Classic graphics #5: Brainbow

Is it presumptuous to award "classic" status to something that's less than a month old? Normally, yes. But sometimes, something is just so stunning that you strongly suspect it will be shown for years to come.

This essay is different from the previous ones, which each focused on a single image. This one pans out to review a veritable gallery of images that will surely just be the first of many galleries.

At the start of this month, Livet and colleagues published a paper in Nature that has arguably the most beautiful pictures of neurons ever taken. And that's a tall order, because most neurons are really beautiful in their own right, particularly when you get a good stain, and you're really able to see their structure in detail under a microscope. But these leave you open mouthed, gaping "The colours, man, check out the colouuuuurs..." like a hippie on an LSD trip in the Summer of Love.

The authors have created mice whose neurons glow a variety of colours. Hence, brain + rainbow = Brainbow.

Unfortunately, in contrast to the beauty of the pictures, the prose of the actual article is not accessible to anyone but real specialists. By specialist, I don't mean, "biologist" or "neurobiologist," I mean, "transgenic mouse neuroscientists." The paper is loaded with cryptic abbreviations ("XFP" means "fluorescent proteins" -- I get the FP, but the X?) and hinges on what the authors call the "widely used Cre/lox recombination system," which I had never heard of, and got sent to a 22 page review when I tried to make heads or tails of it. And even though the word that will probably stick in most peoples' heads when they sit down to search Google Scholar is the neologism "Brainbow," the word "Brainbow" is not in the title.

As far as I can tell, here's what they've done.

It's been a reasonably common trick in biology for some years now to be able to take a gene from one organism and put it into another. These are transgenic organisms, and when they're plants, they're also known as genetically modified (GM) crops. A fairly well known example is to take a gene from a jellyfish that makes then glow called green fluorescent protein (GFP) and introduce that into other animals (like mice), so now that other animal gains the ability to fluoresce, just like the jellyfish.

Now, how were Livet and colleagues able to get neurons to glow a bunch of different colours?

After people were able to put GFP into new organisms, people started tweaking the sequence and found they could make other colours -- like red fluorescent proteins. Other people took genes from other animals that glowed different colours. By doing so, researchers developed a palette of different colours. But as an artist knows, the trick is in combining the colours on the palette.

The authors introduced several of these fluorescent genes (up to four different ones) into mice, and found a way to get each neuron to activate a random selection of these genes using this Cre/lox system. If you remember colour theory, you can mix two colours together to create a third. If you mix three colours, the range of possible new colours is very large indeed. By having these multiple genes activating in unpredictable combinations, each cell glows a particular colour that is shared by few of its neighbours. The authors estimate there are at least 89 distinct colours that they can see.

Now, there is some more genetic trickery involved here that I don't pretend to understand fully. One is that the expression is not automatic in all cases -- it can be turned on in specific regions of the nervous system (Figure 3e in the paper shows neurons "lit" only in the retina of the eye). There's also some jiggery-pokery involving crossbreeding some of these genetically modified mice. Sometimes, the mice gave only the single "primary colours," indicating that only one protein was ever expressed. Some others showed the mixtures, giving many different colours.

The paper goes on to show that the colour of a neuron appears to be consistent throughout its length, an important consideration given that neurons have such long projecting branches. They also show the colours stay stable over time by tracking some neurons for 50 days.

As far as I can tell, this paper is a real technical tour de force. There are a lot of experiments compiled here, that appear to be very thorough. The authors did not just stop and publish when they had a few pretty pictures. ... Okay, make that breathtaking pictures.

It will be very interesting to see how this technology develops, and what it will reveal about neuronal wiring. Because so much research is driven by what we can see.

Meanwhile, here are some more pictures.

References

Livet, J., Weissman, T., Kang, H., Draft, R., Lu, J., Bennis, R., Sanes, J., & Lichtman, J. (2007). Transgenic strategies for combinatorial expression of fluorescent proteins in the nervous system Nature, 450 (7166), 56-62 DOI: 10.1038/nature06293

Supplemental info: http://www.nature.com/nature/journal/v450/n7166/suppinfo/nature06293.html