The latency-associated transcript of herpes simplex virus type 1 promotes survival and stimulates axonal regeneration in sympathetic and trigeminal neurons.

Herpes simplex virus type 1 (HSV-1) primarily infects mucoepithelial tissues of the eye, the orofacial region, and to a lesser extent the genitalia. The virus is retrogradely transported through the axons of sensory and sympathetic neurons to their cell bodies to establishe a life-long latent infection. Throughout this latency period, the viral genome is transcriptionally silent except for a single region encoding the latency-associated transcript (LAT). The function of LAT is still largely unknown. To understand how HSV-1 latency might affect neurons, the authors transfected primary cultures of sympathetic neurons and trigeminal sensory neurons obtained from rat embryos with LAT-expressing plasmids. LAT increased the survival of both sympathetic and trigeminal neurons after induction of cell death by nerve growth factor (NGF) deprivation. Because HSV-1 is transported through axons both after initial infection and during reactivation, the authors considered the possibility that LAT may affect axonal growth. They found that LAT expression increased axonal regeneration by twofold in both types of neurons. Inhibition of the mitogen-activated protein kinase (MAPK) pathway reverses stimulation of both neuronal survival and axonal regeneration, which indicates that these effects are mediated through the MAPK pathway. These data provide evidence that HSV-1 LAT promotes survival of sympathetic as well as trigeminal neurons. The authors show for the first time that LAT stimulates axonal regeneration in both sympathetic and trigeminal neurons.

Herpes simplex virus type-1 latency-associated transcript-induced immunoreactivity of substance P in trigeminal neurons is reversed by bone morphogenetic protein-7.

Herpes simplex virus type-1 (HSV-1) primarily infects mucoepithelial tissues of the eye and the orofacial region. Subsequently, the virus is retrogradely transported through the axons of the trigeminal sensory neurons to their nuclei, where the virus establishes a life-long latent infection. During this latency period, the viral genome is transcriptionally silent except for a single region encoding the latency-associated transcript (LAT). To understand how HSV-1 latency might affect the expression of substance P in sensory neurons, we transfected primary cultures of trigeminal neurons obtained from rat embryos, with LAT expressing plasmids. The expression of LAT increased the percentage of substance P-immunoreactive neurons by two thirds. To examine the effect of bone morphogenetic protein-7 (BMP7) on the LAT-induced increase in substance P expression in trigeminal neurons, cultures transfected with LAT were treated with BMP7. Treatment with BMP7 reversed the effects of LAT on substance P expression in trigeminal neurons. Our data show for the first time that LAT increases substance P expression in trigeminal neurons and BMP7 can reverse these effects of LAT.

Two alphaherpesvirus latency-associated gene products influence calcitonin gene-related peptide levels in rat trigeminal neurons.

Herpes simplex virus type-1 (HSV-1) initially infects mucoepithelial tissues of the eye and the orofacial region. Subsequently, the virus is retrogradely transported through the axons of the trigeminal sensory neurons. HSV-1 establishes a life-long latent infection in these neurons, during which the transcription of the viral genome is silent, except for the sequences encoding the latency-associated transcript (LAT). To determine if HSV-1 latency might affect calcitonin gene-related peptide (CGRP) expression in trigeminal sensory neurons, we transfected primary neuronal cultures of trigeminal ganglia from rat embryos with plasmids expressing LAT. In the presence of Bone Morphogenetic Protein-7 (BMP7), CGRP was expressed in 49% of sensory neurons. However, this percentage was reduced to 19% in neurons transfected with LAT expressing plasmids. We also found that transfection of the IE63 gene of varicella-zoster virus (VZV) reduced the percentage of trigeminal neurons containing CGRP. However, the observed effect of IE63 in contrast to that of LAT was completely reversed by treatment of cultures with MgCl2, which indicates that the effect of IE63 was due to increased release of CGRP from trigeminal neurons. We provide here the first evidence that HSV-1 LAT decreases the level of CGRP in trigeminal neurons. These effects may be important for reducing the neuroinflammatory response, thus protecting host neuronal cells during HSV-1 latency in trigeminal neurons. In contrast, increased release of CGRP in the presence of IE63 protein may contribute to the neuralgias associated with VZV infection.

Herpes simplex virus type-1 latency inhibits dendritic growth in sympathetic neurons.

Herpes simplex virus type-1 (HSV-1) initially infects mucoepithelial tissues of the orofacial region, the eye and to a lesser extent the genitalia. Subsequently, the virus is retrogradely transported through the axons of the sensory and sympathetic neurons to their nuclei, where the virus establishes a life-long latent infection. During this latency period, the viral genome is transcriptionally silent except for a single region encoding the latency-associated transcript (LAT). LAT has been shown to affect apoptosis, but little else is known regarding its effects on neurons. To understand how HSV-1 latency might affect dendrites in sympathetic neurons, we transfected primary cultures of sympathetic neurons obtained from rat embryos, with LAT expressing plasmids. LAT inhibited initial dendritic growth and induced dendritic retraction in sympathetic neurons. Latent HSV-1 infection of cultured sympathetic neurons inhibited dendritic growth indicating that this is likely also a consequence of natural infection.