New Fluorescent Dyes Could Advance Biological Imaging

With a new technique to craft a spectrum of glowing dyes, chemists are no longer chasing rainbows. Pigment Suppliers

Swapping out specific chemical building blocks in fluorescent molecules called rhodamines can generate nearly any color scientists desire—ROYGBIV and beyond, researchers report September 4, 2017 in the journal Nature Methods.

The work offers scientists a way to adjust the properties of existing dyes deliberately, making them bolder, brighter, and more cell-permeable too. Such an expanded palette of dyes could help researchers better illuminate the inner workings of cells, says study leader Luke Lavis, a group leader at the Howard Hughes Medical Institute's Janelia Research Campus in Ashburn, Virginia. His team lit up cell nuclei, made larval fruit fly brains shine, and highlighted visual cortex neurons in mice that had tiny glass windows fitted into their skulls.

Scientists used to concoct different dyes mostly by trial and error, Lavis says. "Now, we've figured out the rules, and we can make almost any color." His team's method could allow chemists to synthesize hundreds of different colors.

A bright history
Until about 20 years ago, scientists relied on chemical fluorescent dyes to make biological molecules visible. For peeking inside cells, staining organelles, and other imaging experiments, "chemistry was king," Lavis wrote in a July 13, 2017 perspective in the journal Biochemistry. And then, the king was kicked off the throne—by a glowing green jellyfish protein called GFP.

In 1994, scientists reported the use of a genetic trick to tack GFP, the green fluorescent protein, onto other cellular proteins; it's like forcing the proteins to hold a glow stick. That trick gave researchers a simpler way to trace proteins' movements under a microscope—without using expensive synthetic dyes. The innovation blazed through the field of biological imaging. In 2007, scientists' mixing of GFP and two other fluorescent proteins let them paint mouse neurons a parade of vivid colors in a technique known as the "Brainbow." A year later, the discovery and development of GFP earned the Nobel Prize in chemistry for three scientists, including the late Roger Tsien, an HHMI investigator.

But GFP has some dark sides too. It's a relatively clunky molecule built out of the limited set of natural amino acids. So GFP isn't always bright enough to reveal what scientists are trying to see.

So researchers turned back to chemistry. Scientists had developed cutting-edge microscopes and new techniques to label cellular contents, Lavis says, but the dyes for marking molecules inside cells were still stuck in the nineteenth century. His team focused on rhodamines, because they're especially bright and cell-permeable—so they easily slip into cells and make them glow. But despite working with rhodamines for more than 100 years, chemists had created only a few dozen colors, and most were similar shades ranging from green to orange.