Varicella-zoster virus VLT-ORF63 fusion transcript induces broad viral gene expression during reactivation from neuronal latency

Abstract

Varicella-zoster virus (VZV) establishes lifelong neuronal latency in most humans world-wide, reactivating in one-third to cause herpes zoster and occasionally chronic pain. How VZV establishes, maintains and reactivates from latency is largely unknown. VZV transcription during latency is restricted to the latency-associated transcript (VLT) and RNA 63 (encoding ORF63) in naturally VZV-infected human trigeminal ganglia (TG). While significantly more abundant, VLT levels positively correlated with RNA 63 suggesting co-regulated transcription during latency. Here, we identify VLT-ORF63 fusion transcripts and confirm VLT-ORF63, but not RNA 63, expression in human TG neurons. During in vitro latency, VLT is transcribed, whereas VLT-ORF63 expression is induced by reactivation stimuli. One isoform of VLT-ORF63, encoding a fusion protein combining VLT and ORF63 proteins, induces broad viral gene transcription. Collectively, our findings show that VZV expresses a unique set of VLT-ORF63 transcripts, potentially involved in the transition from latency to lytic VZV infection.

Aging and Options to Halt Declining Immunity to Virus Infections

“…The mechanisms leading to latency and subsequent reactivation are still to be unraveled. However, a recent study has suggested VLT-ORF63 transcripts from VZV as potential inducers of reactivation (282). In addition, cellmediated immunity appears to play an important role in the immune response to VZV and also latency (283)(284)(285).…”

Multifaceted Roles of ICP22/ORF63 Proteins in the Life Cycle of Human Herpesviruses

“…However, pORF63 did not induce the transcription of any IE, early or late gene, except for the IE promiscuous transactivator pORF61 and the early (E) transcript RNA 16-1 ( Moriuchi et al, 1993 ). These results indicated that gene transcription during VZV reactivation is initiated through VLT-ORF63 transcription/pVLT-ORF63 translation ( Ouwendijk et al, 2020 ). However, whether VLT and ORF63 RNA transcripts are produced by the same or distinct populations of neurons remains unclear, as human TG are composed of dissimilar subtypes of neurons, and how these transcripts may affect ability of VZV to reactivate from latent in neurons is unknown ( Depledge et al, 2018b ).…”

“…Although the expression levels were different, the expression levels of VLT and ORF63 transcripts were significantly correlated, suggesting that they coregulate expression during latent VZV infection ( Ouwendijk et al, 2012 ; Depledge et al, 2018a ). This apparent expression of two distinct viral transcripts during latency is unique among well-studied α-herpesviruses, suggesting that these two transcripts and/or their encoded proteins play an important role in the latency and reactivation of VZV ( Ouwendijk et al, 2020 ). In the human induced pluripotent stem cell–derived sensory neuron (HSN) latency model in vitro , the results of coexpression of VLT and two VLT-ORF63 fusion products, VLT63-1 and VLT63-2, showed that the encoded pVLT-ORF63 fusion protein can act as an initiator of VZV reactivation in infected human TG, providing new insights into the mechanism by which VZV establishes latency and reactivation ( Ouwendijk et al, 2020 ).…”

“…This apparent expression of two distinct viral transcripts during latency is unique among well-studied α-herpesviruses, suggesting that these two transcripts and/or their encoded proteins play an important role in the latency and reactivation of VZV ( Ouwendijk et al, 2020 ). In the human induced pluripotent stem cell–derived sensory neuron (HSN) latency model in vitro , the results of coexpression of VLT and two VLT-ORF63 fusion products, VLT63-1 and VLT63-2, showed that the encoded pVLT-ORF63 fusion protein can act as an initiator of VZV reactivation in infected human TG, providing new insights into the mechanism by which VZV establishes latency and reactivation ( Ouwendijk et al, 2020 ). Moreover, ectopic VLT63-1 induces lytic transcription of viral genes in HSN models of latent VZV infection, suggesting that the pVLT-ORF63 fusion protein, not the corresponding transcript, induces transcriptional activation of VZV genes.…”

“…HSV-1 ICP22 can generally regulate viral and host gene transcription by changing the phosphorylation status of host RNA polymerase II (RNA pol II) ( Bastian and Rice, 2009 ; Lin et al, 2010 ; Zaborowska et al, 2014 ; Fox et al, 2017 ) and can also facilitate the nuclear egress complex (NEC) accurately locate to the nuclear membrane to promote nuclear budding ( Liu et al, 2015 ), whereas VZV ORF63regulates VZV transcription by destabilizing preinitiation complex (PIC) formation ( Di Valentin et al, 2005 ). In addition, both ICP22 and ORF63 transcripts have been suggested to be important in latency ( Kennedy et al, 2015 ; Depledge et al, 2018a ; Matundan et al, 2019 ; Ouwendijk et al, 2020 ; Tormanen et al, 2020 ). By contrast, there are few reports of the participation of ICP22 in the lytic and latent infection of HSV-2.…”