Retrograde axonal transport of herpes simplex virus: Evidence for a singlemechanism and a role for tegument

Herpes simplex virus type I (HSV) typically enters peripheral nerve termina ls and then travels back along the nerve to reach the neuronal cell body, w here it replicates or enters latency. To monitor axoplasmic transport of HS V, we used the giant axon of the squid, Loligo pealei, a well known system for the study of axoplasmic transport. To deliver HSV into the axoplasm, vi ral particles stripped of their envelopes by detergent were injected into t he giant axon, thereby bypassing the infective process. Labeling the viral tegument protein, VP16, with green fluorescent protein allowed viral partic les moving inside the axon to be imaged by confocal microscopy. Viral parti cles moved 2.2 +/- 0.26 mu m/sec in the retrograde direction, a rate compar able to that of the transport of endogenous organelles and of virus in mamm alian neurons in culture. Electron microscopy confirmed that 96% of motile (stripped) viral particles had lost their envelope but retained tegument, a nd Western blot analysis revealed that these particles had retained protein from capsid but not envelope. We conclude that (i) HSV recruits the squid retrograde transport machinery; (ii) viral tegument and capsid but not enve lope are sufficient for this recruitment; and (iii) the giant axon of the s quid provides a unique system to dissect the viral components required for transport and to identify the cellular transport mechanisms they recruit.