Phospholipases A2: Distinctive roles in the regulation of α- synuclein biology and neuronal redox response

Abstract

Iron (Fe) accumulation and α-synuclein (α-syn) overexpression are hallmarks of several neurodegenerative disorders. We have previously reported that Fe-induced oxidative stress activates fatty acid release catalyzed by different phospholipase A2 (PLA2) isoforms in the nervous system. In this work, our aim was to study the involvement of PLA2s in the regulation of α-syn biology and the neuronal redox response to Fe overload. We also investigated the role of glia-secreted factors in the neuronal outcome. For this purpose, we exposed human neuroblastoma cells (IMR-32) to different ferric ammonium citrate concentrations (300–1000 μM) or vehicle for different incubation times (24–72 h). Using these experimental conditions, redox status, α-syn expression and phosphorylation, and the participation of calcium-independent and calcium-dependent PLA2 isoforms (iPLA2 and cPLA2, respectively) in the regulation of these events were studied. IMR-32 neurons exposed to Fe overload showed increased expression levels of iPLA2, concomitantly with an increase in lipid peroxides and reactive oxygen species. The pharmacological blockage of iPLA2 activity increased, even more, the levels of lipid peroxides and the content of reactive oxygen species. On the contrary, the inhibition of cPLA2 showed the opposite effect by promoting a decrease in oxidative stress markers associated with increased neuronal viability. Fe-challenged neurons also displayed increased α-syn expression and phosphorylation. The phosphorylation of α-syn was blocked by the inhibition of iPLA2 activity. To study the role of glia in the neuronal response to Fe, C6 astroglioma cells were challenged with ferric ammonium citrate or vehicle, and the astrocyte-derived media were added to neuronal cultures. Astrocytes exposed to Fe showed an increase in the glial marker S100B and lipid peroxidation levels, thus indicating reactivity to oxidative stress. Neurons incubated with the mentioned astrocyte-derived media displayed lower levels of oxidative injury than neurons only exposed to Fe. Astrocytes were positive for the rate-limiting step enzyme for glutathione biosynthesis. Altogether, our results show specific roles for the different PLA2 isoforms in the neuronal response to Fe-induced injury: whereas iPLA2 showed to be neuroprotective and also to be involved in the regulation of α-syn phosphorylation, cPLA2 appeared to act as a damage promoter. To ascertain the nature of the effect exerted by astrocytes on the neuronal response to oxidative stress, we are currently studying glutathione synthesis and how the isoform-specific PLA2-inhibition could be involved.