SYDNEY—In light of increasing evidence to suggest a “prion-like” mechanism may be at work in Parkinson’s disease, researchers have begun to tease out the questions of why this happens and what the therapeutic implications might be. At the 17th International Congress of Parkinson’s Disease and Movement Disorders, Patrik Brundin, MD, PhD, Head of the Laboratory for Translational Parkinson’s Disease Research and Jay Van Andel Endowed Chair in Parkinson’s Research at the Van Andel Research Institute in Grand Rapids, Michigan, explained that the spreading pathology of Parkinson’s disease throughout the brain shows a “prion-like behavior of alpha-synuclein.”
Some Answers, More Questions
Five years ago, two studies on patients with Parkinson’s disease grafted with immature neurons described how alpha-synuclein forms aggregates in the otherwise young and healthy transplanted cells. Dr. Brundin presented data from a paper he coauthored in 2008 and from another published by Kordower and Olanow the same year demonstrating alpha-synuclein- and ubiquitin-positive Lewy bodies in grafted cells more than 10 years post-transplantation.
The patient, who had undergone transplantation surgeries in two different sessions, passed away in 2005. Lewy bodies were present in 2% of the 12-year-old (implantation surgery performed in 1993) grafted neurons and 5% of the 16-year-old (surgery in 1989) grafted neurons. “Yes, this was rare,” Dr. Brundin said, “but remember, in the substantia nigra in Parkinson’s disease, only about 3% to 4% of the dopamine neurons have Lewy bodies, so maybe there is a steady state where cells that develop Lewy bodies soon die and disappear from the graft, at the same time as new cells get Lewy bodies.”
Dr. Brundin proposed that these studies on patients with neural grafts could suggest that similar cellular mechanisms are at work in the Parkinson’s disease brain. “To model this, one needs cell culture models for a synuclein transfer,” he explained. Dr. Brundin and coinvestigators looked at two different cell lines that expressed alpha-synuclein and tagged them with green fluorescent protein (GFP) and red fluorescent protein (RFP). When these cells were grown side by side in the same culture dish, both colors appeared together in some of them, clearly demonstrating that alpha-synuclein can move between cells.
The next task was to exclude the possibility that the phenomenon might be driven by the fluorescent label itself. The fluorescent proteins are larger than alpha-synuclein and, as they are not normally expressed by neurons, they might behave in an aberrant fashion. The investigators chose to study human melanoma cells known to produce alpha-synuclein. They stained them with an antibody that only detects human alpha-synuclein. They grew these cells next to rodent nerve cells that express no human alpha-synuclein. “The results are striking,” Dr. Brundin reported. “We can easily identify the rat nerve cells, and we see that they have taken up the human alpha-synuclein that has been produced and released by the neighboring human melanoma cells.”
This basic type of research shows that synuclein can pass between cells, leading to the next important question—whether the transmitted synuclein can actually seed aggregation. To answer this question, Dr. Brundin and his colleagues once again turned to cell cultures. Using an advanced microscopy assay called bimolecular fluorescence complementation, they showed that alpha-synuclein derived from neighboring cells made direct contact with alpha-synuclein produced by the cell itself. In an animal transplantation model (mimicking the grafting paradigm in the grafted Parkinson patients described above), they confirmed that alpha-synuclein produced by the grafted neurons could seed onto alpha-synuclein that had been imported, and that was originally produced by cells in the host brain.
Dr. Brundin suggested that the next key cell biology questions that need answers involve mechanisms of alpha-synuclein transfer. Current investigations are looking at the role of exosomes—small vesicles 40 to 90 nm in diameter that appear to have a role in the transfer of alpha-synuclein between cells and released by cells under various forms of stress—which could have important therapeutic implications. This subject, Dr. Brundin said, is a very exciting addition to the emerging field of research into the prion-like behavior of alpha-synuclein.
—Linda Peckel
Contributing Writer