Prions activate a p38 MAPK synaptotoxic signaling pathway

Abstract

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.

Aβ receptors specifically recognize molecular features displayed by fibril ends and neurotoxic oligomers

“…S5). We have shown previously that Aβ oligomer toxicity in this assay is PrP C -dependent 50 . We found that toxicity was correlated with the size of aggregate; ADDLs and 3-day protofibrils induced more marked spine retraction than 7-day protofibrils, while fully polymerized fibrils were relatively inert.…”

“…PrP C also serves as a cell-surface receptor mediating the synaptotoxic activity of PrP Sc , the infectious form of PrP. However, our evidence indicates that the signaling pathways initiated by PrP Sc and Aβ may not be identical 50 .…”

Prion neurotoxicity

“…B . A hippocampal neuronal culture system allows analysis of the acute toxic effects of prions on synaptic structure and function . Left: Neurons are isolated from hippocampi of neonatal pups and cultured at low density on coverslips that are suspended facedown, via wax dots, over a feeder layer of astrocytes.…”

“…Treatment with purified PrP Sc , but not mock‐purified material, leads to retraction of dendritic spines, revealed by staining with fluorescent phalloidin, as well as local increases in p38 MAPK phosphorylation, visualized by staining with antibodies to phospho‐p38 (red) and total p38 (green). Electrophysiological abnormalities in synaptic transmission are also detectable using patch‐clamping techniques . Images of mice were taken from Servier Medical Art ( http://smart.servier.com).…”

“…We have found that PrP Sc produces a strikingly selective synaptotoxic effect, specifically targeting the postsynaptic elements of excitatory synapses . Treatment of hippocampal neurons with purified PrP Sc from infected brains caused a marked reduction in the frequency of miniature excitatory postsynaptic currents (mEPSCs), and a less pronounced but statistically significant decrease in mEPSC amplitude.…”