Best known to biologists as brightly glowing florescent tracers, quantum dots could also have therapeutic powers, says a new study. Work led by Han Seok Ko of Johns Hopkins University in Baltimore and Byung Hee Hong at the University of Seoul, South Korea, reveals that nano-scale carbon lattices bind to and dissolve α-synuclein fibers in vitro. In cells, the graphene quantum dots protect against α-synuclein toxicity, and prevent its spread between neurons. In animals, the dots protect neurons from the ravages of injected synuclein fibrils, or mutant synuclein expression, all without signs of toxicity. The findings are reported in the July 9 Nature Nanotechnology.
- Quantum dots made from graphene disaggregate α-synuclein fibers in vitro.
- In animals, the dots prevent α-synucleinopathy and improve motor function.
- Nanomaterials could provide new approaches to neuroprotection.
“This exciting study presents a novel and very untraditional way of treating synucleinopathy,” said Poul Henning Jensen, Aarhus University in Denmark. “They have done a good job of documenting their observation of a protective effect, but understanding the mechanism will take more work,” he said.
Quantum dots can be made of many materials—those derived from graphene (GQDs) consist of a single layer of carbon atoms arranged in a hexagonal lattice, sometimes with chemical modifications. GQDs look like small scraps of chicken wire, and are being developed as biosensors, imaging agents, and carriers for drug delivery. Interest in these nanostructures as therapeutic agents is on the rise. Some reports suggest they interfere with amyloid fiber formation, and disaggregate Aβ or α-synuclein fibrils (Mahmoudi et al., 2012; Li et al., 2014; Liu et al., 2015; Yang et al., 2015; Liu et al., 2018).
In the new study, co-first authors Donghoon Kim and Je Min Yoo tested the ability of chemically modified GQDs to counteract the aggregation, transmission, and toxicity of α-synuclein. By treating pure carbon fibers with strong acid, the investigators generated 2 nm square snippets of graphene that sport carboxylic acid groups around their edges. When mixed in equal amounts with purified α-synuclein in vitro, these GQDs potently inhibited fibril formation. Even more, they rapidly dissolved preformed fibrils, first melting the structures into shorter fragments and then into monomers. Structural analysis with NMR and other techniques indicated that the GQDs’ negatively charged carboxyl groups bound to the positively charged N-terminal region of α-synuclein in fibrils. A molecular dynamics simulation predicted that hydrophobic interactions between valine residues in α-synuclein and the GQD lattice destroy the β-sheet structure, driving fibril dissociation (see image at right). In agreement with that idea, removing the carboxyl groups destroyed the GQDs’ fibril-busting activity.
Moving to cell-based assays, the scientists demonstrated that GQDs protected primary cortical neurons from α-synuclein toxicity. The dots prevented cell death, restored neurite outgrowth and synaptic proteins, and mitigated mitochondrial toxicity induced by α-synuclein preformed fibrils (PFFs). Quantum dots also reduced the accumulation of phosphorylated and aggregated α-synuclein in response to PFFs, and blocked the transmission of α-synuclein pathology from cell to cell in vitro. Protection appeared to occur inside cells: In live imaging experiments, PFFs and GQDs were spotted together in lysosomes. There, the signal for PFFs decreased and that for GQDs increased over time, suggesting disaggregation of α-syn takes place in that compartment.
The protective effects of GQDs extended to two different PD mouse models. In one, the researchers injected α-syn PFFs into the striata of wild-type mice. After 180 days, six mice lost about half of their dopamine neurons, whereas six animals also given GQD injections twice a week lost only about one-quarter. Treatment also reduced the spread of gliosis, pathologic phosphorylated α-synuclein, and Lewy body-like inclusions (see image below). The animals’ motor function improved too. GQD-treated mice had more control of their paws and balanced better on a pole. In a transgenic PD model, pathology and motor function improved when mice expressing A53T mutated human α-synuclein were treated with the GQDs.
The graphene dots even appeared safe. After four months of dosing, they caused no apparent harm to the mice. They were cleared in the urine. Multiple carboxyl groups packed onto teeny, inert scaffolds provide a potent, nontoxic, anti-aggregation agent, the authors concluded.
How did GQDs get into the brain? The dots readily traversed layers of endothelial cells and astrocytes in an in vitro model of the blood-brain barrier. Fluorescent imaging showed both cell types accumulated GQDs in lysosomes and released them via exosomes. The scientists propose that GQDs are endocytosed by endothelial cells, then released via exosomes, to be taken up by astrocytes, and finally released into the brain. In vivo, the researchers detected GQDs widely in the central nervous system after intraperitoneal injection.
Given studies suggesting they disassemble Aβ fibrils, these quantum dots might have applications beyond tackling synuclein fibrillogenesis. Ko told Alzforum they have confirmed that their GQDs can dissociate Aβ fibrils and oligomers in vitro. They are now testing the dots in a mouse model of AD, he said. At the same time, they are moving forward on the Parkinson’s disease front. Hong is working on safety studies in animals as a prelude to human use. “If all goes well we hope to start trials in people within two years,” Ko said. He believes GQDs will have a universal effect on neurodegenerative disorders that involve accumulation of fibrillar aggregates.
“This paper opens a new research direction on chemical modified nanomaterial for anti-amyloid therapy,” wrote Mingdong Dong, Aarhus University, Denmark, in an email to Alzforum. Dong’s group is evaluating other nanomaterial as anti-amyloid agents (Wang et al., 2018).
Jensen agreed, adding that he hopes the authors will clearly explain the procedure for making the dots. “I’m sure this paper will stimulate at lot of other studies. Their data suggest that it is important to produce the quantum dots in a specific way, and I hope they will describe the method in detail. If there is a trick to the production, it is important to document that,” he said. Jensen also told Alzforum he’d like to see more quantitation of the interaction between GQDs and fibrils, including measures of affinity and specificity for other amyloids, including nonpathogenic, physiological forms.—Pat McCaffrey
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- Kim D, Yoo JM, Hwang H, Lee J, Lee SH, Yun SP, Park MJ, Lee M, Choi S, Kwon SH, Lee S, Kwon SH, Kim S, Park YJ, Kinoshita M, Lee YH, Shin S, Paik SR, Lee SJ, Lee S, Hong BH, Ko HS. Graphene quantum dots prevent α-synucleinopathy in Parkinson's disease. Nat Nanotechnol. 2018 Jul 9; PubMed.