The physiological role of prion protein in neurodegenerative disease

Abstract

The cellular prion protein (Prpc) is a glycosylphosphatidylinositol (GPI)anchored membrane protein, highly conserved in mammalian species, that is expressed most abundantly in the central nervous systenl. Post-translational modification of native PrPC into its pathogenic isoform (PrPSC) is the molecular signature underlying fatal neurodegenerative diseases, known as transmissible spongiform encephalopathies (TSEs). Since generation of PrPSC results from a misfolding of PrPC, there is a marked depletion of PrPC in the brain as the disease progresses, with consequent loss of its normal activity. Thus, identifying the function of PrPC may be crucial to understanding the basis for neurodegeneration in prion diseases. The most salient observations to date as regards prion physiology include: (i) the role of PrPC in copper homeostasis; (ii) the involvement of PrPC in triggering signal transduction pathways; and (iii) the anti-apoptotic and antioxidant properties of PrPC. The true connections between these apparently disparate functions of the prion protein remain however enigmatic, and this question was therefore addressed in the present thesis. In the first part of this work, the N-terminal domain of murine PrPC comprising amino acids 23-106 was expressed intracellularly in yeast, thus

reflecting the physiological generation of a PrP23-11 0/111 fragment (known as NI) during the normal cellular trafficking of mammalian PrPC, It was found that the murine PrP23-1 06 peptide protected S. cerevisiae cells against copper(II) toxicity, but did not modify the growth phenotype in response to zinc(ll) or nickel(II) ions. In addition, it was observed that heterologous expression of PrP23-106 protected neither wild-type yeast nor mutant strains lacking the gene for eu, Zn-superoxide dismutase (SOD), against oxidant toxicity induced by paraquat. Taken together, the results of the yeast work thus suggest a possible role for the mammalian NI fragment in intracellular copper buffering, but not a physiological SOD-like activity of the protein. The aim of the second part of the thesis was to identify a signalling kinase which is activated in both a copper and PrP-dependent fashion, and which acted as a modulator of neuroprotective signalling by PrPC, The collected data strongly indicates a functional link between PrPC expression and phosphatidylinositol 3-kinase (PI3K) activation, a protein kinase that plays a pivotal role in cell survival. Both mouse neuroblastoma N2a cells and immortalized murine hippocampal neuronal cell lines expressing wild-type PrPC had significantly higher PI3K activity levels than their respective controls. Moreover, PI3K activity was found to be elevated in brain lysates from wild-type mice, as compared to prion protein knockout mice. Recruitment of PI3K by PrPC was shown to contribute to cellular survival toward oxidative stress induced by 3-morpholinosydnonimine (SIN-I) and serum deprivation. Moreover, both PI3K activation and cytoprotection by PrPC appeared to rely on copper binding to the N-terminal octapeptide of PrPC. Based on these results, a model is proposed in which copper-bound PrPC, due to its plasma membrane localization, functions as a sensor for extracellular stress with the role of copper being that of triggering metal-dependent signals to PI3K, which in turn acts as a modulator of neuroprotective signalling. Given that conversion of PrPC to PrPSC in prion diseases leads to PrPC deficiency, pharmacological stimulation of lost PrPC signals identified in this work may provide a useful treatment approach for these fatal neurological illnesses. Keywords: prion protein, copper, oxidative stress, superoxide dismutase, Saccharomyces cerevisiae, NI fragment, phosphatidylinositol 3-kinase, neuronal survival.