Synuclein Surprise: Toxicity Linked to Aβ, Plaques?
It may be a bit of a leap, but could α-synuclein toxicity be mediated, at least in part, by amyloid-β (Aβ)? That's one interpretation of a paper in the February 24 Nature Neuroscience online. Researchers led by Thomas Südhof, University of Texas Southwestern Medical Center in Dallas, report that Aβ is significantly elevated, leading to the accumulation of insoluble aggregates, in transgenic mouse models of α-synuclein toxicity. They also found that knocking out ApoE attenuated neurodegeneration in these models and reduced Aβ levels. The study suggests a molecular link between ApoE/Aβ and α-synuclein—pathological players in the two major neurodegenerative disorders, Alzheimer and Parkinson diseases, respectively.
Despite being one of the first-known causes of familial Parkinson disease, α-synuclein has remained a bit of a mystery. Aggregates of the protein appear in Parkinson disease (PD), multiple system atrophy, dementia with Lewy bodies, and even Alzheimer disease (AD)—where the protein is often found as a component of amyloid plaques—but scientists still puzzle over exactly how the protein causes neurodegeneration. Many research groups have turned to transgenic models for the answer. In mice, overexpression of mutant human α-synuclein leads to neurodegeneration primarily in the spinal cord. It was in characterizing one of these models that Südhof and colleagues made the connection with α-synuclein and ApoE.
ApoE, of course, is an established risk factor for Alzheimer disease. By some calculations, having two copies of the ApoE4 variant increases the risk of AD by 15-fold. There is much sparser evidence that ApoE might also be a risk factor for sporadic Parkinson disease. In PD it is the ApoE2 allele that seems to put people at risk, though the effect is slight (see PDGene). Nevertheless, to see if ApoE is involved in neurodegeneration in their transgenic models, first author Gilbert Gallardo and colleagues used quantitative immunoblotting to determine levels of the protein in the mouse spinal cord. They found that there is a fourfold increase in total cord ApoE and a massive (20-fold) increase in ApoE in the sciatic nerve in mice that express human α-synuclein with the A30P mutation. Interestingly, these increases were only detected in symptomatic mice. Transgenic animals that are asymptomatic (the model is not fully penetrant) had no elevation of ApoE, suggesting the lipoprotein increases are intimately linked to pathology in these animals. This is supported by data from a second model using the A53T mutant, where ApoE levels were even higher.
Given that ApoE4 in humans can drive the production of Aβ and the accumulation of Aβ fibrils (see ARF related news story), the researchers looked to see if the massive increase in ApoE had any effect on the peptide. Mouse Aβ is not exactly homologous to the human form and does not readily form fibrils, but when the researchers examined spinal cords they found a modest increase in immunoreactive soluble Aβ40 and Aβ42, a two- to fivefold increase in insoluble Aβ, and an accumulation of thioflavin S-positive deposits, indicative of fibrillar Aβ. The data suggest that, in this mouse model of α-synuclein, the conditions may be just right for Aβ aggregation.
The researchers considered two explanations for what might be going on. Either the rise in ApoE contributes to the neurodegeneration, or it is secondary to the neurodegeneration. To distinguish which might be true, the researchers crossed the α-synuclein mice with ApoE knockout mice. They found that though neurodegeneration was not completely abolished in the absence of ApoE, toxicity started later and the mice survived longer. One reason for the ongoing pathology in the ApoE knockouts may be a sickly ubiquitin proteasome system (UPS). The UPS, which grinds up unwanted proteins, has long been suspected of playing a role in neurodegenerative diseases (see ARF related news story), and the researchers found that it is altered in their synuclein models. Ubiquitinated proteins are increased two- to threefold in asymptomatic mice and four- to fivefold in symptomatic animals. Proteasome subunit composition is also out of balance, with some subunits being increased and others decreased. Interestingly, though Aβ-positive aggregates were reduced in ApoE-negative mice, they were not completely eliminated, though whether this is related to a malfunctioning UPS is not clear. “Taken together, the data show that ApoE contributes to the pathogenesis of α-synuclein induced neurodegeneration,” write the authors.
In a written comment, David Holtzman of Washington University, St. Louis, Missouri, notes that this paper convinces that mouse ApoE somehow mediates α-synuclein toxicity (see below). He adds that it is unclear how or where they may interact, since one is primarily intracellular and the other extracellular, but suggests that the work will open up new areas of investigation. In an interview with ARF, John Trojanowski, University of Pennsylvania, Philadelphia, agreed with this sentiment. “This is a very exciting result and opens up new pathways for investigating the coincidence of α-synuclein pathology and Aβ pathology. It will be important to understand how Aβ fibrillizes more avidly in these mice than otherwise, and it brings up important questions that need to be addressed in human material, too,” Trojanowski said.
The first question that comes to mind, perhaps, is why synucleinopathies are not more frequently associated with Aβ toxicity. As Trojanowski pointed out, amyloid plaques are certainly found in dementia with Lewy bodies, but they are rare in PD, and not found at all in multiple system atrophy. Hopefully, by the next leap day, there will be further advances to report. By then, PIB PET imaging, for one, should have established the extent of Aβ amyloid pathology across the spectrum of PD, DLB, and PD with dementia.—Tom Fagan
- Gallardo G, Schlüter OM, Südhof TC. A molecular pathway of neurodegeneration linking alpha-synuclein to ApoE and Abeta peptides. Nat Neurosci. 2008 Mar;11(3):301-8. PubMed.
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This paper by Sudhof and colleagues is quite interesting. The data are convincing that somehow, murine ApoE is involved in the spinal cord degeneration caused by overexpression of mutant α-synuclein in that the α-synuclein transgenic mice on an ApoE KO background have increased lifespan, decreased synuclein aggregation, decreased UPS activation, and decreased murine Aβ accumulation. It is not clear from genetic studies in humans whether ApoE isoforms influence risk or progression of Parkinson disease. However, this paper strongly suggests that somehow ApoE influences α-synuclein aggregation.
Since ApoE is a secreted protein and most α-synuclein is believed to be cytoplasmic, whether ApoE and synuclein interact directly or indirectly is not yet clear. Synuclein can be found in the extracellular space such as in the CSF. Perhaps it is possible that ApoE and synuclein interact extracellularly. Alternatively, perhaps ApoE, being in lipoprotein particles in the brain, can in some way interact with synuclein after being internalized by cell surface receptors. Synuclein has lipoprotein-like properties and can interact with synaptic vesicle membranes and other intracellular membranes. Future studies may be able to address whether it can directly associate with ApoE via some of these membrane interactions. These findings open up a new area of investigation in relation to ApoE and synucleinopathy.
Washington University in St. Louis
The recent work by Gallardo et al. made several novel and interesting findings that link ApoE and Aβ to α-synuclein-mediated neurodegeneration. Using a mouse model of Parkinson disease in which a mutant form of human α-synuclein is expressed, they show that mice with symptoms of neurodegeneration have increased levels of ApoE in the spinal cord and peripheral nerve compared to non-symptomatic mice. Additionally, the symptomatic mice have increased levels of endogenous mouse Aβ in the spinal cord. To determine whether the increase in ApoE was a potential cause or a secondary effect of neurodegeneration, the investigators crossed their mouse model of Parkinson disease to ApoE knockout mice. Interestingly, they found that deleting ApoE attenuated neurological symptoms and decreased Aβ levels in the spinal cord of their mouse model of Parkinson’s.
These findings suggest that ApoE modifies α-synuclein-mediated neurodegeneration. Since α-synuclein is mainly intracellular and ApoE is primarily extracellular, it seems unlikely that ApoE influences α-synuclein via direct binding. The authors suggest that ApoE alters the ubiquitin/proteasome system (UPS); however, the differences the authors observed in the UPS may be secondary to downstream changes rather than direct effects of ApoE. Interestingly, α-synuclein is less soluble in mice expressing ApoE, suggesting that ApoE may affect either the aggregation of α-synuclein or the lipid associated with α-synuclein. Perhaps ApoE alters cellular lipids in a way that affects the formation of α-synuclein inclusions and subsequent neurodegeneration.
This paper prompts many questions that will need to be answered by further experiments and analysis. How does ApoE affect α-synuclein? Does ApoE play an important role in human synucleinopathies? What is the relevance of Aβ to neurodegeneration in synucleinopathies?
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