RESEARCH HIGHLIGHTS

NEUROSCIENCE It takes two to trans-Tango

Trans-Tango allows trans-synaptic map- nervous system, so that the reporter is only ping of presynaptic and postsynaptic part- activated in cells that are postsynaptic to the ners in Drosophila. starter neurons. Mapping circuits in the brain is a first Barnea and his team chose to use the het- step towards understanding brain func- erologous human glucagon receptor and a tion. Viruses are commonly used as trans- modified version of the glucagon peptide. synaptic tracers in rodents, but these tools Barnea says that this was a very good choice are not necessarily restricted to single syn- for them: “What was really surprising to apses, and they are most efficient in the ret- me…was the signal to noise that was way rograde direction. To address these limita- above my hopes and expectations.” tions, Gilad Barnea from Brown University The researchers validated the trans-Tango and his team wanted to develop an efficient tool in the Drosophila olfactory system, for anterograde tracer that was genetically which the circuitry is well studied, and they encoded. But he quickly realized that the observed the expected connectivity between development of these tools would be tedious olfactory receptor neurons and local inter- in mice. “This is why we started to work in neurons as well as projection neurons. The flies,” says Barnea. researchers also expressed the ligand in Circuit tracing in Drosophila with starter neurons The tool the researchers developed is subsets of olfactory neurons and combined in yellow–green and target neurons in red. based on the Tango system for studying sig- the trans-Tango tool with tools to achieve Image reprinted with permission from Talay et al. naling, which Barnea published as part of mosaic expression. This resulted in sparse- (Elsevier). his postdoctoral work with Richard Axel at ly labeled postsynaptic neurons, which Columbia University. In the Tango system, allowed Barnea and his team to examine reflects the fact that alternative tools for a ligand activates a receptor, which triggers the circuitry in more detail and to further circuit mapping in Drosophila either have proteolytic release of a transcription factor validate the system. high background activity or are difficult from the plasma membrane and the expres- Barnea and his team then turned to the to use. sion of a target reporter gene. fly gustatory system, which is less studied In the meantime, Barnea is refining the To adapt the Tango system to circuit map- than the olfactory system. They revealed trans-Tango system to adapt it for functional ping, “I decided to develop a signaling path- that gustatory receptor neurons involved in studies involving, for example, optogenetic way that will be expressed everywhere in the sweet perception connect to a large number stimulation. In such studies, it would be animal and will be activated by the popu- of second-order neurons in both the central desirable to restrict expression of the opto- lation of neurons that we wish to study,” brain and in the ventral nerve cord. Again, genetic tool to target neurons only and to explains Barnea. The components of the the team refined their mapping strategy by avoid expression in starter neurons. This is trans-Tango system are similar to previous combining trans-Tango with sparse label- currently not possible, as there is the pos- iterations of the Tango system and include ing techniques in order to more precisely sibility of ‘self-activation’ through activa- a ligand, a G-protein-coupled receptor that analyze and describe the connectivity in the tion of the trans-Tango receptor expressed is fused to a transcription factor via a TEV gustatory system. in the starter neurons. Furthermore, Barnea cleavage site, a TEV protease fused to arres- The tool should be useful beyond the has started collaborations to establish trans- tin, and a reporter gene. sensory circuitries that the researchers Tango in other model organisms. He is also In contrast to previous Tango itera- have mapped. “There is no reason why it working on establishing a mouse version, tions, the ligand is attached to Neurexin1 wouldn’t work,” says Barnea, although he which brings him back to his roots in mouse domains, thus tethering it to the presynaptic concedes that “it will take some tweak- olfaction. membrane and preventing diffusion from ing in every system, in every circuit.” In Nina Vogt the synapse. The ligand is only expressed in fact, Barnea has already distributed the the neurons of interest (‘starter’ neurons), trans-Tango system to more than 50 labs RESEARCH PAPERS Talay, M. et al. Transsynaptic mapping of second- while the receptor and other downstream that work on highly diverse questions in order taste neurons in flies by trans-Tango. Neuron components are expressed throughout the Drosophila neuroscience. The high demand 96, 783–795.e4 (2017).

NATURE METHODS | VOL.15 NO.1 | JANUARY 2018 | 11