New role for -synuclein: stabilization of mRNA in bodies

June 12, 2022

Much of the research on α-synuclein, the major component of Lewy bodies in Parkinson’s and other neurodegenerative diseases, has focused on its role in synaptic vesicles and membranes. But could other functions be relevant to disease? In the June 9 Cell, researchers led by Vikram Khurana of Brigham and Women’s Hospital, Boston suggest the same, describing an unexpected role for -synuclein in cytosolic mRNA processing bodies, also called -bodies.

In cultured human neurons, -synuclein slowed down mRNA degradation by physically interacting with key body proteins tasked with dismantling transcripts. In postmortem tissue of the human frontal cortex, aggregates of -synuclein correlated with mRNA accumulation. In addition, genetic analysis linked variants in P body genes to PD risk. Khurana believes the data could shed new light on how the disease starts in some patients, which in turn could guide the development of therapies.

Others praised the work. “It is a tour de force which uses the power of model systems to locate an unexpected connection between -synuclein and bodies, which may have implications for PD,” James Shorter of the University of Pennsylvania, Philadelphia, wrote to Alzforum. Tim Bartels of University College London called the findings convincing. “I think the paper will be a groundbreaking publication in the field,” Bartels wrote (full comment below).

New road to toxicity. normal -synuclein (purple) can associate through its N-terminus with membranes or with mRNA degradation proteins such as Edc4 and Dcp1. Too many -synuclein disrupts de-capping of mRNA, leading to its construction. [Courtesy of Hallacli et al., Cell.]

Previous work on α-synuclein has linked the protein to vesicle transport and fusion, especially at synapses (June 2010 news† October 2014 news† Oct 2016 news† However, while working in the lab of the late Susan Lindquist at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, Khurana also discovered connections to mRNA translation and processing (News from February 2017† How α-synuclein affected these processes was unknown.

To follow up, first author Erinc Hallacli expressed -synuclein in a yeast model system containing labeled RNA-binding proteins that acted as reporters. -synuclein affected the function of several RBPs, especially those that regulated mRNA degradation. To determine how α-synuclein did this, the scientists turned to a fly model of synucleinopathy. A genetic study found that knocking out nine different P-body genes exacerbated synuclein toxicity and reduced the flies’ ability to move. A particularly strong effect came from the knockdown of the ribonuclease Xrn1, which cleaves mRNA. Again, this suggested an interaction between -synuclein and mRNA degradation genes.

Was this interaction direct? To investigate this, the authors immunoprecipitated α-synuclein from HEK293 cells and identified 29 bound proteins by mass spectrometry. Four of these were P-body proteins, with Edc4 as the strongest interactor. This scaffold protein helps assemble the complex responsible for snipping the 5′ cap on mRNA, allowing Xrn1 to eat the rest. The other three – Edc3, Dcp1, and Dcp2 – are also part of this digestion complex.

The authors dug deeper and found that α-synuclein is only associated with small soluble P bodies, not the larger macro P bodies scientists typically study. Intriguingly, some evidence suggests that larger P bodies are specialized to store mRNA, while the smaller bodies break it down (Corbet and Parker, 2019† The findings help explain how the association of -synuclein with these structures has gone undetected.

Next, the authors used human neurons generated from iPSCs to investigate the function of -synuclein in bodies. The iPSCs are made from familial PD patients who carry four copies of the -synuclein gene. In these neurons, which expressed twice the normal amount of -synuclein, more of the protein bound to Edc4. The more α-synuclein there was, the less Edc4 interacted with Dcp1, indicating interference with normal P-body function. In support of this, neurons with twice the normal amount of α-synuclein had only half as many macro-P bodies as control neurons, and they degraded mRNA more slowly. The authors noted that larger P bodies can be in equilibrium with smaller ones. Several of the most affected transcripts, such as SCARB2, VPS13A and PIKFYVE, have been previously associated with PD and other neurodegenerative diseases (july 2011 news† News from February 2018† April 2021 news†

Similarly, in postmortem dorsolateral prefrontal cortex samples from the ROSMAP observation cohort, the more Lewy bodies there were, the more these same P body transcripts had accumulated. In brain tissue from people who carried a -synuclein A53T variant or a gene duplication, the more -synuclein-Edc4 interactions there were, the fewer Edc4-Dcp1 interactions.

Genetic analyzes reinforced the argument that these changes in P bodies are relevant to disease. Parkinson’s GWAS previously had a P-body gene, LSM7 (Chang et al., 2017† The authors fished for others by examining the cumulative effect of mutations in P body genes. They found an association between these mutations and PD in seven different case-control cohorts. In a separate analysis, people with PD were more likely to carry rare variants predicted to impair the function of P body genes than healthy controls, but had the same number of silent variants in these genes as controls. Khurana noted that the finding needs to be confirmed in larger data sets.

How do these findings fit with the function of -synuclein in synaptic vesicles? α-synuclein binds vesicular membranes through its N-terminus, which takes the form of a corkscrew in the hydrophobic environment of the lipid bilayer. Curiously, the N-terminus is also responsible for binding de-capping proteins in the P body. When the authors mutated α-synuclein in cultured cells to make the protein more hydrophobic and promote membrane binding, it disappeared from bodies. The data indicate that membranes and P bodies can compete to bind α-synuclein, creating a kind of intracellular crosstalk between these compartments.

Bartels found this enlightening. His lab and others have linked α-synuclein’s tendency to bind membranes to its toxicity, but the mechanism was unclear. “This study would imply that aberrant lipid binding is not real” [cause toxicity] through the membrane, but by preventing the proper interaction between -synuclein-P body,” Bartels noted. “If that’s true, there will be a plethora of new exploratory studies…possibly starting an entirely new field of -synuclein biology.”

Hallacli noted that this previously unrecognized function of -synuclein in P bodies could complicate the considered therapeutic strategies that would prevent the protein from clustering on membranes and impair vesicle transport. If too much -synuclein gets into the cytosol, it can affect mRNA decay and cause toxicity.” In the future, there may be a combined therapeutic strategy where you also push it off Edc4 to fine-tune the effect,” Hallacli suggested.

Khurana believes that -synuclein membrane and -body pathologies may reflect different pathways to disease vulnerability in patients. His lab is investigating a possible physiological role for α-synuclein in P bodies. If it has one, that could affect strategies that try to suppress the protein via antisense oligonucleotides or immunotherapy (Conference news of May 2018† Conference news April 2021† Conference news April 2022† “Understanding the functions of -synuclein will be important to pinpoint unwanted effects on the target of such approaches,” Khurana wrote to Alzforum. – Madolyn Bowman Rogers