The case for cell-to-cell transmission of Parkinson disease (PD) pathology just got stronger, based on a new study showing that purified alpha-synuclein fibrils are endocytosed and induce the formation of Lewy bodies in previously healthy neurons, ultimately killing them.
The Oct. 6 study in Neuron by Virginia Lee, PhD, and colleagues, strengthens earlier conclusions from both animal models and human patients suggesting that alpha-synuclein pathology released from dying neurons is transmitted to adjacent cells, which experts say is similar to the spread of misfolded prion protein in Creutzfeldt-Jakob disease. The study also presents a simple new cell-based model of sporadic PD for testing new therapies to stop alpha-synuclein transmission or misfolding.
Dr. Lee, professor of pathology and lab medicine at the University of Pennsylvania School of Medicine in Philadelphia, is hesitant to describe the transmission process as prion-like. While pathology spreads from cell to cell, she said, “there is absolutely no evidence that Parkinson disease can be passed from one person to another. We really don't want to create public fear.”
The first strong evidence that PD may spread within the nervous system came in 2003, with careful post-mortem examinations by Heiko Braak, MD. On the basis of these exams, he proposed that alpha-synuclein pathology begins in the anterior olfactory nucleus and elsewhere, ascends through the brainstem to the substantia nigra — at which time motor features develop — and ultimately spreads into the cortex.
Then, in 2008, autopsies of several patients who had undergone fetal transplants revealed that Lewy bodies had developed in the fetal tissue within 15 years of implantation. “I think the whole Parkinson's disease research community was fascinated by this observation, which was unprecedented,” Dr. Lee said. “It wasn't until the results of the fetal grafts were published that people really began to entertain that there was some truth in what Braak was saying.”
There were two possibilities to explain these results, Dr. Lee said. Either Lewy bodies were developing in response to signals from the “bad neighborhood” of the patient's diseased brain, or the pathology was directly spreading from cell to cell. The case for the latter model was strengthened in 2009 by a study showing that both mouse embryonic stem cells and human dopaminergic cells would take up alpha-synuclein from neighboring cells that over-expressed it. But most neurodegenerative diseases develop in the context of normal levels of protein expression, she noted, making it crucial to determine if the same phenomenon could occur with wild-type levels of alpha-synuclein.
Therefore, Dr. Lee exposed wild-type mouse hippocampal cells in culture to purified human alpha-synuclein fibrils derived from bacteria. The fibrils form spontaneously at high concentrations of protein monomers, she said. The cells took up the fibrils, and within two weeks, had developed Lewy body-like aggregates. The same aggregates were seen in cortical and dopaminergic neurons treated similarly. It was clear that the exogenous fibrils required the host cell's own alpha-synuclein to form aggregates, since alpha-synuclein knockout cells did not form them. Like Lewy bodies in PD patients, the aggregates were ubiquitinated and phosphorylated, both modifications occurring after exposure to the fibrils.
By studying the cells at successive times after exposure, Dr. Lee showed that the earliest aggregates appeared in the axon terminal, where alpha-synuclein is normally concentrated, and only later appeared in the cell body, by which time axonal aggregates had grown larger. The temporal pattern suggests that the pathology propagates down the length of the axon, rather than forming in the terminal and being transported elsewhere.
In other words, the Lewy bodies do not move from the terminal to cell body, but rather early in the terminal they seed the formation of others further along the axon, resulting in new Lewy bodies growing in the cell body.
The development of aggregates correlated with increasing neuronal dysfunction and death, with a decrease in synchronized neuronal activity within four days of exposure, and the death of two-thirds of cells by 14 days.
These results, Dr. Lee said, directly support the hypothesis that Lewy body pathology can spread from cell to cell, as suggested by the Braak autopsy results, and that the development of Lewy bodies in fetal grafted cells arose from direct uptake of alpha-synuclein fibrils released by dying neurons in the striatum of the patients receiving the grafts.
The results do not yet close the case on whether Lewy bodies themselves are toxic, or represent an inadequate protective mechanism. “I like to believe they are toxic,” Dr. Lee said, “but it still remains to be seen how aggregates compromise other cell functions and pathways. We don't really know the cause of death in these neurons.”
Multiple other questions remain unanswered as well, including the exact process of the uptake of the fibrils, and whether toxicity is inherent in any alpha-synuclein species, or only in fibrils of a certain size.
Dr. Lee noted that the rapid effect of exposure to fibrils in this system, versus the slow degenerative process in human disease, may be in part due to concentration of fibrils; indeed, she showed that reducing the concentration slowed the formation of aggregates and cell death in culture.
One of the most immediate consequences of this study is that it presents a new and simple disease model unencumbered by the need for transgenic cells or organisms. “We have developed a very easy-to-use system for studying sporadic Parkinson disease. We have created PD pathology in a dish,” she said. Her lab is currently optimizing the system for rapid identification of compounds that can slow the development of pathology. “And lots of others can do this easily in their labs,” Dr. Lee said. “The more the merrier.”
‘A NEW PIECE OF THE PUZZLE’
According to Warren Olanow, MD, this work represents “an additional piece of the puzzle that supports the idea that protein misfolding is a very important part of the neurodegenerative process in Parkinson disease, and probably in other neurodegenerative diseases as well.” Dr. Olanow, who coauthored one of the studies on the fetal transplant patients, is professor of neurology and neuroscience at the Mount Sinai School of Medicine in New York.
“What they showed that couldn't be shown readily in vivo is that alpha-synuclein forms insoluble aggregates in the nerve terminal, and then the pathology spreads back to the cell body. Many of us suspected that, but they actually proved it,” Dr. Olanow said. This result is consistent with new data showing nerve terminal dysfunction in PD patients, he added.
The work further supports the model of disease spread put forward by Dr. Braak. “It starts to make a compelling story,” said Dr. Olanow.
Whether protein misfolding is the beginning of the pathogenic cascade is not yet clear, Dr. Olanow said, adding that he suspects it is. “You can argue for mitochondrial dysfunction, oxidative stress, or other contributors, but many labs have shown that if you do damage to one component of the cascade, others are damaged as well. All these are probably integrated, but it sure is appealing to me to think it is starting with abnormal handling of proteins.”
While Dr. Lee is reluctant to invoke prions, Dr. Olanow thinks the parallel is there. “You have a protein particle that gets into a cell, causes natively proteins to misfold into amyloid sheets, and then moves out. This is characteristic of prions,” he said.
“There are many ways you could attack a prion-like system therapeutically,” he said, including drugs that facilitate clearing of protein, or block uptake, or interfere with conformational change. “Based on this model, it suggests a whole new therapeutic approach to Parkinson disease.”