In Vivo Cytotoxicity of the Prion Protein Fragment 106–126

Abstract

Transmissible spongiform encephalopathies are fatal neurological diseases characterized by astroglyosis, neuronal loss, and by the accumulation of the abnormal isoform of the prion protein. The amyloid prion protein fragment 106 -126 (P106 -126) has been shown to be toxic in cultured hippocampal neurons (1). Here, we show that P106 -126 is also cytotoxic in vivo. Taking advantage of the fact that retina is an integral part of the central nervous system, the toxic effect of the peptide was investigated by direct intravitreous injection. Aged solutions of P106 -126 induced apoptotic-mediated retinal cell death and irreversibly altered the electrical activity of the retina. Neither apoptosis nor electroretinogram damages were observed with freshly diluted P106 -126, suggesting that the toxicity is linked to the aggregation state of the peptide. The retina provides a convenient in vivo system to look for potential inhibitors of cytotoxicity associated with spongiform encephalopathies. Transmissible spongiform encephalopathies (TSE)1 are neurological disorders having common pathological signs like vacuolization of the neurophils, astrocytosis, and loss of neurons (2). TSE are characterized by the accumulation of large aggregates of the prion protein, PrP-res, in both human and animal brains. The abnormal PrP-res isoform has a high content of ␤-sheet secondary structure, forms amyloid fibrils, and is partially resistant to proteolysis (3-5). The precursor of the pathological isoform PrP-res is a normal host glycoprotein (termed PrP-sen) widely expressed in the central nervous system and in peripheral tissues. Unlike PrP-res, PrP-sen is a protease-sensitive protein, soluble in mild detergents, and has mainly ␣-helix structures (6 -8). Interestingly, the development of spongiform pathologies requires the presence of both isoforms PrP-res and PrP-sen (9), the level of PrP-sen expression being an essential factor in the pathology (10). In experimental transmissions with transgenic mice, it was found that increasing the number of PrP gene copies led to a decreased incubation time of the disease (11, 12).The in vivo mechanisms leading to the synaptic loss and neuronal death are not elucidated, but the possible involvement of apoptotic processes has been postulated and is consistent with the lack of inflammation stigmata in TSE-affected brains. The neural tissue damages could be due exclusively to PrP-res deposits, and/or to disruption of PrP-sen physiological functions, and/or to cytotoxic effects induced by PrP fragments or PrP-res aggregates. PrP-res and a synthetic peptide fragment (P106 -126) have been shown to be cytotoxic in vitro likely via the programmed cell death pathway (1, 13). Like PrP-res, the P106 -126 peptide is partially resistant to proteinase K, presents a high ␤-sheet enriched structure, and forms amyloid fibrils in vitro (14). Hope et al. (15) have shown that the expression of PrP-sen is necessary for P106 -126 to exert its cytotoxic effects.Whether the apoptotic process could account for in vivo neuro...

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