LAG3 helps to transmit
fibrils in Parkinson’s disease
Published January 05 2016 Fibrillar alpha-synuclein (brown) shows up in Parkinson’s disease IMAGE COURTESY OF WIKIMEDIA
“There’s gotta be a way it’s getting into cells,” says Ted Dawson of the Johns Hopkins University School of Medicine. He’s referring to fibrillar alpha-synuclein, the culprit of Parkinson’s disease, which is the second-most common neurological disorder.
More than 60,000 people get diagnosed with Parkinson’s disease in the U.S. each year. The disease is one of several brain disorders where the root cause is the transmission of a protein in the form of aggregates through neurons. The aggregated protein in Parkinson’s disease is alpha-synuclein. Although usually monomeric with a function that’s not known, alpha-synuclein can misfold and form clumps that cause neuronal cell death.
In a study published Sept. 30 in the journal Science, members of the Dawson laboratory identified LAG3 as a receptor for the pathological alpha-synuclein. LAG3 preferentially bound the alpha-synuclein fibrils, suggesting that LAG3 acted as a doorway into cells for alpha-synuclein aggregates and permitted their transmission.
For their experiments, the investigators used preformed fibrils, or PFFs. PFFs are an experimental tool that mimics alpha-synuclein fibrils. They were developed by Virginia Lee’s laboratory at the University of Pennsylvania. It was the initial study using PFFs by the Lee group, published in 2011 in the journal Neuron, that motivated the Dawson group.
“It was just a beautiful experiment,” says Dawson. The Lee group added PFFs to wild-type neurons and got the “disease in the dish,” says Dawson. In cultured neurons in which alpha-synuclein was knocked out, no hallmarks of Parkinson’s disease were observed. This suggested that fibrils somehow bound and entered cells to cause toxicity. So how exactly did these fibrils gain access to cells?
The Dawson group screened a library of 352 proteins for binding partners of PFFs. The library, which had been used by Stephen Strittmatter’s group at Yale University, consisted of transmembrane proteins. The Dawson group found three proteins that bound fibrillar alpha-synuclein. Lymphocyte activation gene 3, or LAG3, had the highest affinity for the fibrils.
When the Dawson group investigated LAG3, it preferred to bind fibrils rather than monomers. It was expressed on neurons but not on astrocytes or microglia. The number of internalized PFFs was significantly lower in LAG3-knockout neurons.
The investigators next used a microfluidic device. In this device, three adjacent chambers of cultured neurons were connected by grooves. Initially, wild-type neurons were cultured in all three chambers and PFFs were added to the first chamber. PFFs were observed to be transmitted to the third chamber. However, when LAG3-knockout neurons were cultured in the second chamber, fewer PFFs were transmitted to the third chamber. When LAG3 was added back to the second chamber, transmission of PFFs was restored. These experiments “showed that LAG3 was really responsible for the cell-to-cell communication,” says Dawson.
New questions now arise. First, why is LAG3, an immune-system protein, expressed on neurons at all? Second, LAG3 contains an intracellular domain that may signal when bound to fibrillar alpha-synuclein. “It’s got to be doing something,” says Dawson. But, he adds, no one knows what LAG3’s intracellular domain does. Third, while LAG3 may be important for taking in alpha-synuclein, other pathways of entry are possible. Dawson underscores that knocking out LAG3 does not completely halt fibrillar transmission.
Preliminary studies in LAG3-knockout mice support the use of antibodies against LAG3 as therapeutics; a more in-depth study is underway. Dawson is quick to note that it’s still unclear if these antibodies can cross the blood–brain barrier.
LAG3 is a new piece of the Parkinson’s disease puzzle. It requires more attention to understand its role as the partner in crime for alpha-synuclein.
is a graduate student at the Johns Hopkins University School of Medicine.