TRACR: an anterograde transneuronal tracing system for genetic access across synapses and longitudinal circuit analysis

Following neural signals as they converge onto and diverge from individual neurons is central to understanding circuit function and disease-related dysfunction. However, existing approaches to map connected structural reconstruction or functional recordings, or rely on cytotoxic viral tracers. To address these limitations, we adapted synthetic Notch designs to create TRanssynaptic Anterograde Circuit Readout (TRACR). Binding of the engineered ligand-receptor across synapses induces reporter transcription, enabling characterization and manipulation of postsynaptic neurons. By applying TRACR at multiple synapses in the mouse visual system, we show that TRACR labels postsynaptic partners of sensory neurons, long-range projections and local inhibitory interneurons. TRACR provides segregated genetic access to pre- and postsynaptic populations for expression of markers, sensors, or effectors. Finally, TRACR signaling is reversible and fails to activate when synapses are absent or disrupted. TRACR is an accessible, AAV-deliverable transneuronal reporting tool for longitudinal analysis of circuit assembly, organization, and degeneration.