Rainbows in the Brain DOI: 10.1038/Nrn2296 How Are the Millions of Brain Cells Constructs Are Often Inserted in Tan- Connected to Achieve the Different Dem

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T e c h n o lo gy Rainbows in the brain DOI: 10.1038/nrn2296 How are the millions of brain cells constructs are often inserted in tan- connected to achieve the different dem. Indeed, the authors confirmed facets of neuronal function? Until that the colour diversity was caused now, we have been limited to studying by up to 16 tandem repeats of the only a few dye-filled or fluorescent- Brainbow construct. Individual cells protein-expressing neurons at a expressed distinct combinations of time. Reconstruction of connectivity the four fluorescent proteins, result- from three-dimensional electron ing in the multitude of colours that microscope sections is cumbersome enabled the authors to distinguish and limited to the sectioned area. To single neurons and their processes study connectivity on a large scale, within dense cell clusters. Livet et al. have generated a set of The authors then tested several genetic constructs, termed Brainbow, applications of Brainbow expression. that colour-tag each individual cell First, they studied connectivity in the with one random colour from a inner granular layer of the cerebel- pool of colours, allowing research- lum. They demonstrated that each labelled cells remained constant ers to trace dendrites and axons of postsynaptic granule cell received over extended periods and that, individual neurons and to study the synaptic inputs from multiple therefore, Brainbow can be used for interactions between cells even in presynaptic mossy fibre neurons, longitudinal studies of circuits and densely packed areas of the brain. answering a long-standing question interactions in vivo. The Brainbow constructs were in cerebellar circuitry. The Brainbow constructs pre- designed by combining Cre/Lox- Some of the transgenic mouse sented in this study can be seen as the dependent DNA excision or lines also showed Brainbow expres- first generation of a series of new and inversion with up to four different sion in astrocytes, allowing the study powerful tools for studying connec- fluorescent proteins (XFPs). Upon of the interaction of these cells with tivity and the interaction of cells in Cre- recombination, stochastic choice neurons. For example, the authors vivo. Choosing different promotors of XFP expression produces up to demonstrated that multiple cerebellar and fluorescent proteins are just two four different fluorescent colours. Bergman glia cells ensheathed the options for the further development To generate transgenic mice, the same part of Purkinje-cell dendrites, of this tool, which is by no means expression of Brainbow was placed extensively interdigitating in their limited to neuroscience applications. under the control of the Thy-1 target areas. Combining the Brainbow technology promotor, which drives strong gene In time-lapse studies of with other methods will without expression in a number of neuronal Brainbow-expressing Schwann cells doubt profoundly influence cell types. The authors generated at adult-mouse neuromuscular junc- neuroscience research in the future. 19 transgenic mouse lines which, tions, the authors showed that the Claudia Wiedemann surprisingly, expressed up to 100 glial sheath is dynamic, moving back Original Research Paper Livet, J. et al. different colours in neuronal cells. and forth on the nerve terminal. Transgenic strategies for combinatorial However, it is a well-known These experiments also verified expression of fluorescent proteins in the nervous phenomenon in transgenic mice that that the intensity and colour of the system. Nature 450, 56–62 (2007)