viruses Review Infection and Transport of Herpes Simplex Virus Type 1 in Neurons: Role of the Cytoskeleton Monica Miranda-Saksena 1,* ID , Christopher E. Denes 1 ID , Russell J. Diefenbach 2 ID and Anthony L. Cunningham 1,* ID 1 Centre for Virus Research, The Westmead Institute for Medical Research, The University of Sydney, Westmead NSW 2145, Australia; [email protected] 2 Department of Biomedical Sciences, Faculty of Medicine and Health Sciences, Macquarie University, Sydney NSW 2109, Australia; [email protected] * Correspondence: [email protected] (M.M.S.); [email protected] (A.L.C.); Tel.: +612-8627-3624 (M.M.S.) Received: 24 January 2018; Accepted: 20 February 2018; Published: 23 February 2018 Abstract: Herpes simplex virus type 1 (HSV-1) is a neuroinvasive human pathogen that has the ability to infect and replicate within epithelial cells and neurons and establish a life-long latent infection in sensory neurons. HSV-1 depends on the host cellular cytoskeleton for entry, replication, and exit. Therefore, HSV-1 has adapted mechanisms to promote its survival by exploiting the microtubule and actin cytoskeletons to direct its active transport, infection, and spread between neurons and epithelial cells during primary and recurrent infections. This review will focus on the currently known mechanisms utilized by HSV-1 to harness the neuronal cytoskeleton, molecular motors, and the secretory and exocytic pathways for efficient virus entry, axonal transport, replication, assembly, and exit from the distinct functional compartments (cell body and axon) of the highly polarized sensory neurons. Keywords: herpes simplex virus; neurons; axonal transport; cytoskeleton; microtubules; actin 1. Introduction Viruses are acellular, obligate intracellular parasites that can only reproduce within living hosts, and hence have evolved to utilize different mechanisms of viral reproduction. All the viruses have a capsid, which contains either double-stranded or single-stranded DNA or RNA. The shape of the capsid itself varies between viruses, and capsids are either enclosed within an envelope or naked (lacking an envelope). In order to reproduce, the virus must enter the host cell. Once inside the cell, the virus may use the host cell cytoskeleton, that is the actin and microtubule networks, for navigation through the cell cytoplasm [1,2]. The viral life cycle begins with the attachment of the virus to the host cell membrane, which allows for the virus or viral particles to enter the cell. The viral genome then needs to be translocated to the nucleoplasm or cytoplasm where replication of the viral genome, and the synthesis of viral proteins, takes place. These then move to sites where assembly of viral particles occurs, and finally the virus is transported back to the cell plasma membrane. This process usually involves the virus using the cell’s secretory pathways and the cytoskeleton, which are important for long distance transport of cellular cargo and organelles [1,3]. 2. Herpes Simplex Viruses The herpes viruses are DNA viruses able to cause lytic and latent infections in both human and animal populations. All of the herpesviruses (more than 100 known viruses) share a common virion morphology and a group of conserved genes that are required for virus replication [4]. Herpes simplex Viruses 2018, 10, 92; doi:10.3390/v10020092 www.mdpi.com/journal/viruses Viruses 2018, 10, 92 2 of 20 Viruses 2018, 10, x FOR PEER REVIEW 2 of 19 virus type 1 and 2 (HSV-1 and HSV-2) are human neuroinvasive pathogens and are amongst the most amongstcommonly the studied most memberscommonly of thestudied alphaherpesviruses. members of Thethe spreadalphaherpesviruses. of alphaherpesviruses The spread through of alphaherpesvirusesthe peripheral nervous through system the is criticalperipheral for establishingnervous system a life-long is critical infection for establishing within sensory a life-long neurons infectionof the host. within HSV-1 sensory is approximately neurons of 225the nmhost. in HSV-1 diameter is andapproximately consists of 225 four nm layers: in diameter the episomal and consistsDNA genome, of four capsid, layers: tegumentthe episomal and DNA envelope genome, [5]. At capsid, the center tegument is a large and linear envelope double-stranded [5]. At the center DNA is(dsDNA) a large linear genome double-stranded (152 kbp) that codes DNA for (dsDNA) more than genome 80 open (152 reading kbp) that frames codes (ORFs) for more enclosed than within80 open a readingT = 16 icosahedral frames (ORFs) capsid. enclosed Surrounding within the a capsidT = 16 isicosahedral a dense proteinaceous capsid. Surrounding layer called the the capsid tegument is a denseconsisting proteinaceous of 23 tegument layer called proteins the that tegument link the consisting capsid to of the 23 host tegument cell-derived proteins lipid-bilayer that link the envelope capsid tocontaining the host cell-derived 16 different lipid-bilayer membrane proteins envelope [6 containing–8]. 16 different membrane proteins [6–8]. Primary HSV-1 infection is usually established within the stratifiedstratified squamous epithelium (epidermis) of the oral and anogenitalanogenital mucosamucosa andand isis commonlycommonly asymptomaticasymptomatic (Figure(Figure1 1).). UponUpon infectioninfection of of sensory sensory nerves nerves innervating innervating the the skin skin or or mucosa, the the virus undergoes retrograde axonal transporttransport to to the the neuronal neuronal cell cell body body where where it can establish a life-long latent infection within the trigeminaltrigeminal and dorsal rootroot gangliaganglia (DRG)(DRG) [9[9,10].,10]. AfterAfter reactivation, reactivation, anterograde anterograde transport transport of of HSV-1 HSV-1 is isusually usually directed directed towards towards the the peripheral peripheral epithelial epithelial cells cells where where it typically it typically presents presents as mild as oralmild or oral labial or labiallesions lesions in an immunocompetent in an immunocompetent host (Figure host1 )[(Fig11ure]. In 1) neonates [11]. In or neonates immunocompromised or immunocompromised individuals, individuals,however, lesions however, are morelesions severe are more and severe persistent, and persistent, often causing often serious causing and serious life-threating and life-threating diseases, diseases,such as herpes such as simplex herpes encephalitis simplex encephalitis and disseminated and disseminated herpes. Even herpes. in immunocompetent Even in immunocompetent individuals, individuals,HSV-1 keratitis, HSV-1 which keratitis, is an infection which is of an the infection cornea in of the the eye, cornea can result in the in eye, blindness can result [12]. in Prior blindness genital [12].infection Prior with genital HSV-2 infection also enhances with HSV-2 HIV also acquisition enhances 3–5 HIV fold acquisition [13]. 3–5 fold [13]. Figure 1. HerpesHerpes simplex simplex virus virus type type 1 1 (HSV-1) (HSV-1) life cycle in the human host. After After primary primary infection in thethe epidermis,epidermis, HSV-1 HSV-1 enters enters sensory sensory nerves nerves innervating innervating the skin the or skin mucosa or andmucosa undergoes and undergoes retrograde retrogradeaxonal transport axonal to transport the neuronal to the cell neuronal body where cell itbody establishes where ait life-longestablishes latent a life-long infection latent within infection sensory withinneurons. sensory During neurons. reactivation, During HSV-1 reactivation, travels via anterogradeHSV-1 travels axonal via transport anterograde towards axonal the peripheraltransport towardsepidermis the to peripheral cause recurrent epidermis herpes to orcause to be recu asymptomaticallyrrent herpes or to shed. be asymptomatically shed. 3. Dorsal Root Ganglia Neurons Sensory DRG neurons have pseudounipolar axons that bifurcate outside the DRG to innervate the spinal cord and the epithelium (Figure 1). DRG neurons are encapsulated by a glial cell sheath, which is formed from closely applied satellite cells, which are in turn encased in a basal lamina [14]. The neuron-satellite cell unit is separated from other similar units by collagen fibrils and other Viruses 2018, 10, 92 3 of 20 3. Dorsal Root Ganglia Neurons Sensory DRG neurons have pseudounipolar axons that bifurcate outside the DRG to innervate the spinal cord and the epithelium (Figure1). DRG neurons are encapsulated by a glial cell sheath, which is formed from closely applied satellite cells, which are in turn encased in a basal lamina [14]. The neuron-satellite cell unit is separated from other similar units by collagen fibrils and other extracellular matrix molecules. The structure of a DRG neuron consists of a cell body connected to the axon by the axon hillock (Figure1). The cell body contains the nucleus, which is where viral DNA replication, transcription, and capsid assembly takes place during HSV-1 infection (Figure1). The axon serves to conduct the transmission of action potentials from the cell body to synapse. It also provides a physical conduit for the transport of essential biological materials from the cell body to the synapse, which is necessary for the function and viability of neurons. The axon hillock is a specialized neck region that contains key organelles and a specialized dense cytoskeletal organization, which controls the axonal transport of proteins and mRNA. The axon hillock also forms a barrier, where organelles such as Golgi bodies and lysosomes accumulate in