Anterograde Viral Tracer Herpes Simplex Virus 1 Strain H129 Transports Primarily as Capsids in Cortical Neuron Axons.

The features of herpes simplex virus 1 (HSV-1) strain 129 (H129), including natural neurotropism and anterograde transneuronal trafficking, make it a potential tool for anterograde neural circuitry tracing. Recently anterograde polysynaptic and monosynaptic tracers were developed from H129 and have been applied for the identification of novel connections and functions of different neural circuitries. However, how H129 viral particles are transported in neurons, especially those of the central nervous system, remains unclear. In this study, we constructed recombinant H129 variants with mCherry-labeled capsids and/or green fluorescent protein (GFP)-labeled envelopes and infected the cortical neurons to study axonal transport of H129 viral particles. We found that different types of viral particles were unevenly distributed in the nucleus, cytoplasm of the cell body, and axon. Most H129 progeny particles were unenveloped capsids and were transported as capsids rather than virions in the axon. Notably, capsids acquired envelopes at axonal varicosities and terminals where the sites forming synapses are connected with other neurons. Moreover, viral capsids moved more frequently in the anterograde direction in axons, with an average velocity of 0.62 ± 0.18 μm/s and maximal velocity of 1.80 ± 0.15 μm/s. We also provided evidence that axonal transport of capsids requires the kinesin-1 molecular motor. These findings support that H129-derived tracers map the neural circuit anterogradely and possibly transsynaptically. These data will guide future modifications and improvements of H129-based anterograde viral tracers.