Article In Brief
A unique molecular structure—evident in induced pluripotent stem cells taken from people with young-onset Parkinson's disease—suggests that the defects may be present throughout patients' lives, and that they could therefore be used as diagnostic markers.
Induced pluripotent stem cells (iPSCs) taken from patients with young-onset Parkinson's disease (YOPD) and grown into dopamine-producing neurons displayed a “molecular signature” that was corrected in vitro, as well as in the mice striatum, by a drug already approved by the US Food and Drug Administration (FDA), a study published in the January 27 online edition of Nature Medicine found.
Although the patients had no known genetic mutations associated with PD, the neurons grown from their iPSCs nonetheless displayed abnormally high levels of soluble alpha-synuclein—a classic phenotype of the disease, but one never before seen in iPSCs from patients whose disease developed later in life. Surprisingly, for reasons not yet understood, the cells also had high levels of phosphorylated protein kinase C-alpha (PKC).
In addition, the cells also had another well-known hallmark of PD: abnormally low levels of lysosomal membrane proteins, such as LAMP1. Because lysosomes break down excess proteins like alpha-synuclein, their reduced levels in PD have long been regarded as a key pathogenic mechanism.
When the study team tested agents known to activate lysosomal function, they found that a drug previously approved by the FDA as an ointment for treating precancerous lesions, PEP005, corrected all the observed abnormalities in vitro: it reduced alpha-synuclein and PKC levels while increasing LAMP1 abundance. It also decreased alpha-synuclein production when delivered to the mouse striatum.
Unexpectedly, however, PEP005 did not work by activating lysosomal function; rather, it caused another key protein-clearing cellular structure, the proteasome, to break down alpha-synuclein more readily.
The findings suggest that the defects seen in the iPSCs are present throughout patients' lives, and that they could therefore be used as diagnostic markers. Moreover, the drug PEP005 should be considered a potentially promising therapeutic candidate for YOPD and perhaps even for the 90 percent of PD patients in whom the disease develops after the age of 50, according to the study's senior author, Clive Svendsen, PhD, director of the Cedars-Sinai Board of Governors Regenerative Medicine Institute and professor of biomedical sciences and medicine at Cedars-Sinai.
“These findings suggest that one day we may be able to detect and take early action to prevent this disease in at-risk individuals,” said study coauthor Michele Tagliati, MD, FAAN, director of the movement disorders program and professor of neurology at Cedars-Sinai Medical Center.
But the study still raises questions regarding the biological mechanisms, and certainly does not warrant off-label prescribing of PEP005 at this time, said Marco Baptista, PhD, vice president of research programs at the Michael J. Fox Foundation, who was not involved with the study.
“Repurposing PEP005 is a long way away,” Dr. Baptista said. “This is not something that neurologists should be thinking about prescribing or recommending to their patients.”
Study Design, Findings
Accumulation of alpha-synuclein has been seen in iPSC-derived dopaminergic cultures taken from patients with known genetic defects, but such defects account for only about 10 percent of the PD population. In those without known mutations, on the other hand, no defects in iPSC-derived dopamine-producing neurons have been seen. Until now, however, such studies had been conducted only in patients who had developed PD after age 50.
“My idea was why to look in young-onset patients,” said Dr. Svendsen.
The idea paid off more richly than he expected. “We were shocked to find a very, very prominent phenotype, a buildup of alpha-synuclein, in the neurons of these patients who are genetically normal,” Dr. Svendsen said. “None of the controls had a buildup of synuclein, and all but one of the early PD patients had a twofold increase in it.”
The signature is so consistent, he said, that it offers a natural model that can be interrogated to further understand its workings.
Because high levels of PKC were also seen, Dr. Svendsen said, “We picked a bunch of drugs known to reduce PKC. We found one, PEP005, which is actually extracted from the milkweed plant, and it completely reduced synuclein levels almost to normal in dopaminergic neurons. And it also increased dopamine levels in those cells, so we got two for one.”
After observing the effects of PEP005 in vitro, “We put it into the mouse brain and found it reduced synuclein in vivo,” Dr. Svendsen said. “But we had to infuse it right into the brain. We're now trying to work out how to get it across the blood-brain barrier more efficiently.”
To determine how PEP005 lowers cellular levels of alpha-synuclein, his group tested whether it was activating the lysosome, but found to their surprise that it did not do this until after the synuclein had already been degraded.
“Then we asked whether it could be the proteosome, which also breaks down proteins but normally doesn't break down synuclein,” Dr. Svendsen said. “But when we applied PEP005, it did activate the proteasome. So we think that might be the mechanism.”
Because the drug is currently applied externally, Dr. Svendsen said, the next step will be to see if it crosses the blood-brain barrier when applied to the skin of mice, and whether that results in a lowering of synuclein levels in dopaminergic neurons.
Justin Ichida, PhD, the Richard N. Merkin assistant professor of stem cell biology and regenerative medicine at the USC Keck School of Medicine, said the findings are “quite important in the field. The potential diagnostic tools they made could be important in clinical care. And identifying a drug that may very effectively reverse the disease in neurons is a very important discovery.”
He wondered, however, whether the increase in alpha-synuclein is truly specific to Parkinson's neurons or if it would also be seen in iPSC neurons from patients with Alzheimer's disease or amyotrophic lateral sclerosis.
“I wonder if alpha-synuclein accumulating is a sign of PD in a dish or is a consequence of neurodegeneration or impaired protein degradation in general,” Dr. Ichida said. “That's a key question if you want to use this molecular signature as a diagnostic tool.”
He also questioned if proteins other than alpha-synuclein, such as tau, would also be seen to accumulate in the iPSCs of YOPD patients.
“If one of the protein-clearance mechanisms in the cell is working poorly, you would imagine that other things would also accumulate,” Dr. Ichida said.
In response, Dr. Svendsen said that while some proteins other than alpha-synuclein were reported in the paper at increased levels, “We did not look at tau specifically, but are in the process of looking right now. It could be that synuclein and some other proteins are somehow altered to evade them from being degraded by the lysosome, or that there is a general lysosomal problem.”
Patrik Brundin, MD, PhD, director of the Center for Neurodegenerative Science and Jay Van Andel Endowed Chair at Van Andel Research Institute in Grand Rapids, MI, called the paper “very interesting and thought-provoking. If these findings hold up, they could shift our understanding of young-onset PD. They imply that there is a strong genetic component that has not been picked up in prior genetic studies.”
Dr. Brundin said he would like to see the results replicated in another lab using different sets of reagents. “It is so intriguing and rather unexpected that one wonders if the observations really apply, as the study states, to 95 percent of all YOPD.”
He also questioned whether all the young-onset PD patients are similar. “Clearly the iPSCs studied here are not monogenetic PD, so they must be very diverse genetically and still all have the same alpha-synuclein change.”
Dr. Brundin also asked why the abnormalities seen in YOPD neurons have not previously been seen in older cases of PD. “Is there a specific cutoff regarding age-of-onset when these purposed genetic differences apply?” he asked.
Dr. Svendsen responded: “We don't know why the YO have this phenotype or exactly what the cut off is. We have, however, looked at one adult-onset case that did not show this phenotype. Also, one of our YO cases did not show this phenotype. Thus some patients even with early onset may not have it. We are currently testing many more cases from older-onset patients.”
Dr. Brundin also wanted to know whether non-dopaminergic neurons have the same deficits described in the study.
“We don't know which neurons specifically have the protein deficit as we cannot do single-cell proteomics,” Dr. Svendsen answered. “It could be a little in all cells or a lot in a small set. Immunocytochemistry is not quantitative but showed that it is more likely a general increase in synuclein and not specific to dopaminergic neurons.”
While the findings in iPSCs suggest that the abnormal levels of alpha-synuclein must be present at birth, Dr. Brundin said, “I do not know how to reconcile the present findings with genetic data.”
The absence of previously described mutations in the YOPD patients means only that more work must be done to uncover the genetic underpinnings, Dr. Svendsen said.
“We're just at the tip of the iceberg with understanding the genome,” he said. “It's such a bizarrely complex beast. Perhaps there are a thousand different proteins interacting to stop the synuclein from being degraded. In 10 years, we probably will be clever enough to see it. We know it must be there. Now the genome guys will go after it.”
Dr. Baptista from the Michael J. Fox Foundation said he agreed with the view that there must be genetic alterations underpinning the defects seen in the iPSCs.
“Just because we call something non-genetic could simply reflect the current ignorance of the field,” he said. “I think the discoveries are simply difficult to make.”
He added that he wished that the main comparator in the study was not healthy controls, and that there were more older-onset iPSCs to compare against YOPD patients' samples.
Dr. Svendsen said it could be that the iPSCs from older-onset patients might yet be found with additional study to display abnormalities similar to those seen in YOPD.
“Right now we only see it in young onset,” he said. “We may need to leave the cultures longer to see in the older-onset patients. We are doing those experiments now.”
Drs. Tagliati and Svendsen disclosed that an intellectual patent is pending for diagnostic and drug screening for molecular signatures of early-onset Parkinson's disease. Dr. Ikeda is a co-founder of AcuraStem Inc. Dr. Brundin has received commercial support as a consultant from Renovo Neural, Inc., Lundbeck A/S, AbbVie, Fujifilm-Cellular Dynamics International, Axial Biotherapeutics, and Living Cell Technologies. He has also received commercial support for research from Lundbeck A/S and Roche and has ownership interests in Acousort AB and Axial Biotherapeutics. Dr. Baptista had no disclosures.