Transcript: A signaling cascade initiated by the infectious prion protein leads to dendritic spine degeneration

[[RP_SECTION:prion-induced-synaptic-collapse|Prion induced synaptic collapse]]

Sam: PrPSc, the infectious form of the prion protein, doesn't just cause structural damage. It switches on a specific signaling cascade that tells the neuron to actively dismantle its own synapses. And notably, that process turns out to be reversible even after the damage has started. This is work from Cheng Fang and colleagues at Boston University, published in PLoS Pathogens.

Alex: So the collapse isn't just physical wear and tear. The cell is following instructions to take itself apart?

Sam: That's the picture. Think of a dendritic spine as a tent held up by an actin pole. PrPSc acts like a saboteur that trips an alarm — the p38 MAPK pathway — and that alarm tells the cell to knock the pole down. The spine collapses because it's been ordered to. [[RP_SECTION:signaling-pathway-and-reversal|Signaling pathway and reversal]]

Alex: If it's a signaling pathway rather than direct damage, that implies you could interrupt it before the neuron is actually harmed.

Sam: That's the central finding. Blocking either the NMDA receptor or p38 MAPK with inhibitors stopped spine retraction from happening in the first place. But the more interesting result: when they applied the p38 inhibitor *after* spines had already retracted, the spines re-emerged. The structure recovered.

Alex: That's a strong claim. p38 MAPK is a fairly general pathway — how do they rule out that they're just seeing some broad side effect of shutting it down?

Sam: Fair concern for any kinase inhibitor. They backed up the pharmacology with a genetic approach — a dominant-negative p38 mutant — which reproduced the protective effect without the off-target risk that comes with small-molecule drugs. And the toxicity itself was narrow: excitatory synapses were affected, but inhibitory synapses and presynaptic markers like synaptophysin were untouched. [[RP_SECTION:actin-cytoskeleton-and-function|Actin cytoskeleton and function]]

Alex: What's actually happening downstream of p38 that causes the physical collapse?

Sam: It comes down to the actin cytoskeleton — the tent pole in that analogy. When they used a compound called SiR-actin to stabilize the actin filaments directly, it completely blocked PrPSc-induced retraction, even with the upstream signaling intact. So p38 is the signal, and actin depolymerization is the executioner. They also confirmed this mattered functionally, not just structurally — they measured miniature synaptic currents, and stabilizing actin prevented the drop in both signal size and frequency that PrPSc normally causes. That's a fairly clean line from signaling, to structure, to function. [[RP_SECTION:comparison-with-alzheimer-models|Comparison with Alzheimer models]]

A signaling cascade initiated by the infectious prion protein leads to dendritic spine degeneration

Cheng Fang, Bei Wu, Nhat T T Le, Thibaut Imberdis, Robert C C Mercer, David A Harris

Sep 20, 2018·4:15·zotero·Original Paper

Prion synaptotoxicityNMDA receptorsp38 MAPK signalingDendritic spine degenerationPrPC-dependent signalingPrion diseases

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4 min

Briefing

Research Question

Prion diseases are characterized by early synaptic degeneration, yet the specific molecular mechanisms driving this process remain largely unknown. This study sought to dissect the signaling pathways by which the infectious prion protein (PrPSc) induces the retraction of dendritic spines in cultured hippocampal neurons, a process known to be dependent on the endogenous cellular prion protein (PrPC).

Approach

Using a previously established neuronal culture system, the researchers treated hippocampal neurons with purified PrPSc and employed a combination of pharmacological inhibitors and genetic tools to map the signaling cascade. They monitored dendritic spine morphology, intracellular calcium levels, and synaptic transmission (via patch-clamp electrophysiology) to identify the sequence of events leading to synaptic damage. They also compared these findings with the pathways activated by Alzheimer’s disease-associated Aβ oligomers.

Main Findings

PrPSc initiates a highly specific, stepwise toxic pathway: binding to PrPC triggers the activation of NMDA and AMPA receptors, resulting in an influx of calcium. This calcium surge stimulates p38 MAPK (specifically the α isoform) and downstream kinases (MK2/3), which ultimately cause the actin cytoskeleton within dendritic spines to collapse. The researchers demonstrated that inhibiting any step in this cascade—or stabilizing actin filaments—prevents spine retraction. Remarkably, p38 MAPK inhibitors were able to reverse spine retraction even after the degenerative process had already begun. Furthermore, while both PrPSc and Aβ oligomers require PrPC to exert toxicity, they activate distinct signaling pathways, as evidenced by their differential sensitivity to mGluR5 and p38 MAPK inhibitors.

Why It Matters

These findings provide a mechanistic explanation for early synaptic loss in prion disease and identify several druggable targets for potential therapeutic intervention. By demonstrating that the synaptic damage is reversible in this model, the study suggests that there may be a viable therapeutic window for treating patients even after the onset of early symptoms. Additionally, the identification of distinct pathways for PrPSc and Aβ highlights the complexity of neurodegenerative disease mechanisms and suggests that different proteinopathies may require tailored therapeutic strategies.

Key Terms and Definitions

PrPSc — The infectious, misfolded, and protease-resistant form of the prion protein that drives disease progression.
PrPC — The normal, endogenous cellular prion protein that acts as a receptor or mediator for the toxic signals generated by PrPSc.
Dendritic Spines — Small, actin-rich protrusions on dendrites that serve as the primary sites for excitatory synaptic connections and are critical for learning and memory.
p38 MAPK — A stress-activated protein kinase that, in this context, acts as a key signaling mediator in the pathway leading from PrPSc exposure to synaptic collapse.
Excitotoxicity — A pathological process where excessive activation of glutamate receptors (like NMDA receptors) leads to an influx of calcium and subsequent neuronal or synaptic damage.

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.

A signaling cascade initiated by the infectious prion protein leads to dendritic spine degeneration | Cheng Fang et al. | ResearchPod

A signaling cascade initiated by the infectious prion protein leads to dendritic spine degeneration: Research Paper Podcast