Microtubule-Dependent Trafficking of Alphaherpesviruses in the Nervous System: The Ins and Outs

Abstract

The Alphaherpesvirinae include the neurotropic pathogens herpes simplex virus and varicella zoster virus of humans and pseudorabies virus of swine. These viruses establish lifelong latency in the nuclei of peripheral ganglia, but utilize the peripheral tissues those neurons innervate for productive replication, spread, and transmission. Delivery of virions from replicative pools to the sites of latency requires microtubule-directed retrograde axonal transport from the nerve terminus to the cell body of the sensory neuron. As a corollary, during reactivation newly assembled virions must travel along axonal microtubules in the anterograde direction to return to the nerve terminus and infect peripheral tissues, completing the cycle. Neurotropic alphaherpesviruses can therefore exploit neuronal microtubules and motors for long distance axonal transport, and alternate between periods of sustained plus end- and minus end-directed motion at different stages of their infectious cycle. This review summarizes our current understanding of the molecular details by which this is achieved.

Keywords: anterograde axonal transport; herpes simplex virus; microtubules; motors; neurons; pseudorabies virus; retrograde axonal transport.

Figure 1

Alphaherpesvirus entry into neurons. Capsids are represented as red discs and the UL36p/UL37p inner tegument as a gray capsid-bound layer. Microtubules are blue rods with the + end indicated. Virions replicate and assemble in infected epithelial cells (green) (1) and exocytosis (2) releases infectious enveloped particles (3) that fuse at the surface of adjacent sensory neurons (4). Tegument partially disassembles (grey discs) (5), and the capsid with associated inner tegument attaches to the plus end of axonal microtubules (6). The tegument-bound capsid then recruits dynein/dynactin and proceeds by MT-directed retrograde axonal transport (7), eventually reaching the MTOC (purple disc) (8). The capsid then switches to an anterograde trafficking mode (9) to deliver the viral genome to the cell nucleus (10).

Figure 2

Structure of alphaherpesvirus particles that utilize MTs during assembly and traffic. Left: The naked capsid. After assembly and packaging in the nucleus the capsid (red sphere) is delivered to the cytoplasm. The inner tegument proteins UL36p and UL37p (dark grey layer) provides the foundation for attachment of outer tegument subunits (yellow) prior to or during cytoplasmic envelopment (not shown). Right: the organelle-enclosed enveloped virion (OEV). Budding of the naked capsid into the lumen (light grey space) of a cytoplasmic organelle is accompanied by completion of the outer tegument (yellow) and generates a mature infectious enveloped virion within the organellar lumen. Multiple virally encoded membrane proteins (green bars) reside in the viral envelope and surrounding organellar membrane. For the purposes of this review three virally encoded membrane proteins are shown in detail: the gE/gI glycoprotein heterodimer (subunits shown in light blue and dark blue) and the US9p protein (orange bar). Note that US9p has essentially no lumenal domain. US9p and the gE/gI complex in the organellar bounding membrane project tails into the cell cytoplasm.

Figure 3

The UL36p and UL37p inner tegument proteins. Upper bar: The UL36p polypeptide. Regions discussed in this review are annotated as follows: DUB (Deubiquitinase) domain: yellow. Ub (Ubiquitin) attachment site: purple lollipop. WD/WE (tryptophan-acidic) motifs: red text. Proline-rich region: green. CBD (carboxy terminal capsid binding domain: black (additional capsid binding domains exist in UL36p but are not shown). Region sufficient for dynein/dynactin interaction is bracketed. Lower bar: UL37p polypeptide. R2 surface region: red. Site of dystonin binding: blue. Broken black line connects regions of UL36p and UL37p implicated in their association.

Figure 4

Alphaherpesvirus anterograde traffic in neurons. Capsids (red discs) are assembled and packaged with DNA in the nucleus, then emerge into the cytoplasm. Steps 1–6: married model. Naked capsids (1) acquire inner tegument and recruit kinesin to traffic along MTs (2) to their site of envelopment in the cell body (3). The resulting OEVs (4) then utilize MTs for delivery into and along the axon (5) eventually arriving at the nerve terminal for exocytosis and spread (6). Steps a–e: separate model. Naked capsids (a) acquire inner tegument and recruit kinesin to traffic along MTs (b) into the axon and along axonal MTs as naked capsids [c(i)]. In a variant of the separate model the naked capsid hitch-hikes on the surface of an anterograde trafficking carrier vesicle [c(ii)]. In both cases, the naked capsid moves down the axon separately from the lipids and proteins of its envelope. After arriving at the nerve terminal (d), the capsid is enveloped (e) to generate organelle-bound infectious virions indistinguishable from those delivered by the married model (6).

Figure 5

The alphaherpesvirus envelopment intermediate is non-motile. (a) Naked capsids (red disc) with inner tegument (grey layer) are capable of anterograde traffic along MTs (blue arrow). (b) Following delivery to the envelopment organelle capsids dock and bud into the lumen to assemble infectious enveloped virions. (c) The ESCRT apparatus and the Vps4 ATPase (green cylinder) catalyzes scission at the bud neck to pinch the enveloped virus into the organellar lumen. This envelopment intermediate is non-motile until scission is completed, and can be irreversibly trapped using a dominant negative allele of Vps4 (red text). (d) If scission is successful, the resulting organelle, containing a lumenal enveloped alphaherpesvirus particles, acquires the ability to traffic along MTs.