The interaction of human fetal neurons and epidermal cells in vitro

Experimental models to study varicella-zoster virus infection of neurons

Varicella zoster virus (VZV) infection results in the establishment of latency in human sensory neurons. Reactivation of VZV leads to herpes zoster which can be followed by persistent neuropathic pain, termed post-herpetic neuralgia (PHN). Humans are the only natural host for VZV, and the strict species specificity of the virus has restricted the development of an animal model of infection which mimics all phases of disease. In order to elucidate the mechanisms which control the establishment of latency and reactivation as well as the effect of VZV replication on neuronal function, in vitro models of neuronal infection have been developed. Currently these models involve culturing and infecting dissociated human fetal neurons, with or without their supporting cells, an intact explant fetal dorsal root ganglia (DRG) model, neuroblastoma cell lines and rodent neuronal cell models. Each of these models has distinct advantages as well as disadvantages, and all have contributed towards our understanding of VZV neuronal infection. However, as yet none have been able to recapitulate the full virus lifecycle from primary infection to latency through to reactivation. The development of such a model will be a crucial step towards advancing our understanding of the mechanisms involved in VZV replication in neuronal cells, and the design of new therapies to combat VZV-related disease.

[Visualization of viruses in living cells]

Live-cell imaging of cells infected with recombinant viruses expressing fluorescently tagged structural viral proteins enables to visualize virion maturation pathways at a virion particle level in the same cells as infection progresses. This technology have gradually unveiled previously unreported aspects of the pathways. This article focuses on live-cell imaging technology of herpes simplex virus 1 (HSV-1) and reviews the up-to-date topics of HSV-1 maturation pathway including our recent progress.

Two viral kinases are required for sustained long distance axon transport of a neuroinvasive herpesvirus

Axonal transport is essential for the successful establishment of neuroinvasive herpesvirus infections in peripheral ganglia (retrograde transport) and the subsequent spread to exposed body surfaces following reactivation from latency (anterograde transport). We examined two components of pseudorabies virus (US3 and UL13), both of which are protein kinases, as potential regulators of axon transport. Following replication of mutant viruses lacking kinase activity, newly assembled capsids displayed an increase in retrograde motion that prevented efficient delivery of capsids to the distal axon. The aberrant increase in retrograde motion was accompanied by loss of a viral membrane marker from the transported capsids, indicating that the viral kinases allow for efficient anterograde transport by stabilizing membrane-capsid interactions during the long transit from the neuron cell body to the distal axon.

Reciprocal transmission of herpes simplex virus type 1 (HSV-1) between corneal epithelium and trigeminal neurites in an embryonic chick organ culture

Reciprocal transmission between epithelia and sensory neurons of the peripheral nervous system is a crucial step in the life cycle of herpes simplex virus type 1 (HSV-1) and related alphaherpesviruses. In searching for an easy-to-perform and generally applicable experimental approach that enables the direct analysis of virus transfer between primary epithelial cells and sensory neurites, we investigated the spread of HSV-1 in a dual-chamber organ model comprising chick embryonic corneal epithelia and trigeminal sensory neurons. Embryonic chick corneal and trigeminal tissues were found to be permissive for productive infection with HSV-1. Our data show that HSV-1 efficiently enters neurites re-innervating the cornea and reaches the ganglion explant by retrograde axonal transport, with the first antigen-positive cells being detectable approximately 12 h postinfection. After direct infection of trigeminal tissues, the virus is transported by anterograde axonal transport to the corneal epithelium, causing a visible cytopathic effect approximately 48 h postinfection. These results suggest that the organ model presented in this study holds particular promise for the direct observation and molecular analysis of herpes simplex virus spread between primary epithelia and sensory neurons and that it may be an attractive alternative to current experimental approaches based on laboratory animals or human fetal tissues.

Anterograde transport of herpes simplex virus type 1 in cultured, dissociated human and rat dorsal root ganglion neurons

The mechanism of anterograde transport of herpes simplex virus was studied in cultured dissociated human and rat dorsal root ganglion neurons. The neurons were infected with HSV-1 to examine the distribution of capsid (VP5), tegument (VP16), and glycoproteins (gC and gB) at 2, 6, 10, 13, 17, and 24 h postinfection (p.i.) with or without nocodazole (a microtubule depolymerizer) or brefeldin A (a Golgi inhibitor). Retrogradely transported VP5 was detected in the cytoplasm of the cell body up to the nuclear membrane at 2 h p.i. It was first detected de novo in the nucleus and cytoplasm at 10 h p.i., the axon hillock at 13 h p.i., and the axon at 15 to 17 h p.i. gC and gB were first detected de novo in the cytoplasm and the axon hillock at 10 h p.i. and then in the axon at 13 h p.i., which was always earlier than the detection of VP5. De novo-synthesized VP16 was first detected in the cytoplasm at 10 to 13 h p.i. and in the axon at 16 to 17 h p.i. Nocodazole inhibited the transport of all antigens, VP5, VP16, and gC or gB. The kinetics of inhibition of VP5 and gC could be dissociated. Brefeldin A inhibited the transport of gC or gB and VP16 but not VP5 into axons. Transmission immunoelectron microscopy confirmed that there were unenveloped nucleocapsids in the axon with or without brefeldin A. These findings demonstrate that glycoproteins and capsids, associated with tegument proteins, are transported by different pathways with slightly differing kinetics from the nucleus to the axon. Furthermore, axonal anterograde transport of the nucleocapsid can proceed despite the loss of most VP16.

Anterograde transport of herpes simplex virus proteins in axons of peripheral human fetal neurons: an immunoelectron microscopy study

Herpes simplex virus (HSV) reactivates from latency in the neurons of dorsal root ganglia (DRG) and is subsequently transported anterogradely along the axon to be shed at the skin or mucosa. Although we have previously shown that only unenveloped nucleocapsids are present in axons during anterograde transport, the mode of transport of tegument proteins and glycoproteins is not known. We used a two-chamber culture model with human fetal DRG cultivated in an inner chamber, allowing axons to grow out and penetrate an agarose barrier and interact with autologous epidermal cells in the outer chamber. After HSV infection of the DRG, anterograde transport of viral components could be examined in the axons in the outer chamber at different time points by electron and immunoelectron microscopy (IEM). In the axons, unenveloped nucleocapsids or focal collections of gold immunolabel for nucleocapsid (VP5) and/or tegument (VP16) were detected. VP5 and VP16 usually colocalized in both scanning and transmission IEM. In contrast, immunolabel for glycoproteins gB, gC, and gD was diffusely distributed in axons and was rarely associated with VP5 or VP16. In longitudinal sections of axons, immunolabel for glycoprotein was arrayed along the membranes of axonal vesicles. These findings provide evidence that in DRG axons, virus nucleocapsids coated with tegument proteins are transported separately from glycoproteins and suggest that final assembly of enveloped virus occurs at the axon terminus.

Neutralizing antibodies inhibit axonal spread of herpes simplex virus type 1 to epidermal cells in vitro

The ability of antibodies to interfere with anterograde transmission of herpes simplex virus (HSV) from neuronal axons to the epidermis was investigated in an in vitro model consisting of human fetal dorsal root ganglia innervating autologous skin explants in a dual-chamber tissue culture system. The number and size of viral cytopathic plaques in epidermal cells after axonal transmission from HSV type 1 (HSV-1)-infected dorsal root ganglionic neurons were significantly reduced by addition to the outer chamber of neutralizing polyclonal human sera to HSV-1, of a human recombinant monoclonal group Ib antibody to glycoprotein D (gD), and of rabbit sera to HSV-1 gB and gD but not by rabbit anti-gE or anti-gG. A similar pattern of inhibition of direct infection of epidermal cells by these antibodies was observed. High concentrations of the monoclonal anti-gD reduced transmission by 90%. Rabbit anti-gB was not taken up into neurons, and human anti-gD did not influence spread of HSV in the dorsal root ganglia or axonal transport of HSV antigens when applied to individual dissociated neurons. These results suggest that anti-gD and -gB antibodies interfere with axonal spread of HSV-1, possibly by neutralizing HSV during transmission across an intercellular gap between axonal termini and epidermal cells, and thus contribute to control of HSV spread and shedding. Therefore, selected human monoclonal antibodies to protective epitopes might even be effective in preventing epidermis-to-neuron transmission during primary HSV infection, especially neonatal infection.