The stable 2-kb LAT intron of herpes simplex stimulates the expression of heat shock proteins and protects cells from stress

Multifunctional Non-Coding RNAs Mediate Latent Infection and Recurrence of Herpes Simplex Viruses

Herpes simplex viruses (HSVs) often cause latent infection for a lifetime, leading to repeated recurrence. HSVs have been engineered as oncolytic HSVs. The mechanism of the latent infection and recurrence remains largely unknown, which brings great challenges and limitations to eliminate HSVs in clinic and engineer safe oHSVs. Here, we systematically reviewed the latest development of the multi-step complex process of HSV latency and reactivation. Significantly, we first summarized the three HSV latent infection pathways, analyzed the structure and expression of the LAT1 and LAT2 of HSV-1 and HSV-2, proposed the regulation of LAT expression by four pathways, and dissected the function of LAT mediated by five LAT products of miRNAs, sRNAs, lncRNAs, sncRNAs and ORFs. We further analyzed that application of HSV LAT deletion mutants in HSV vaccines and oHSVs. Our review showed that deleting LAT significantly reduced the latency and reactivation of HSV, providing new ideas for the future development of safe and effective HSV therapeutics, vaccines and oHSVs. In addition, we proposed that RNA silencing or RNA interference may play an important role in HSV latency and reactivation, which is worth validating in future.

HSV1 latent transcription and non-coding RNA: A critical retrospective

Virologists have invested great effort into understanding how the herpes simplex viruses and their relatives are maintained dormant over the lifespan of their host while maintaining the poise to remobilize on sporadic occasions. Piece by piece, our field has defined the tissues in play (the sensory ganglia), the transcriptional units (the latency-associated transcripts), and the responsive genomic region (the long repeats of the viral genomes). With time, the observed complexity of these features has compounded, and the totality of viral factors regulating latency are less obvious. In this review, we compose a comprehensive picture of the viral genetic elements suspected to be relevant to herpes simplex virus 1 (HSV1) latent transcription by conducting a critical analysis of about three decades of research. We describe these studies, which largely involved mutational analysis of the notable latency-associated transcripts (LATs), and more recently a series of viral miRNAs. We also intend to draw attention to the many other less characterized non-coding RNAs, and perhaps coding RNAs, that may be important for consideration when trying to disentangle the multitude of phenotypes of the many genetic modifications introduced into recombinant HSV1 strains.

Stable intronic sequence RNAs (sisRNAs): a new layer of gene regulation

Upon splicing, introns are rapidly degraded. Hence, RNAs derived from introns are commonly deemed as junk sequences. However, the discoveries of intronic-derived small nucleolar RNAs (snoRNAs), small Cajal body associated RNAs (scaRNAs) and microRNAs (miRNAs) suggested otherwise. These non-coding RNAs are shown to play various roles in gene regulation. In this review, we highlight another class of intron-derived RNAs known as stable intronic sequence RNAs (sisRNAs). sisRNAs have been observed since the 1980 s; however, we are only beginning to understand their biological significance. Recent studies have shown or suggested that sisRNAs regulate their own host's gene expression, function as molecular sinks or sponges, and regulate protein translation. We propose that sisRNAs function as an additional layer of gene regulation in the cells.

The half-life of the HSV-1 1.5-kb LAT intron is similar to the half-life of the 2.0-kb LAT intron

Herpes simplex virus type 1 establishes a latent infection in the sensory neurons of the peripheral nervous system of humans. Although about 80 genes are expressed during the lytic cycle of the virus infection, essentially only one gene is expressed during the latent cycle. This gene is known as the latency-associated transcript (LAT), and it appears to play a role in the latency cycle through an anti-apoptotic function in the 5' end of the gene and miRNA encoded along the length of the transcript which downregulate some of the viral immediate-early gene products. The LAT gene is about 8.3 kb long and consists of two exons separated by an unusual intron. The intron between the exons consists of two nested introns. This arrangement of introns has been called a twintron. Furthermore, the larger (2 kb) intron has been shown to be very stable. In this study, we measure the stability of the shorter 1.5-kb nested intron and find its half-life is similar to the longer intron. This was achieved by deleting the 0.5-kb overlapping intron from a plasmid construct designed to express the LAT transcript from a tet-inducible promoter and measuring the half-life of the 1.5-kb intron in tissue culture cells. This finding supports the hypothesis that it is the common branch-point region of these nested introns that is responsible for their stability.

Investigation of the mechanism by which herpes simplex virus type 1 LAT sequences modulate preferential establishment of latent infection in mouse trigeminal ganglia

We previously demonstrated that herpes simplex virus type 1 (HSV-1) preferentially establishes latent infection in monoclonal antibody (MAb) A5-positive ganglionic neurons and that a 2.8-kb portion of the HSV-1 genome, corresponding to the 5' end of the LAT (latency-associated transcript) coding region, is responsible for this phenotype (38, 65). In the current study we carried out further genetic mapping of this latency phenotype and investigated some of the mechanisms that might be responsible. Studies with the chimeric virus HSV-1 17syn+/LAT2, an HSV-1 virus engineered to express HSV-2 LAT, demonstrated that this virus exhibited an HSV-2 latency phenotype, preferentially establishing latency in MAb KH10-positive neurons. This result is complementary to that previously described for the chimeric virus HSV-2 333/LAT1 and indicate that the HSV-1 latency phenotype can be changed to that of HSV-2 by substitution of a 2.8-kb piece of complementary viral DNA. Sequential studies in which we evaluated the pattern of HSV-1 latent infection of the mouse trigeminal ganglion following ocular inoculation with viruses with deletions of functional thymidine kinase, glycoprotein E, ICP0, and US9 protein demonstrate that preferential establishment of HSV-1 latent infection in A5-positive neurons is not a consequence of (i) differential access of HSV-1 to A5-positive neurons,(ii) differential cell-to-cell spread of HSV-1 to A5-positive neurons, (iii) differential "round-trip" spread of HSV-1 to A5-positive neurons, or (iv) expression of ICP0. Additional mapping studies with the HSV-1 LAT deletion viruses dLAT371, 17DeltaSty, and 17Delta348 indicate that most of the LAT 5' exon is not required for HSV-1 to preferentially establish latent infection in A5-positive neurons.

Efficient quiescent infection of normal human diploid fibroblasts with wild-type herpes simplex virus type 1

Quiescent infection of cultured cells with herpes simplex virus type 1 (HSV-1) provides an important, amenable means of studying the molecular mechanics of a nonproductive state that mimics key aspects of in vivo latency. To date, establishing high-multiplicity nonproductive infection of human cells with wild-type HSV-1 has proven challenging. Here, we describe simple culture conditions that established a cell state in normal human diploid fibroblasts that supported efficient quiescent infection using wild-type virus and exhibited many important properties of the in vivo latent state. Despite the efficient production of immediate early (IE) proteins ICP4 and ICP22, the latter remained unprocessed, and viral late gene products were only transiently and inefficiently produced. This low level of virus activity in cultures was rapidly suppressed as the nonproductive state was established. Entry into quiescence was associated with inefficient production of the viral trans-activating protein ICP0, and the accumulation of enlarged nuclear PML structures normally dispersed during productive infection. Lytic replication was rapidly and efficiently restored by exogenous expression of HSV-1 ICP0. These findings are in agreement with previous models in which quiescence was established with HSV mutants disrupted in their expression of IE gene products that included ICP0 and, importantly, provide a means to study cellular mechanisms that repress wild-type viral functions to prevent productive replication. We discuss this model in relation to existing systems and its potential as a simple tool to study the molecular mechanisms of quiescent infection in human cells using wild-type HSV-1.

Stable cell lines expressing high levels of the herpes simplex virus type 1 LAT are refractory to caspase 3 activation and DNA laddering following cold shock induced apoptosis

The herpes simplex virus type 1 (HSV-1) latency associated transcript (LAT) gene's anti-apoptosis activity plays a central, but not fully elucidated, role in enhancing the virus's reactivation phenotype. In transient transfection experiments, LAT increases cell survival following an apoptotic insult in the absence of other HSV-1 genes. However, the high background of untransfected cells has made it difficult to demonstrate that LAT inhibits specific apoptotic factors such as caspases. Here we report that, in mouse neuroblastoma cell lines (C1300) stably expressing high levels of LAT, cold shock induced apoptosis was blocked as judged by increased survival, protection against DNA fragmentation (by DNA ladder assay), and inhibition of caspase 3 cleavage and activation (Western blots). To our knowledge, this is the first report providing direct evidence that LAT blocks two biochemical hallmarks of apoptosis, caspase 3 cleavage and DNA laddering, in the absence of other HSV-1 gene products.

Spontaneous reactivation of herpes simplex virus type 1 in latently infected murine sensory ganglia

Careful studies of mouse trigeminal ganglia (TG) latently infected with herpes simplex virus type 1 (HSV-1) indicate the presence of productive cycle viral gene products and persistent immune response, suggesting ongoing spontaneous viral reactivation in these tissues. In the present study we set out to determine whether infectious virus is present in murine TG latently infected with HSV-1 (KOS). At 37 days after ocular inoculation we found a small amount of infectious virus in ca. 6% of latently infected murine TG. Furthermore, the amount of infectious virus that we detected (PFU per viral antigen-positive neuron) was similar to that detected in acutely infected ganglia. We conclude that spontaneous reactivation of infectious HSV-1 occurs in the mouse TG and is likely the principle cause of viral protein expression in these tissues. We next examined the role of latency-associated transcript (LAT) in spontaneous ganglionic reactivation by examining ganglia latently infected with KOS dlLAT1.8, a LAT deletion virus. Through the use of immunocytochemistry we found that KOS dlLAT1.8 had a rate of spontaneous ganglionic reactivation very similar to that of HSV-1 (KOS). Studying spontaneous ganglionic reactivation of HSV in the mouse TG allows a direct study of viral reactivation from latently infected neurons without the potential confounders and complicating downstream events that accompany the study of viral reactivation by explantation or peripheral viral shedding. Since most cases of human viral shedding and reactivation are not associated with a known trigger, spontaneous ganglionic reactivation of HSV-1 may be a better model of human disease than existing models.