Exosomes carried RNA across the brain’s protective barrier and blocked inflammatory neuronal death in mice
Exosomes carried RNA across the brain’s protective barrier and blocked inflammatory neuronal death in mice
On 11 July, Advanced Science published a study of siRNA, a short RNA molecule that causes a cell to stop producing a selected protein. The authors loaded siRNA targeting RIPK3 into exosomes, injected them intravenously into mice, and detected the exosomes in the brain, suppression of the target in neurons, and better performance on memory tests in a mouse model of Alzheimer’s disease.
Designing a molecule against a brain disease can be easier than delivering it to a neuron. The blood-brain barrier consists of tightly connected vascular cells and allows almost no large RNA molecules to pass from the blood. Free siRNA is rapidly degraded. Without a delivery system, even a precisely targeted RNA cannot enter the cell where it is needed.
A team from Harbin Medical University used exosomes, natural vesicles through which cells exchange material. They placed the RVG peptide on the exosome surface because it binds to receptors on the walls of cerebral blood vessels and on neurons. They loaded the exosomes with siRNA targeting RIPK3, a protein involved in initiating necroptosis, an inflammatory form of cell death. During necroptosis, a neuron ruptures and releases distress signals that increase inflammation in the surrounding tissue.
The authors tested each step in this process. In a barrier model, a fluorescent label entered the “brain” chamber. After injection into mice, the exosome signal reached the brain in approximately six hours. In neurons, the siRNA escaped from lysosomes, intracellular vesicles that normally digest foreign material, entered the cytoplasm, and reduced the activity of RIPK3 and its partner MLKL.
The researchers then tested the system in a disease model. Triple APP/PS1/TAU mice carry mutations that cause early accumulation of amyloid and pathological tau protein, followed by loss of synapses and memory. After intravenous administration of the siRNA-loaded exosomes, the animals were better able to recognize a novel object and locate the former platform position in a water maze. In the brain, the proportion of cells with active RIPK3 fell from 28,48% to 12,19%, while the proportion with active MLKL fell from 24,42% to 12,45%. In human cortical organoids, the system also reduced the necroptosis signal and restored the synaptic protein GRIA1.
The study addresses two separate problems: blocking a cell-death pathway and delivering a gene-silencing agent to neurons. Necroptosis has long been suspected of contributing to neuronal loss in Alzheimer’s disease. In a 2023 experiment, researchers protected human neurons in the mouse brain by suppressing RIPK1, RIPK3, or MLKL. The new study tests the delivery problem by asking whether a gene-silencing agent can travel from a vein directly into a vulnerable neuron.
Organoids lack blood vessels and a complete immune system, and the exosomes used in these experiments were obtained from mouse cell lines. A fluorescent signal in the brain also does not measure the exact proportion of the administered dose that would reach human tissue. Human trials will require data from primates on long-term safety, distribution among organs, and the production of consistent exosome batches.