Different Molecular Mechanisms Mediate Direct or Glia-Dependent Prion Protein Fragment 90–231 Neurotoxic Effects in Cerebellar Granule Neurons

Glia over-stimulation associates with amyloid deposition contributing to the progression of central nervous system neurodegenerative disorders. Here we analyze the molecular mechanisms mediating microglia-dependent neurotoxicity induced by prion protein (PrP)90–231, an amyloidogenic polypeptide corresponding to the protease-resistant portion of the pathological prion protein scrapie (PrPSc). PrP90–231 neurotoxicity is enhanced by the presence of microglia within neuronal culture, and associated to a rapid neuronal [Ca++]i increase. Indeed, while in “pure” cerebellar granule neuron cultures, PrP90–231 causes a delayed intracellular Ca++ entry mediated by the activation of NMDA receptors; when neuron and glia are co-cultured, a transient increase of [Ca++]i occurs within seconds after treatment in both granule neurons and glial cells, then followed by a delayed and sustained [Ca++]i raise, associated with the induction of the expression of inducible nitric oxide synthase and phagocytic NADPH oxidase. [Ca++]i fast increase in neurons is dependent on the activation of multiple pathways since it is not only inhibited by the blockade of voltage-gated channel activity and NMDA receptors but also prevented by the inhibition of nitric oxide and PGE2 release from glial cells. Thus, Ca++ homeostasis alteration, directly induced by PrP90–231 in cerebellar granule cells, requires the activation of NMDA receptors, but is greatly enhanced by soluble molecules released by activated glia. In glia-enriched cerebellar granule cultures, the activation of inducible nitric oxide (iNOS) and NADPH oxidase represents the main mechanism of toxicity since their pharmacological inhibition prevented PrP90–231 neurotoxicity, whereas NMDA blockade by d(−)-2-amino-5-phosphonopentanoic acid is ineffective; conversely, in pure cerebellar granule cultures, NMDA blockade but not iNOS inhibition strongly reduced PrP90–231 neurotoxicity. These data indicate that amyloidogenic peptides induce neurotoxic signals via both direct neuron interaction and glia activation through different mechanisms responsible of calcium homeostasis disruption in neurons and potentiating each other: the activation of excitotoxic pathways via NMDA receptors and the release of radical species that establish an oxidative milieu.