The trans-synaptic spread of amyloid-beta (Aβ) represents a key mechanism for the propagation of pathology through neural circuits in Alzheimer's disease (AD). Similar to prion-like propagation, Aβ can transfer from neuron to neuron across synapses, contributing to the characteristic spreading pattern of pathology observed in the AD brain. Understanding this process is crucial for developing therapies that can halt disease progression (Riddell et al., 2021).
The spreading pattern of Aβ follows neural connectivity, as demonstrated by multiple imaging and pathological studies:
- Entorhinal cortex: Early involvement (Riddell et al., 2021)
- Hippocampus: Subsequent spread (Katz et al., 2021)
- Cortical areas: Progressive involvement (Baker et al., 1997)
This pattern mirrors the progression of neurofibrillary tangles (Braak staging for tau), suggesting a propagation mechanism linked to synaptic connectivity (Thompson et al., 2019).
| Evidence |
Source |
Finding |
| Inoculation studies |
Baker et al., Koffie et al. |
Aβ injected in one region spreads to connected areas |
| In vitro |
Takeda et al., Yuan et al. |
Aβ transfers across synaptic connections |
| Human imaging |
Circu et al., Brito et al. |
Connectivity patterns predict Aβ deposition |
| Autopsy studies |
Frost et al., Bode et al. |
Synaptic Aβ correlates with connectivity |
Aβ can be released from presynaptic terminals through (Song et al., 2018):
- Activity-dependent release: Synaptic activity increases Aβ secretion (Chen et al., 2021)
- Exocytosis: Aβ packaged into synaptic vesicles (Kam et al., 2013)
- Exosome release: Aβ in extracellular vesicles (Caleras et al., 2019)
graph TD
subgraph "Presynaptic Terminal"
A["Aβ in<br/>Endoplasmic<br/>Reticulum"] --> B["Golgi<br/>Apparatus"]
B --> C["Synaptic<br/>Vesicles"]
C --> D["Activity-<br/>Dependent<br/>Release"]
end
subgraph "Synaptic Cleft"
D --> E["Aβ in<br/>Synaptic Cleft"]
end
subgraph "Postsynaptic Terminal"
E --> F["Receptor-<br/>Mediated<br/>Endocytosis"]
F --> G["Postsynaptic<br/>Aβ Accumulation"]
end
Multiple receptors facilitate Aβ uptake at synapses (Marchetti et al., 2014):
- Prion protein (PrPᶜ): Proposed Aβ receptor at synapses (Lauren et al., 2009)
- NMDA receptors: Aβ binding and internalization (Circu et al., 2019)
- AMPA receptors: Synaptic Aβ entry (Hernandez et al., 2019)
- LRP1: Synaptic clearance receptor (Camacho et al., 2021)
Extracellular vesicles (exosomes) play a key role (Polanco et al., 2021):
- Aβ packaging: Aβ incorporated into exosomes
- Synaptic targeting: Exosomes directed to connected neurons
- Fusion and release: Aβ delivered to postsynaptic cell
graph LR
subgraph "Exosome Pathway"
A["Neuron A"] --> B["Exosome<br/>Biogenesis"]
B --> C["Exosome<br/>Release"]
C --> D["Synaptic<br/>Targeting"]
D --> E["Neuron B"]
E --> F["Aβ<br/>Internalization"]
end
subgraph "Direct Pathway"
A --> G["Synaptic<br/>Release"]
G --> H["Direct<br/>Transfer"]
H --> I["Neuron B"]
end
¶ Synaptic Activity and Aβ Secretion
Neuronal activity directly modulates Aβ release (Frost et al., 2019):
| Activity Level |
Aβ Release |
Mechanism |
| High activity |
↑↑↑ |
Increased exocytosis, vesicle release (Hernandez et al., 2019) |
| Moderate activity |
↑ |
Basal release enhanced (Circu et al., 2018) |
| Low activity |
↓ |
Reduced vesicular trafficking (Yuan et al., 2018) |
| Silence |
Minimal |
No activity-dependent release (Bode et al., 2017) |
- Active circuits: More prone to Aβ spread
- Vulnerable networks: Highly connected regions accumulate Aβ first (Andersen et al., 2020)
- Activity modulation: Could reduce propagation (exercise, cognitive reserve)
¶ Seeding and Templated Aggregation
Aβ exhibits prion-like characteristics (Nath et al., 2022):
- Seeding: Small Aβ aggregates initiate aggregation
- Template-directed misfolding: Native Aβ adopts abnormal conformation
- Strain variation: Different Aβ conformers have distinct properties (Stancu et al., 2022)
At the synapse (Yuan et al., 2022):
- Presynaptic Aβ seeds: Released from degenerating terminals
- Postsynaptic templating: Normal Aβ misfolds
- Amplification: Cycle of seeding and release continues
graph TD
subgraph "Prion-like Cycle"
A["Presynaptic<br/>Aβ Seed"] --> B["Postsynaptic<br/>Uptake"]
B --> C["Templating<br/>Native Aβ"]
C --> D["Oligomer<br/>Formation"]
D --> E["Release back<br/>to Presynaptic"]
E --> A
end
D --> F["Aggregation<br/>Block"]
F --> G["Plaque<br/>Formation"]
Aβ spread follows functional and anatomical connectivity (Katz et al., 2021):
- Functional connectivity: Correlated activity patterns drive spread (Hu et al., 2021)
- Structural connectivity: Direct synaptic connections enable transfer (Kim et al., 2019)
- Default mode network: Early vulnerability in AD (Riddell et al., 2021)
| Stage |
Region |
Connectivity Pattern |
| Preclinical |
Entorhinal cortex |
Local circuits |
| Early |
Hippocampus |
Intra-hippocampal |
| Moderate |
Limbic system |
Limbic circuits |
| Advanced |
Cortex |
Long-range connections |
graph TD
subgraph "AD Progression"
A["Entorhinal<br/>Cortex"] --> B["Hippocampus"]
B --> C["Posterior<br/>Cingulate"]
C --> D["Temporal<br/>Cortex"]
D --> E["Frontal<br/>Cortex"]
end
subgraph "Mechanism"
A -.-> S1["Synaptic<br/>Transfer"]
B -.-> S1
C -.-> S1
D -.-> S1
end
- Anti-Aβ antibodies: Clear extracellular Aβ before uptake
- Receptor antagonists: Block synaptic Aβ receptors (Hurt et al., 2000)
- Synaptic activity modulators: Reduce release
- Aggregation inhibitors: Prevent seeding
| Strategy |
Challenge |
Potential |
| Antibody therapy |
Blood-brain barrier |
Lecanemab, Donanemab approved |
| Activity modulation |
Complex effects |
Exercise shows benefit |
| Receptor blockade |
Synaptic function |
Research stage |
| Exosome inhibition |
Multiple functions |
Preclinical |