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Abstract

During herpes simplex virus 1 (HSV1) egress in neurons, viral particles
travel from the neuronal cell body along the axon towards the synapse.
Whether HSV1 particles are transported as enveloped virions as proposed
by the 'married' model or as non-enveloped capsids suggested by the
'separate' model is controversial. Specific viral proteins may form a
recruitment platform for microtubule motors that catalyze such
transport. However, their subviral location has remained elusive. Here
we established a system to analyze herpesvirus egress by cryo electron
tomography. At 16 h post infection, we observed intra-axonal transport
of progeny HSV1 viral particles in dissociated hippocampal neurons by
live-cell fluorescence microscopy. Cryo electron tomography of
frozen-hydrated neurons revealed that most egressing capsids were
transported independently of the viral envelope. Unexpectedly, we found
not only DNA-containing capsids (cytosolic C-capsids), but also capsids
lacking DNA (cytosolic A-/B-capsids) in mid-axon regions. Subvolume
averaging revealed lower amounts of tegument on cytosolic A-/B-capsids
than on C-capsids. Nevertheless, all capsid types underwent active
axonal transport. Therefore, even few tegument proteins on the capsid
vertices seemed to suffice for transport. Secondary envelopment of
capsids was observed at axon terminals. On their luminal face, the
enveloping vesicles were studded with typical glycoprotein-like spikes.
Furthermore, we noted an accretion of tegument density at the concave
cytosolic face of the vesicle membrane in close proximity to the
capsids. Three-dimensional analysis revealed that these assembly sites
lacked cytoskeletal elements, but that filamentous actin surrounded
them and formed an assembly compartment. Our data support the 'separate
model' for HSV1 egress, i.e. progeny herpes viruses being transported
along axons as subassemblies and not as complete virions within
transport vesicles.