Generating and Imaging Multicolor Brainbow Mice
Fluorescent microscopy works by labeling cells with specific markers that cause them to glow certain colors when bathed in a special wash of chemical agents (fluorophores). These “markers” are usually genetic markers, and by tinkering with the genome of a host animal, the markers – and thus the colors produced by cells under the microscope – can be altered. Driven by a desire to map the vast web of neural connections in the mouse brain, Jeff Lichtman and his team at Harvard developed a fluorescent staining technique affording them a sizable palette with which to paint neurons.
The genetic system they used is called the Cre/lox system. Cre is an enzyme responsible for deleting sections of DNA that are adjacent to lox alleles. By splicing in a handful genetic markers that are responsible for different fluorescent colors (green, yellow, red, etc) in various places near the lox sites, a game of genetic roulette was played – depending on the position of different fluorescent color-producing genes in relation to the lox enzymes, a myriad of colors would ultimately be produced in the target neurons (i.e. red green green yellow, red red red green, red yellow yellow yellow, etc).
Lichtman cleverly dubs the technique “Brainbow,” and explains its application to discovering neuron connections:
“The ability of the Brainbow system to label uniquely many individual cells within a population may facilitate the analysis of neuronal circuitry on a large scale… This labeling appears well suited for visualization and tracing of large numbers of neurons and their connectivity…color differences between neurons provide a way to sort their processes while tracing through sections, to directly visualize their putative synaptic interactions, and to distinguish the neurons that converge onto a postsynaptic cell.”
The gallery above shows a sampling of the lush, elegant views of neural networks provided by the Brainbow technique.
(Images: Jean Livet, Tamily A. Weissman, Hyuno Kang, Ryan W. Draft, Ju Lu, Robyn A. Bennis, Joshua R. Sanes & Jeff W. Lichtman.)