Kim, E.J. et al. Cell Rep. 15, 692–699 (2016).

Direct inputs into particular neurons can be identified through retrograde monosynaptic tracing with rabies-virus-based tools. To improve the efficiency of trans-synaptic tracing, Kim et al. optimized both trans-synaptic spreading and initial expression levels in starter neurons. The trans-synaptic infection process depends on virus particle packaging in the postsynaptic cell and on uptake in the presynaptic cell, which involves viral glycoprotein G. The researchers tested the trans-synaptic spread of rabies virus in the presence of chimeric G proteins that consisted of the extracellular portion of various rabies strains and the transmembrane and cytoplasmic parts of the commonly used G protein of the SAD B19 strain. Furthermore, they increased expression levels through optimized codon usage. The resulting optimized G protein (oG) resulted in up to twentyfold more efficient trans-synaptic tracing.