Caltech biologists have modified a harmless virus in such a way that it can successfully enter the adult mouse brain through the bloodstream and deliver genes to cells of the nervous system. The virus could help researchers map the intricacies of the brain and holds promise for the delivery of novel therapeutics to address diseases such as Alzheimer's and Huntington's. In addition, the screening approach the researchers developed to identify the virus could be used to make additional vectors capable of targeting cells in other organs.
To sneak genes past the blood-brain barrier, the researchers used a new variant of a small, harmless virus called an adeno-associated virus (AAV). The researchers developed a high-throughput selection assay, CREATE (Cre REcombinase-based AAV Targeted Evolution), that allowed them to test millions of viruses in vivo simultaneously and to identify those that were best at entering the brain and delivering genes to a specific class of brain cells known as astrocytes.
They started with the AAV9 virus and modified a gene fragment that codes for a small loop on the surface of the capsid—the protein shell of the virus that envelops all of the virus' genetic material. Using a common amplification technique, known as polymerase chain reaction (PCR), they created millions of viral variants.
Then they used their novel selection process to determine which variants most effectively delivered genes to astrocytes in the brain. Importantly, the new process relies on strategically positioning the gene encoding the capsid variants on the DNA strand between two short sequences of DNA, known as lox sites. These sites are recognized by an enzyme called Cre recombinase, which binds to them and inverts the genetic sequence between them. By injecting the modified viruses into transgenic mice that only express Cre recombinase in astrocytes, the researchers knew that any sequences flagged by the lox site inversion had successfully transferred their genetic cargo to the target cell type—here, astrocytes.
After one week, the researchers isolated DNA from brain and spinal cord tissue, and amplified the flagged sequences, thereby recovering only the variants that had entered astrocytes.
Next, they took those sequences and inserted them back into the modified viral genome to create a new library that could be injected into the same type of transgenic mice. After only two such rounds of injection and amplification, a handful of variants emerged as those that were best at crossing the blood-brain barrier and entering astrocytes.
Through this selection process, the researchers identified a variant dubbed AAV-PHP.B as a top performer. To test AAV-PHP.B, the researchers used it to deliver a gene that codes for a protein that glows green, making it easy to visualize which cells were expressing it. They injected the AAV-PHP.B or AAV9 (as a control) into different adult mice and after three weeks used the amount of green fluorescence to assess the efficacy with which the viruses entered the brain, the spinal cord, and the retina.
"We could see that AAV-PHP.B was expressed throughout the adult central nervous system with high efficiency in most cell types," says Gradinaru. Indeed, compared to AAV9, AAV-PHP.B delivers genes to the brain and spinal cord at least 40 times more efficiently.
The research was published in the journal Nature Biotechnology.