidin fluorescence. We also present evidence that EMF exposure of Raji cells can interfere with protein phosphorylation. Our observations confirm the hypothesis that electric and magnetic fields may modify the plasma membrane structure and interfere with the initiation of the signal cascade pathways.

Modulation by protein kinase C of nitric oxide and cyclic GMP poffation in cultured cerebellar granule cells

The possible modulation of nitric oxide (NO) synthase (NOS) activity by protein kinase C (PKC) was investigated in primary cultures of rat cerebellar neurons. Incubation of the cells with L-arginine and nicotinamide-adenine dinucleotide phosphate (NADPH) produced detectable levels of NO, as quantified by photometric assay [0.14 +/- 0.03 nmol/h/dish (2.5 x 10(6) cells)]. The NO producing activity was paralleled by concomitant accumulation of cyclic GMP (cGMP) (0.12 +/- 0.02 pmol/dish). Downregulation of PKC by prolonged treatment with phorbol esters or inhibition of the kinase by treatment with 4taurosporine raised the basal levels of NO and cGMP five fold. When granule cells were incubated in the absence of extracellular Mg2+, N-methyl-D-aspartate and to a lesser extent, glutamate became effective in enhancing NO formation and cGMP accumulation with respect to the control. The NO and cGMP increases induced by the two agonists were almost doubled by treatment of the cells with staurosporine or depletion of PKC. Calphostin C. an inhibitor of the regulatory domain of PKC, was as effective as staurosporine in increasing the formation of NO in both resting and excited cells. These results indicate that downregulation or inhibition of PKC increase NOS activity in cerebellar neurons, and suggest that phosphorylation of NOS by PKC negatively modulates the catalytic activity of the enzyme in these cells.

Role of membrane fluidity in Epstein Barr virus (EBV) infectivity on Akata cell line

Infection of Epstein Barr virus (EBV) to its host cells is initiated by the attachment of the glycoprotein gp 350/220 to the CR2 molecule. We used the sensitivity at the polar environment of the fluorescent probe Laurdan to study the membrane viscosity distribution from single leaving cells on two lymphoid cell lines Raji and Akata. Lipid analysis on both cells line demonstrated a lower cholesterol to phospholipid molar ratio on Akata than Raji cells. Cell fluidity analysis by Laurdan generalized emission polarization (GP) or by DPH polarization, indicated that membrane viscosity of Akata was lower than Raji cells. This difference was correlated to the increased susceptibility of Akata cells in expressing EBV early antigens (EA) after EBV superinfection.