Excitotoxicity Through NMDA Receptors Mediates Cerebellar Granule Neuron Apoptosis Induced by Prion Protein 90-231 Fragment

Prion diseases recognize, as a unique molecular trait, the misfolding of CNS-enriched prion protein (PrPC) into an aberrant isoform (PrPSc). In this work, we characterize the in vitro toxicity of amino-terminally truncated recombinant PrP fragment (amino acids 90-231, PrP90-231), on rat cerebellar granule neurons (CGN), focusing on glutamatergic receptor activation and Ca2+ homeostasis impairment. This recombinant fragment assumes a toxic conformation (PrP90-231(TOX)) after controlled thermal denaturation (1 h at 53 A degrees C) acquiring structural characteristics identified in PrPSc (enrichment in beta-structures, increased hydrophobicity, partial resistance to proteinase K, and aggregation in amyloid fibrils). By annexin-V binding assay, and evaluation of the percentage of fragmented and condensed nuclei, we show that treatment with PrP90-231(TOX), used in pre-fibrillar aggregation state, induces CGN apoptosis. This effect was associated with a delayed, but sustained elevation of [Ca2+](i). Both CGN apoptosis and [Ca2+](i) increase were not observed using PrP90-231 in PrPC-like conformation. PrP90-231(TOX) effects were significantly reduced in the presence of ionotropic glutamate receptor antagonists. In particular, CGN apoptosis and [Ca2+](i) increase were largely reduced, although not fully abolished, by pre-treatment with the NMDA antagonists APV and memantine, while the AMPA antagonist CNQX produced a lower, although still significant, effect. In conclusion, we report that CGN apoptosis induced by PrP90-231(TOX) correlates with a sustained elevation of [Ca2+](i) mediated by the activation of NMDA and AMPA receptors.