Overcoming two major hurdles, scientists have delivered small interfering RNA (siRNA) into the brains of mice with a tail vein injection and protected them from deadly viral encephalitis. The researchers designed a molecular package that crossed the blood–brain barrier and then spread through the brain to treat the disease.
The results, published online June 17 in Nature, suggest that it may be possible to direct promising gene-silencing therapies to the brain. In one experiment to test the specificity of the brain-targeting system, for example, the researchers knocked down SOD1, the most commonly mutated gene in the inherited form of amyotrophic lateral sclerosis (ALS), a common, fatal motor neuron disorder with no effective treatment.
“It’s a potentially big breakthrough,” said John Rossi, chair of molecular biology at the Beckman Research Institute in City of Hope, Calif., author of an accompanying commentary for the print edition. “It’s the first real clear-cut case of delivering siRNA into the brain without injecting directly into the brain.”
The project was conducted by postdoctoral fellow Priti Kumar in the lab of Premlata Shankar and Manjunath Swamy, HMS assistant professors of pediatrics at the Immune Disease Institute (formerly the CBR Institute for Biomedical Research).
The RNA fragment works against the fatal viral encephalitis by silencing a shared genetic sequence in the Japanese encephalitis virus and West Nile virus, two related mosquito-borne flaviruses, the researchers showed last year (see Focus, March 10, 2006). The big drawbacks were that the siRNA only worked in the localized infected cells near the injection site and could not travel through the brain like the viruses.
For the latest study, Kumar and her colleagues tried a lot of ways to deliver the siRNA to the brain through blood vessels.
tiny part of the rabies virus glycoprotein (RVG), first mapped by Yale researchers nearly 30 years ago, showed the most impressive diffusion throughout the mouse brains. The peptide binds to nicotinic acetylcholine receptors plentiful on the endothelial lining of brain capillary cells and on the neurons themselves. The authors believe RVG passes through capillary cells and travels into neurons.
Unfortunately, the peptide does not bind to nucleic acids. With the help of collaborators in Seoul, Korea, the team fused RVG to another cell-penetrating peptide composed of nine arginine (9R) residues that can carry siRNA into cells. Seven out of nine mice challenged with Japanese encephalitis virus survived after an intravenous injection of the RVG-9R carrying the siRNA, compared with none of the untreated infected mice.
“It is proof of principle, but there is a long list of things to do before it will be ready for clinical trials in people,” Swamy said.
The list includes further toxicology and bioavailability studies on the RVG-targeting peptide (named CORVUS by the institute’s technology development officer) as well as devising a better packaging system to ensure more siRNA arrives in the brain, Kumar said.
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