New neuronal labelling method invented

Scientists have taken advantage of viral vectors to deliver genetic information past the blood-brain barrier, also providing an alternative method to label neurons. This breakthrough could see the vectors used as gene therapy to treat neurodegenerative diseases affecting the central nervous system such as Huntington’s disease. Researchers have long struggled to deliver drugs past the blood-brain barrier (BBB), a highly selective semi-permeable membrane.

Vivian Gradinaru, Assistant Professor at the California Institute of Technology and co-author of the study, said: "We have now developed a new collection of viruses and tools to study the central and peripheral nervous systems. We are now able to get highly efficient brain-wide delivery with just a low-dose systemic injection, access neurons in difficult-to-reach regions, and precisely label cells with multiple fluorescent colours to study their shapes and connections."

Travelling high and low

Gradinaru’s team have developed two variants based on an adeno-associated virus (AAV) discovered in 2016. AAV-PHP.eB can carry genetic materials pass the BBB while AAV-PHP.S can be picked up by peripheral neurons outside the brain. Ben Deverman, senior research scientist and involved in the research, said: “Neurons outside of the central nervous system have many functions, from relaying sensory information to controlling organ function, but some of these peripheral neural circuits are not yet well understood.

“The AAV-PHP.S vector that we developed could help researchers study the activity and function of specific types of neurons within peripheral circuits using genetically-encoded sensors and tools to modulate neuronal firing with light or designer drugs, respectively."

The new AAV vectors can also deliver genes that code for fluorescent proteins, making them useful for identifying and labelling cells. To achieve this, numerous AAVs with different colours are injected in the bloodstream. When the AAVs reach specific neurons, each neuron will uptake a unique combination of vectors, and therefore colours, allowing for easier identification. To make this process easier, the researchers also developed a method to limit the number of neurons labelled – so researchers can visualise individual neurons shapes.

Dr Ken Chan, also from the University and first author, said: "Usually, when researchers want a mouse or other animal model to express fluorescent proteins in certain cells, they need to develop genetically modified animals that can take months to years to make and characterise. Now with a single injection, we can label specific cells with a variety of colours within weeks after the injection.”

The most exciting implication of this research, according to Dr Gradinaru, is that when paired with appropriate activity modulator genes, these vectors could enable non-invasive deep brain modulation. Deep brain modulation is a surgical procedure implanting an electrical node into the brain and is used in patients that do not respond well to medication, or suffer severe side effects. The researchers’ AAVs could provide the same effect, without the need for surgery. The study was published in Nature Neuroscience.