Identification of Herpes Simplex Virus Type 1 Latency-Associated Transcript Sequences That both Inhibit Apoptosis and Enhance the Spontaneous Reactivation Phenotype

Abstract

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene is essential for the high spontaneous and induced reactivation phenotype of HSV-1 in the rabbit ocular model and for the high induced reactivation phenotype in the mouse ocular model. Recently we showed that LAT has an antiapoptosis function, and we hypothesized that LAT's ability to inhibit apoptosis played an important role in LAT's ability to enhance the reactivation phenotype. Expression of just the first 1.5 kb of the 8.3-kb LAT gene is sufficient for both inhibition of apoptosis in an in vitro transient-transfection assay and the high spontaneous reactivation phenotype in vivo. Here we show the results of more complex mapping studies in which inhibition of apoptosis and the enhanced spontaneous reactivation phenotype also appear to be linked. The HSV-1 mutant virus dLAT371 has a high spontaneous reactivation phenotype in rabbits, suggesting that the LAT region deleted in this mutant (LAT nucleotides 76 to 447) is not required for this phenotype. The LAT3.3A viral mutant (which expresses LAT nucleotides 1 to 1499) also has a high spontaneous reactivation phenotype, suggesting that the region of LAT not expressed by this mutant (LAT nucleotide 1500 to the end of LAT) is also not required for this phenotype. Surprisingly, LAT2.9A, which is a combination of dLAT371 and LAT3.3A (i.e., it expresses LAT nucleotides 1 to 76 and 447 to 1499), has a low spontaneous reactivation phenotype indistinguishable from that of LAT null mutants. We report here that consistent with the low spontaneous reactivation phenotype of LAT2.9A, a plasmid expressing the identical LAT RNA did not inhibit caspase 9-induced apoptosis. In contrast, plasmids containing the same deletion but able to transcribe up to or past LAT nucleotide 2850 (rather than just up to LAT nucleotide 1499) inhibited caspase 9-induced apoptosis, consistent with the high spontaneous reactivation phenotype of dLAT371. Thus, LAT2.9A may have a low spontaneous reactivation phenotype because the LAT RNA that is made cannot block apoptosis, and dLAT371 apparently has a high spontaneous reactivation phenotype because the LAT RNA made has significant antiapoptosis activity. Furthermore, LAT appeared to have at least two regions capable of interfering with caspase 9-induced apoptosis. One region partially overlaps LAT nucleotides 76 to 447. The second region is partially (or completely) downstream of LAT nucleotide 1499.

Herpes simplex virus type 1 latency-associated transcript inhibits apoptosis and promotes neurite sprouting in neuroblastoma cells following serum starvation by maintaining protein kinase B (AKT) levels

“…It is also possible that AKT promotes cell survival after serum withdrawal and additional signalling pathways promote neurite outgrowth. Several studies have demonstrated that LAT inhibits apoptosis (Hamza et al, 2007;Inman et al, 2001;Jin et al, , 2002Jin et al, , , 2003Jin et al, , , 2004Peng et al, 2003Peng et al, , 2004Perng et al, 2000), but this study was the first to link AKT to the anti-apoptosis functions encoded within LAT coding sequences.…”

“…Consequently, LAT can inhibit caspase 8-or caspase 9-induced apoptosis (Henderson et al, 2002;Jin et al, 2003;Peng et al, 2004), the two major apoptotic pathways (Krueger et al, 2001;Schmitz et al, 2000;Wang, 2001). The anti-apoptosis functions of LAT correlate with promoting spontaneous reactivation (Inman et al, 2001;Jin et al, 2003) and appear to be the crucial function of LAT because three different anti-apoptosis genes restore wt levels of spontaneous reactivation to a LAT null mutant virus (Jin et al, 2005(Jin et al, , 2008Mott et al, 2003;Perng et al, 2002b). LAT also represses productive viral gene expression in TG of mice during acute infection (Chen et al, 1997b;Garber et al, 1997).…”

Introducing point mutations into the ATGs of the putative open reading frames of the HSV-1 gene encoding the latency associated transcript (LAT) reduces its anti-apoptosis activity

“…LAT plays an important role in the HSV latency-reactivation cycle since LAT null mutants have significantly reduced reactivation phenotypes in mice and rabbits while otherwise having wild-type like replication [5][6][7][8][9][10][11]. LAT has anti-apoptosis activity [12][13][14][15][16][17][18][19]. Mutants in which LAT is replaced by an alternative anti-apoptosis gene have efficient, wild-type like reactivation phenotypes [20][21][22], indicating that LAT's anti-apoptosis activity is sufficient to account for LAT's ability to support the wild-type reactivation phenotype in experimental animal models.…”

Influence of Herpes Simplex Virus Latency-Associated Transcript (LAT) on the Distribution of Latently Infected Neurons

“…Recent studies have demonstrated that HSV-1 LAT inhibits apoptosis of neurons, which is hypothesized to enhance the establishment of latency (2,13,36,45,76,77,81). However, HSV-2…”