Brain cells, and many other cell types communicate by releasing vesicles, which are very small bubbles of the cell’s membrane. These vesicles contain cargo, proteins or RNA, which generally function as signals of the cell’s state. These authors have engineered a pretty clever system for tracking when brain cells exchange these vesicles.
First they insert two genes into mice. The first is called tdTomato and is a red fluorescent protein. Crucially, they insert this gene backwards. I’ll explain that in a second. Directly on either side of this gene, they also include a specific section of DNA called a LoxP site. This LoxP site is recognized by a protein called Cre. Cre binds to two LoxP sites, pulls them together, cuts out the DNA between them, flips the direction of the DNA, then reinserts it backwards. So, normally the backwards tdTomato can’t be expressed as a protein, but if Cre protein is introduced, it will flip the tdTomato and suddenly the cell will start generating this red fluorescing protein.
The authors also engineered a virus to carry the Cre protein. Specifically they chose a lentivirus, which inserts its own genome into its host’s genome. Here this means the virus inserts the DNA encoding the Cre protein. They injected this virus into one area of the mouse brain, the striatum. Now the striatum cells express Cre, flip the tdTomato, and glow red. The lentivirus is not capable of expanding, so it stays isolated to the injection site in the striatum.
But, the most important part is that the striatum cells also sometimes package up Cre mRNA in their vesicles. They export those vesicles and other brain cells take them up. Then those other brain cells express the Cre, and as a result, glow a pleasing 581nm wavelength red. This tells the authors that red glowing cells received a vesicle from a cell in the striatum.
Very clever! As on example of an application, this could help scientists better understand how brain tumor cells are affecting other cells in the brain through vesicles.
