Transneuronal spread of alpha herpesviruses: Mechanisms and applications

Alpha herpesviruses are pantropic, neuroinvasive pathogens that establish latency in the peripheral nervous system of their natural host. Development of herpes disease relies on spread of infection between the surface epithelia and the nervous system. The capacity for synaptically-defined, transneuronal spread of infection makes alpha herpesviruses a useful tool for determining the connectivity of neural circuits. We investigated the molecular mechanisms that underlie transneuronal spread of alpha herpesviruses to better understand the development of herpesvirus disease, and to design novel herpesvirus strains for neural circuit tracing studies.;We demonstrate that the pseudorabies virus strain Bartha (PRV Bartha) exhibits a partial retrograde transneuronal spread defect owing to mutations in the UL21 locus. It is likely that mutations in the Bartha U L21 gene confer defects that reduce infectious particle production, causing a delay in spread to pre-synaptic neurons and amplification of infection. These events manifest as slower kinetics of retrograde viral spread in a neural circuit. Repair of the UL21 locus in PRV Bartha resulted in a more efficient retrograde-restricted neural circuit tracer, PRV 326.;Further, we demonstrate that the viral glycoprotein B (gB), essential for transneuronal spread of PRV, is not required for sorting virions into axons of infected neurons, anterograde transport, or virion release from the axon. Rather, this component of the viral fusion machinery is required for viral penetration of the postsynaptic membrane. We show that spread of the PRV strain harboring deletions in gB and US9 loci is restricted to first-order neurons in vitro, indicating its potential as a retrograde-restricted, monosynaptic neural tracer.