Blocking the activity of sirtuin 2 (SIRT2), one of a family of proteins that regulate longevity, may offer protection from Parkinson’s disease, according to new findings by HMS researchers. Writing in the June 21 online Science, Aleksey Kazantsev, HMS assistant professor of neurology at Massachusetts General Hospital, and colleagues report that small molecule inhibitors of SIRT2 protect cell and animal models against the toxicity of mutated alpha-synuclein. The same mutations in humans cause degeneration of dopaminergic neurons in a specific region of the brain, the substantia nigra, and are responsible for certain inherited forms of Parkinson’s disease.
“This discovery suggests that we can develop neuroprotective compounds that block cells from dying and help them retain their functionality,” said Kazantsev. Current therapies for Parkinson’s supplement dopamine levels, but do not address the inherent degenerative nature of the disease.
Kazantsev and colleagues discovered the SIRT2 inhibitors by using high-throughput, phenotypic screens to identify molecules that promote protein aggregation. Last year, after screening around 37,000 small molecules, the researchers reported that a lead compound called B2 promotes the formation of inclusion bodies, intracellular aggregates of toxic proteins such as alpha-synuclein and mutant huntingtin, which causes Huntington’s disease.
Inclusion bodies are believed to be protective because they sequester neurotoxic intermediates that form protein aggregates. The researchers then faced the daunting challenge of modifying that lead compound so it would work in animal models and eventually humans. “Lead development is a very tedious and difficult process, and it really requires that you have a molecular target,” said Kazantsev. Unfortunately, one of the problems with cell-based screens is that the molecular target is initially unknown, he explained.
To address this, the researchers tested B2 directly on a variety of proteins implicated in neurodegenerative diseases, including proteases, chaperones, and histone deacetylases like SIRT2. The compound weakly, though consistently, inhibited SIRT2. “This was very exciting for us because this class of histone deacetylase has been implicated in the pathology of both Parkinson’s and Huntington’s diseases,” said Kazantsev.
With this B2 target identified, the researchers next set about optimizing the molecule. They generated about 200 structural analogs. One of these, AGK2, was about 10-fold more potent against SIRT2 and appeared quite specific, having very little inhibitory action on SIRT1 or SIRT3 (there are seven known human SIRT proteins). Tiago Fleming Outeiro, a postdoctoral fellow in collaborator Bradley Hyman’s lab at MGH and first author on the Science paper, found that AGK2 inhibited SIRT2 in human cells and that the compound promoted the formation of large inclusion bodies in neuroglioma cells overexpressing alpha-synuclein. It also protected dopaminergic neurons against cell death caused by a Parkinson’s disease–associated variant of the protein. When fed to fruit flies (in a collaborative experiment with Mel Feany’s lab at Brigham and Women’s Hospital), AGK2 also had a dramatic effect on alpha-synuclein–driven loss of dopaminergic neurons. At the highest doses tested, neuronal loss was only about 10 percent versus the 70 percent seen in untreated animals.
“Our next step will be to develop a safe SIRT2 inhibitor for use in human phase I clinical trials for Parkinson’s disease, first testing various compounds in mouse models,” said Kazantsev. Because inclusion bodies are also hallmarks of Huntington’s and Alzheimer’s diseases, Kazantsev plans to test SIRT2 inhibitors in animal models of those diseases as well.
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