Understanding Neural Networks: Identifying Monosynaptic Connections

Inventors: Edward Callaway, Ian Wickersham, and John Young
Potential Uses: Drug Discovery, CNS, Gene Expression

Understanding the complexity of neural circuits and how neural circuits generate perception and behavior is one of the most important aspects for studying neural networks. To date, the best available tools to do this are transsynaptic tracers. These tracers are dependent on cellular machinery, which has several inherent problems, including traveling across synapses at different rates, being unable to distinguish the difference between strong, indirect connections and weak direct ones, and not being able to label any connected neurons when starting from a single cell.

We have identified a novel method for understanding neural networks by using a synaptic tracer that crosses only one synaptic step, unambiguously identifying cells directly presynaptic to the starting population so-called, the transcomplemented transsynaptic tracing method. To do this, we inject the cell population of interest with a deletion-mutant tracing virus that is missing one or more genes required for transsynaptic spread, and then complement this deletion by providing the missing viral genes in trans in the initially infected neurons only. With all the viral genes present in the starting cells, the virus can spread from them to cells in monosynaptic contact. Because those genes are not in the secondarily infected cells, however, the virus can't spread beyond them. Based on the rabies virus, this tracer is genetically targetable, allows high-level expression of any gene of interest in the synaptically coupled neurons, and is powerful enough to label neurons that connect to a single starting cell.