'Floating' gene-silencing drugs into the brain with exosomes

Many new drugs fail because they can’t get into the brain – a major hurdle to developing new Huntington’s disease treatments. Now, researchers have used exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells – tiny bubbles produced naturally by some cells, to ‘float’ gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein drugs across the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream.

The blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream

It’s been said that the human brain is the most complex object in the universe. That complexity is what makes humans unique as a species, and each of us unique as individuals. But it comes at a cost: to keep working, the brain needs to maintain very strict control of its own environment – tiny changes in temperature, blood flow and the chemicals it lets in and out can cause big changes in the brain’s functioning.

The blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream is one way the brain keeps itself protected from the constantly changing outside world. The barrier is made from special reinforcements to the walls of blood vessels in the brain. Many chemicals can be absorbed from the stomach into the bloodstream, but hardly any can enter the brain from the blood – because of the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream.

The problem of delivery

While it might be good for us as a species, the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream is a nightmare for ‘drug hunters’. It means drugs that work well in the laboratory often don’t work at all when tested in humans – a familiar source of frustration for anyone who follows Huntington’s disease drug research.

There are several tricks already that can be tried, to get a drug to enter the brain, but none of them is perfect. One option is to alter the drug chemically so that it can cross the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream, but changing drugs in this way can reduce their ability to work once they’re in the brain. Another tactic is to inject the drug directly into the brain, or into the fluid that surrounds it – but that requires potentially dangerous brain surgery and the drug still might not spread throughout the whole brain. Viruses can be used to help drugs spread further, but can come with their own problems, like activating the immune system.

All in all, drug delivery and the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream are big problems for scientists working on any disease that affects the brain.

Exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells

Step forward, exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells. Exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells are tiny particles, produced naturally by some cells. They’re used to transfer chemicals to other cells. To produce exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells, the cell ‘squeezes’ a bit of its membrane then snips it off, a bit like a small bubble being generated from a larger bubble.

Cells can load the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells with cargo, and the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells can deliver that cargo into a cell it meets later. When we say exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells are tiny, we really mean it – they are over a thousand times smaller than the width of a human hair.

So how might exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells be able to help us to get drugs across the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream? These tiny bubbles can ‘float’ across the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream, so Dr Matthew Wood of Oxford University had the idea of using exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells to deliver gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein drugs to the brain.

Gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein drugs

Our HDBuzz gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein primer will give you the full lowdown on gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein in Huntington’s disease. Put simply, gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein involves making drugs that consist of specially designed messenger molecules, that tell cells not to make a certain protein. In the case of HD, that would be the huntingtin proteinhuntingtin protein The protein produced by the HD gene.. One approach to gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein is called RNA interferenceRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene or RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene.

Gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein has great potential for helping to protect brains against the effects of the Huntington’s disease mutation. But the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream is a big problem for RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene drugs – if they are given by injection into the bloodstream, they don’t get into the brain. Even if they’re injected directly into the brain, they don’t spread very far.

Using exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells to deliver RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene drugs

“The exosomes zipped across the blood-brain barrier and switched off the right gene in the right place”

Like any package, a drug delivery needs three things: a container, a destination address and a cargo.

Wood’s team started with the container – the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells. They set up an exosomeexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells production line using cells purified from mouse bone marrowbone marrow The gooey stuff in the middle of bones, that manufactures blood cells. When eaten, gives dogs a healthy coat and vitality, called dendritic cellsdendritic cell A type of cell that comes from bone marrow and is part of the immune system. Dendritic cellsdendritic cell A type of cell that comes from bone marrow and is part of the immune system produce lots of exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells naturally and are able to stay under the radar of the immune system, hopefully meaning that their exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells will be similarly stealthy. The team grew the dendritic cellsdendritic cell A type of cell that comes from bone marrow and is part of the immune system in the lab, then collected and purified the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells.

Next, the researchers needed to label the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells with a ‘destination address’ that would make sure they got to where they were needed. To target brain tissue, they genetically altered the dendritic cellsdendritic cell A type of cell that comes from bone marrow and is part of the immune system so that they produced a protein from the virus that causes rabiesrabies A virus that infects the brain. Because rabiesrabies A virus that infects the brain is great at targeting the brain, the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells produced by these altered cells should be good at attaching to brain cells.

Finally, the researchers needed to load the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells with cargo. They did this by mixing the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells with an RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene molecule and zapping the mixture with just the right amount of electricity. That made the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells absorb the drug molecules.

With the container, address and cargo in place, Wood’s team tested their RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene-delivering exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells on brain cells in the lab and found that they successfully delivered the drug to the right cells and switched off the right gene. But would they work in living animals?

In short – yes. When injected into the bloodstream of mice, the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells were found to be quite safe and, as hoped, didn’t cause a bad immune reaction. The targeting system worked well, too. Exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells addressed to the brain zipped across the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream and switched off the right gene in the right place, and avoided other organs, even when their RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene cargo could have switched off genes elsewhere too.

The real test came when the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells were used to deliver an RNAiRNA interference A type of gene silencing treatment in which specially designed RNA molecules are used to switch off a gene drug aimed at switching off a disease-causing gene. It wasn’t the huntingtin gene they chose, but a gene involved in the development of Alzheimer’s disease, called BACE1BACE1 The gene for a protein called beta secretase 1, which thought to be involved in the development of Alzheimer's disease. They chose BACE1BACE1 The gene for a protein called beta secretase 1, which thought to be involved in the development of Alzheimer's disease because reducing its activity is likely to protect against Alzheimer’s, but so far, all the drugs that act on BACE1BACE1 The gene for a protein called beta secretase 1, which thought to be involved in the development of Alzheimer's disease can’t cross the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream.

Again, the exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells performed well. When injected into the bloodstream, they reached the brain, switched off the gene and even reduced levels of amyloidamyloid The main protein that builds up in the brains of Alzheimer's disease patients protein, which builds up in brains affected Alzheimer’s disease.

Summary

All in all, this research adds an important new piece to the gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein jigsaw. So far, exosomeexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells delivery hasn’t been tested in a model of Huntington’s disease, and it will need to be tested thoroughly to make sure it’s safe, before it can be tried in humans.

But the idea of injecting a gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein drug into the bloodstream, and knowing that it will cross the blood-brain barrierblood-brain barrier A natural barrier, made from reinforcements to blood vessels, that prevents many chemicals from getting into the brain from the bloodstream and spread throughout the brain, is very appealing. HD researchers will doubtless be turning their attention to exosomesexosome Tiny bubble-like particles produced by cells that can transfer chemicals to other cells as one possible way of making gene silencinggene silencing An approach to treating HD that uses targeted molecules to tell cells not to produce the harmful huntingtin protein a reality for Huntington’s disease patients.

Learn more

• Nature Biotechnology article by Wood and colleagues (full article requires payment or subscription)
• Dr Wood’s homepage at Oxford University