Cellular prion protein and Alzheimer disease

Abstract

Soluble oligomeric amyloid-β (Aβ) has been suggested to impair synaptic and neuronal function, leading to neurodegeneration that is clinically observed as the memory and cognitive dysfunction characteristic of Alzheimer disease, while the precise mechanism(s) whereby oligomeric Aβ causes neurotoxicity remains unknown. Recently, the cellular prion protein (PrP^C) was reported to be an essential co-factor in mediating the neurotoxic effect of oligomeric Aβ. Our recent study showed that Prnp^−/− mice are resistant to the neurotoxic effect of oligomeric Aβ in vivo and in vitro. Furthermore, application of an anti-PrP^C antibody or PrP^C peptide was able to block oligomeric Aβ-induced neurotoxicity. These findings demonstrate that PrP^C may be involved in neuropathologic conditions other than conventional prion diseases, i.e., Creutzfeldt-Jakob disease.

Keywords: :

• Amyloid-β
• NMDA receptor
• PrPC
• neurotoxicity
• oligomer

This article refers to:

Alzheimer disease (AD) is a progressive neurodegenerative disorder representing the most common cause of dementia in the elderly. Characterized clinically by memory loss, dementia, and eventual decline in motor skills, AD is only definitively diagnosed after post-mortem demonstration of the neuropathological hallmarks of the disease. These hallmarks include intracellular aggregates of phosphorylated tau in neurofibrillary tangles, neuronal cell loss in specific populations within the brain, and extracellular deposits of the Aβ protein in senile plaques. Although oligomeric Aβ, out of all of the Aβ species, is suggested to play an important role in the neuronal loss, the mechanisms through which Aβ causes neuronal death are still unclear. The pathological relevance of oligomeric Aβ has been substantiated by its disease-specific accumulation in transgenic mouse AD models¹ and by the accumulation of structurally equivalent oligomers in the human brain and cerebrospinal fluid. There is also great interest in understanding the mechanisms whereby oligomeric Aβ affects synaptic functions involved in learning and memory. Such knowledge may provide significant insight into AD pathogenesis and potentially lead to better strategies for the prevention and/or treatment of AD.

Recent observations suggested the possibility of a connection between AD and prion diseases.^2–6 Initially, Lauren et al. identified PrP^C as a high affinity receptor for oligomeric Aβ and showed that expression of PrP^C was essential for oligomeric Aβ-induced synaptic toxicity as determined by loss of long-term potentiation (LTP)² and memory impairment in transgenic Alzheimer mice.³ However, three independent studies failed to confirm that PrP^C played a critical role either in vivo or in vitro.^4–6 Balducci et al. reported that oligomeric Aβ is responsible for cognitive impairment in AD and that PrP^C is not required.⁵ Another paper suggested that ablation or overexpression of PrP^C had no effect on the impairment of hippocampal synaptic plasticity in APP/PS1 mice.⁴ At the same time, independent studies, including one from our group, provided additional experimental support for the hypothesis that PrP^C acts as a mediator of Aβ-induced toxicity. Recent studies confirmed that an anti-PrP antibody targeted to PrP^C residues 93–102 blocks LTP induced by Aβ-containing AD brain extract.^7,8 The cross-linking of PrP^C by oligomeric Aβ triggers abnormal activation of cPLA2 and synapse damage.⁹

Our recent study found that Prnp^−/− mice are resistant to the neurotoxic effect of oligomeric Aβ in vivo and in in vitro hippocampal slice cultures. Furthermore, blocking the binding between PrP^C and oligomeric Aβ using either an anti-PrP^C antibody or a competitive PrP^C peptide prevents Aβ oligomer-induced neurotoxicity.¹⁰ These studies support the hypothesis that the PrP^C/Aβ interaction is necessary for neuronal cell loss. Actually, other groups have also recently provided additional evidence that PrP^C can act as a mediator of Aβ-induced toxicity.^11,7 The expression of PrP^C sensitizes cells to the toxic effects of other β-sheet-rich conformers, such as the yeast prion protein Sup35 or designed β-sheet peptides, in addition to Aβ.¹¹

The molecular mechanisms of neurotoxicity caused by oligomeric Aβ through PrP^C remain unclear. NMDA receptor-mediated excitotoxicity might be the downstream mediator of Aβ neurotoxicity, since our data and others showed that an NMDA antagonist blocks the neurotoxicity.¹⁰ Oligomeric Aβ was found to directly or indirectly bind the NMDA receptor^{12, 11} and PrP^C is also reported to interact with the NR2D subunit, which is a key regulatory subunit of the NMDA receptor.¹³ Interestingly, we found that Aβ-induced neurotoxicity was significantly reduced by general pharmacological blockage of the NMDA receptor and by specifically blocking the NR2B subunit (our unpublished data). Consistent with our result, oligomeric Aβ induces early neuronal dysfunction largely by activation of NR2B-containing NMDA-receptors.^{14, 15} Several studies indicate that NR2A is generally found at the synapse, whereas NR2B is predominantly localized to extrasynaptic sites.¹⁶ Enhanced activation of extrasynaptic NR2B-containing NMDARs is common in AD and leads to excessive influx of Ca²⁺ into the cell, resulting in inappropriate activation of enzymes (such as calpains and other calcium-regulated enzymes) and mitochondrial dysfunction, culminating in apoptosis.^{16, 17} Collectively, these data suggest that abnormal NMDA receptor function may contribute to the neurotoxicity of oligomeric Aβ through PrP^C.

The connection between the NR2 subunit of the NMDA receptor and PrP^C is far from being understood. This may involve interaction of PrP^C with the protein assemblies and additional cellular factors required for signal transmission. Fyn kinase, a member of the Src family of tyrosine kinases (SFK), is a likely candidate being a signaling molecule that links the two proteins; it is a well-known kinase of the NR2 subunit^19,18 and is activated by stimulation of PrP^C in bioaminergic cells.²⁰ In addition, Fyn has been suggested to play roles in AD pathogenesis and Aβ-induced neurotoxicity.^22,21 Since the molecular mechanisms by which Fyn contributes to neurotoxicity are unclear, it would be interesting to study the role of Fyn kinase in Aβ-induced neurotoxicity through PrP^C and its mechanism. Indeed, Um et al. recently reported that the activation of Fyn kinase by Aβ and its neurotoxicity requires PrP^C expression suggesting the PrP^C-Fyn pathway is the main pathway of Aβ neurotoxicity.²³ Future study is required to delineate the connection between the NR2 subunit and PrP^C.

Another important issue regarding the PrP^C-Aβ interaction is to identify the binding sites on PrP^C linked to neurotoxic signaling by oligomeric Aβ. Pretreatment with the antibody 6D11, which binds PrP^C_93–109, prevents neuronal cell death by oligomeric Aβ, while another anti-PrP^C antibody, 6H4, which recognizes PrP^C_144–152, failed to block oligomeric Aβ-induced neuronal toxicity.¹⁰ Consistent with the antibody blocking experiments, addition of the peptide corresponding to residues PrP^C_98–107 reduced the neurotoxicity of oligomeric Aβ in the hippocampal slice cultures, whereas the peptide corresponding to residues PrP^C_213–230 had no effect on Aβ-induced neurotoxicity. These data indicate that the essential region in PrP^C that mediates the Aβ-PrP^C interaction falls within residues 98–107. Further support for this finding is provided by a previous study² that identified the amino acid sequence 95–105 as a binding site on PrP^C for Aβ. Treatment with an antibody binding this region prevented the Aβ-PrP^C interaction and Aβ-induced LTP² and improved cognitive deficits in aged AD transgenic mice.²⁴ Additional recent studies also demonstrated that an anti-PrP antibody targeted to PrP^C_93–102 blocks LTP induced by Aβ-containing AD brain extract.^{7, 8} Furthermore, PrPN, a mutant lacking a large portion of the N-terminal domain, is impaired in both binding to Aβ and mediating its toxic effects.¹¹ Collectively, these findings strongly suggest that PrP^C_93–107 contains the critical amino acid sequence for oligomer Aβ-induced synaptic impairment and neuronal cell death. To further refine the binding site, a study using overlapping peptides as competitors for Aß binding would be required. Although an increasing number of observations support the role of PrP^C as a receptor for Aβ-induced neuronal loss, further research is required to identify the neurotoxic mechanisms that act through PrP^C.

Neurotoxic signaling through PrP^C might explain, at least in part, mechanisms underlying the neurodegeneration seen in AD. In addition, it is possible that the Aß/PrP^C interaction has a function in normal neuronal function that is subverted in the disease state. Future studies should provide further clarification of the important roles of PrP^C in normal neuronal physiology as well as in the neurotoxic mechanism.

Acknowledgments

Work in the authors’ laboratories is supported by the National Institutes of Health (AG028679 to H.G.L.) and the Alzheimer’s Association (IIRG-11–173471 to H.G.L.).