Jellyfish green fluorescent protein (GFP) can used to label living cells or proteins without exogenous cofactors or substrates, and has been a boon to neuroscientists who study neuronal development and plasticity. In addition to GFP, four other spectral variants—yellow (YFP), cyan (CFP) and red (RFP) fluorescent proteins have been developed and shown to label neurons in vivo. In the October issue of Neuron, Feng et al. report the generation of transgenic mice expressing CFP, YFP, GFP, or RFP under the control of neuron-specific elements from the thy1 gene. Fluorescent neurons were found in many parts of the peripheral nervous system, including sympathetic and sensory ganglia, retina, forebrain, midbrain, cerebellum and spinal cord. Non-neuronal cells, such as muscle cells, were not labeled. The authors also showed that repeated imaging of YFP did not have toxic effects, and that this approach can be used to image neurons in living animals over long periods of time. Furthermore, fluorescently-labeled axons can survive, grow, and even form synapses during illumination. “We plan to exploit this potential to follow synapse formation over time in wild-type animals and in mutants known to have synapse formation defects”, says Joshua Sanes, senior author of the study.

Feng et al. also crossed thy-1-CFP transgenic mice with thy1-YFP transgenic mice to create a CRP+YFP double transgenic. The picture shows periperhal ganglia of these mice, in which CFP- (dark blue), YFP- (green) and CFP + YFP double labeled (light blue) neurons are readily distinguishable. This technology will allow researchers to specifically label distinct neuronal subsets, such as motor or sensory neurons, and study their development, interaction, or response to different agents. “Now we can also easily identify and isolate specific neuronal cell types for gene expression profiling”, says Sanes.