One in 10 people with Parkinson’s start noticing signs of the disease between the ages of 20 and 50. Most have no family history, and few carry known autosomal-dominant PD mutations. Can induced pluripotent stem cell (iPSC) modeling help scientists understand the cause of young-onset PD? Clive Svendsen and colleagues at Cedars-Sinai Medical Center in Los Angeles believe so. In the January 27 Nature Medicine, they report a dearth of lysosomal proteins, along with an overabundance of α-synuclein, in YOPD-derived dopaminergic (DA) neurons. By boosting protein degradation pathways with an FDA-approved drug, the investigators reduced α-synuclein in these cells, hinting at a possible therapeutic avenue.
- Dopaminergic neurons derived from people with young-onset PD.
- The cells are low on some lysosomal proteins and accumulate α-synuclein.
- A repurposed drug stimulates their degradation of α-synuclein.
“Overall, this is impressive work that shows the potential of iPSC modeling,” said Birgitt Schüle, Stanford University, California. By deriving dopaminergic cells from YOPD iPSCs and applying transcriptomics, they were able to identify disrupted pathways that turned out to be druggable. “That could be a new way to tackle sporadic disease,” she said.
“This kind of approach, correlating gene expression with protein aggregation, has the potential to be a very powerful tool in helping to understand other genetic forms of Parkinson’s and parkinsonism,” said Patrick Lewis, University of London.
Deriving dopamine neurons from iPSCs can provide researchers with a cellular model of PD. This has been done previously for sporadic late-onset and familial forms of the disease (Byers et al., 2011; Sanchez-Danes et al., 2012; Woodward et al., 2014). In derived neurons from the latter, but not the former, α-synuclein accumulates (Soldner et al., 2009). No one has derived neurons from people with young-onset PD who have no family history or causative genetic variant. Svendsen and colleagues wondered if such neurons might reveal something about the cause.
Co-first authors Samuel Sances, Alexander Laperle, and Nur Yucer induced DA neurons from the iPSCs of three people diagnosed with YOPD and from three age-matched controls. The former were all in their 30s, and had no family history of parkinsonism. The researchers analyzed whole-genome sequencing data for each, and found no mutations in nine established PD genes: EIFG1, PARK2, LRRK2, GBA, SNCA, PINK1, PARK7, VPS35, and ATP13A2. No extra copies of SNCA, the gene encoding α-synuclein, were found either. All told, the data suggest no known genetic cause for this early form of PD, though Lewis pointed out that other potential YOPD genes have been discovered that might contribute to these people’s genetic risk (Puschmann 2017).
To look for other perturbations in these DA neurons, Sances and colleagues turned to α-synuclein. Its messenger RNA levels were on par with those of control neurons, but its protein levels were double. Why? Measuring total transcript and protein concentrations, the researchers found low levels of lysosomal proteins such as lysosomal-associated membrane protein 1 (LAMP1) and the hydrolase glucocerebrosidase, though their mRNA levels were normal. This finding held up in dopamine neurons derived from nine additional YOPD cases and seven controls.
When the researchers blocked all RNA translation with cycloheximide, α-synuclein levels promptly fell in control neurons, but continued to accrue in YOPD neurons. Breakdown of α-synuclein in control cells appeared to be lysosomal, because blocking the proteasome had no effect on α-synuclein protein levels. Dysfunction of the lysosome has been tied to Parkinson’s disease previously (Jun 2012 news).
Next, the authors tested a lysosomal stimulating compound called PEP005—an agonist of protein kinase C (PKC)—to see if it would reduce α-synuclein levels in these neurons. Within eight hours of adding the compound to the medium, α-synuclein dropped by half. Surprisingly, the scientists weren’t able to measure a bump in lysosomal activity until three days after PEP005 was added, suggesting the drug brought down α-synuclein levels another way. Epoxomicin, a proteasome inhibitor, nullified the effect of PEP005, suggesting PEP005 somehow recruited the proteasome.
PEP005 also increased the amount of tyrosine hydroxylase, which catalyzes production of dopamine, by 20 to 30 percent (see image above).
To see if PEP005 works in mice, Laperle and colleagues injected one 215 ng dose directly into the striatum on one side of the brain of wild-type. The drug reduced α-synuclein relative to the uninjected side.
Good Riddance. In control dopamine neurons (green) α-synuclein (red) is broken down by lysosomes (yellow). In YOPD neurons (pink), dysfunctional lysosomes lead to synuclein accumulation. Treating them with PEP005 (yellow) stimulates the proteasome (green) to degrade α-synuclein. TH production also increases (orange). [Image courtesy of Laperle et al., 2020.]
Lewis noted that the authors only examined α-synuclein, not other aggregation-prone proteins such as tau. Doing so would show whether these cells have a broader dysfunction of proteostasis, or something specific to α-synuclein.
The results suggest a large genetic component to YOPD, Sances told Alzforum. Because iPSC-derived neurons are stripped of most of the epigenetic changes brought on by their environment, the altered protein signature largely reflects the genotype. “Up to this point the cause of young-onset PD has been thought of as a mix of environmental and genetic risk,” said Sances.
PEP005, aka ingenol mebutate, is derived from the sap of the plant Euphorbia peplus, and known to induce necrosis. The FDA has approved a topical PEP005 gel preparation as a treatment for precancerous skin lesions. This means some toxicology has been done on the compound. Its ability to safely reach an intended target in the brain, much less inside neurons, is unknown, Sances told Alzforum.—Gwyneth Dickey Zakaib
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