When ground squirrels hibernate, they dramatically reduce the flow of blood to their brains. Yet the squirrels suffer no ill effects, thanks to a cellular process called SUMOylation. By burrowing into the SUMOylation secrets hoarded by squirrels, scientists hope to retrieve molecular boons for people who suffer ischemic stroke, which typically results from a blood clot, constrains blood flow to part of the brain, and deprives brain cells of oxygen and nutrients.
SUMOylation refers to a kind of protein modification that regulates protein function in various cellular processes. In hibernating ground squirrels, SUMOylation is heightened—a hint that SUMOylation tags, small ubiquitin-like modifiers (SUMOs), may protect brain cells when oxygen and glucose become scarce.
By identifying molecules that can target the SUMO system, scientists based at the NIH have identified a potential drug that could allow stroke patients to emulate the cellular changes that protect hibernating animals. The drug, like proteins found in the hibernating ground squirrel, inhibits molecular processes that diminish SUMOylation.
The potential drug was evaluated in an article (“Quantitative High-Throughput Screening Identifies Cytoprotective Molecules That Enhance SUMO Conjugation via the Inhibition of SUMO-Specific Protease (SENP)2”) that appeared November 16 in The FASEB Journal. According to this article, post-translational modifications such as SUMOylation have emerged as critical molecular regulatory mechanisms in states of both homeostasis and ischemic stress.
“For decades scientists have been searching for an effective brain-protecting stroke therapy to no avail,” said Francesca Bosetti, Ph.D., Pharm.D., program director at the NIH's National Institute of Neurological Disorders and Stroke (NINDS). “If the compound identified in this study successfully reduces tissue death and improves recovery in further experiments, it could lead to new approaches for preserving brain cells after an ischemic stroke.”
At present, the only way to minimize stroke-induced cell death is to remove the clot as soon as possible. A treatment to help brain cells survive a stroke-induced lack of oxygen and glucose could dramatically improve patient outcomes, but no such neuroprotective agents for stroke patients exist.
Recently, researchers led by John Hallenbeck, M.D., an NINDS senior investigator and co-senior author of the current study, explored the possibility that enhanced SUMOylation helped animals' brains survive the reduced blood flow caused by hibernation. In experiments in cells and mice, the researchers confirmed their suspicions.
“If we could only turn on the process hibernators appear to use to protect their brains, we could help protect the brain during a stroke and ultimately help people recover,” said Joshua Bernstock, a graduate student in Dr. Hallenbeck's lab and the study's first author.
SUMOylation occurs when an enzyme attaches a molecular tag called a SUMO to a protein, altering its activity and location in the cell. Other enzymes called SUMO-specific proteases (SENPs) can then detach those tags, thereby decreasing SUMOylation. In the current study, Bernstock and his colleagues teamed up with researchers from the NIH's National Center for Advancing Translational Sciences (NCATS) to examine whether any of over 4000 molecules from the NCATS small-molecule collections could boost SUMOylation by blocking a SENP called SENP2, which would theoretically protect cells from a shortage of life-sustaining substances.
“Herein,” wrote the authors of the current study, “we describe a process capable of identifying and characterizing small molecules with the potential of targeting the SUMO system through inhibition of SUMO deconjugation in an effort to develop novel stroke therapies.”
The researchers first used an automated process to examine whether the compounds prevented SENP2 from severing the connection between a tiny metal bead and an artificial SUMO protein created in the lab of Wei Yang, Ph.D., the study's other senior author and an associate professor at Duke University in Durham, NC. This system, along with computer modeling and further tests performed both in and outside of cells, whittled the thousands of candidate molecules down to eight that could bind to SENP2 in cells and were nontoxic. Two of those—ebselen and 6-thioguanine—were then found to both boost SUMOylation in rat cells and keep them alive in the absence of oxygen and glucose.
A final experiment showed that ebselen boosted SUMOylation in the brains of healthy mice more than a control injection. 6-Thioguanine was not tested because it is a chemotherapy drug with side effects that make it unsuitable as a potential stroke treatment. The researchers now plan to test whether ebselen can protect the brains of animal models of stroke.
Because SUMOylation affects a variety of molecules, Bernstock believes his group's approach could inspire similar attempts to treat neurological conditions by targeting pathways with wide-ranging effects. He also hopes it will prompt others to look to natural models, as he and Dr. Hallenbeck did with the ground squirrel.
“As a physician–scientist, I really like to work on projects that have clear relevance for patients,” Bernstock noted. “I always want outcomes that can lend themselves to new therapeutics for people who are in need.”