Date Published: September 20, 2018
Publisher: Public Library of Science
Author(s): Cheng Fang, Bei Wu, Nhat T. T. Le, Thibaut Imberdis, Robert C. C. Mercer, David A. Harris, Umberto Agrimi.
Synaptic degeneration is one of the earliest pathological correlates of prion disease, and it is a major determinant of the progression of clinical symptoms. However, the cellular and molecular mechanisms underlying prion synaptotoxicity are poorly understood. Previously, we described an experimental system in which treatment of cultured hippocampal neurons with purified PrPSc, the infectious form of the prion protein, induces rapid retraction of dendritic spines, an effect that is entirely dependent on expression of endogenous PrPC by the target neurons. Here, we use this system to dissect pharmacologically the underlying cellular and molecular mechanisms. We show that PrPSc initiates a stepwise synaptotoxic signaling cascade that includes activation of NMDA receptors, calcium influx, stimulation of p38 MAPK and several downstream kinases, and collapse of the actin cytoskeleton within dendritic spines. Synaptic degeneration is restricted to excitatory synapses, spares presynaptic structures, and results in decrements in functional synaptic transmission. Pharmacological inhibition of any one of the steps in the signaling cascade, as well as expression of a dominant-negative form of p38 MAPK, block PrPSc-induced spine degeneration. Moreover, p38 MAPK inhibitors actually reverse the degenerative process after it has already begun. We also show that, while PrPC mediates the synaptotoxic effects of both PrPSc and the Alzheimer’s Aβ peptide in this system, the two species activate distinct signaling pathways. Taken together, our results provide powerful insights into the biology of prion neurotoxicity, they identify new, druggable therapeutic targets, and they allow comparison of prion synaptotoxic pathways with those involved in other neurodegenerative diseases.
Prion diseases are a group of fatal, infectious neurodegenerative diseases affecting humans and animals. The infectious agent, or prion, is composed of PrPSc, a conformationally altered form of a normal, cell-surface glycoprotein designated PrPC. Prions propagate themselves by a highly specific templating process in which PrPSc molecules impose their unique, β-sheet-rich conformations on endogenous PrPC substrate molecules [1–4]. Consistent with this model, PrP knockout mice, in which PrPC expression is absent, are completely resistant to prion infection [5, 6]. Moreover, these mice do not display symptoms of a prion disease , indicating that the disease phenotype is due primarily to a gain-of-function attributable to PrPSc or a related toxic species, rather than to a loss of the normal function of PrPC. Therefore, it is important to understand the molecular mechanism of PrPSc neurotoxicity.
Although the molecular templating process underlying the infectivity of prions is now well understood, the mechanisms by which prions cause neurodegeneration, in particular, damage to synapses, remain poorly understood. In a previously published study , we established a neuronal culture system that recapitulates one of the earliest events in prion synaptotoxicity, PrPSc-induced retraction of dendritic spines. In the present work, we have exploited the simplicity of this system to dissect the cellular pathways underlying the toxic effects of PrPSc on synapses. Our results uncover a multi-step signaling cascade that begins with binding of PrPSc to PrPC on the cell surface, and is followed by activation of NMDA and AMPA receptors, calcium influx, stimulation of the stress-inducible MAPK, p38, and finally collapse of the actin cytoskeleton, retraction of dendritic spines, and a decrease in excitatory neurotransmission (Fig 13). This work provides new insights into the mechanisms of synaptic degeneration in prion diseases, it identifies novel molecular targets for treatment of these disorders, and it allows comparison with pathologic mechanisms operative in other neurodegenerative disorders such as Alzheimer’s disease.