Regulation of herpes simplex virus type 1 thymidine kinase gene expression by thyroid hormone receptor in cultured neuronal cells

Abstract

SUMMARY Herpes simplex virus type-1 (HSV-1) undergoes acute primary infection in epithelial cells followed by establishment of latency in the neurons of trigeminal ganglia. The latent virus maintains a life-long dormant state and can reactivate spontaneously in a small fraction of host cells, suggesting transcriptional silencing occurs in neurons. Computer data mining analyses identified a thyroid hormone response element (TRE), the binding site for the thyroid hormone receptor (TR), in the promoter of HSV-1 thymidine kinase (TK). TRs are transcription factors whose activity is dependent on their ligand thyroid hormone (TH or T3). We hypothesize that TR and T3 exert regulation on HSV-1 gene expression in neurons. A neuroblastoma cell line expressing the TR isoform β (N2aTRβ) was utilized for initial in vitro investigation. Liganded TR repressed TK promoter activity but unliganded TR relieved the inhibition. Chromatin immunoprecipitation (ChIP) assays showed that TRs were recruited to TK TRE regardless of the status of T3; hyperacetylated histone H3 at lysine 9 was associated with transcriptionally active promoters. In addition, ChIP results indicated that a repressive mono-methylated H3 modified at lysine 9 was enriched in TK promoter in the presence of TR and T3. T3-treated N2aTRβ cells were suppressive to TK expression and release of infectious viruses at low moi. RT-PCR and plaque assays showed that the TK can be de-repressed and the virus release was increased when the T3 was removed. These results suggest that T3 could regulate HSV-1 gene expression through its receptor via histone modification in cultured neuronal cells.

A retrospective cohort analysis of the relationship between thyroid hormone level and herpes simplex virus-1 activation

“…First, it has been reported that TH interacted with its nuclear receptor TRs exert regulatory function on key HSV-1 genes [4,5]. Second, TH and TR are present in ganglia neurons [13,14] and can influence the survival, differentiation, maturation of neurons [15] thus affecting the HSV-1 latency status.…”

“…Endocrine dysfunctions have been shown to cause viral infection [3]. Recent studies suggested that TH and TRs played roles on HSV-1 gene silencing/activation and DNA replication and may have implication during latency/reactivation [4,5].…”

A Novel Thyroid Hormone Mediated Regulation of HSV-1 Gene Expression and Replication is Specific to Neuronal Cells and Associated with Disruption of Chromatin Condensation

“…TK is not required for lytic infection. However, it is essential for viral replication in neurons to generate dNTP since it is in a resting state [15]. Besides, viral replication, although occurred later during the lytic infection, is critical for efficient α and β expressions in neurons during reactivation [31].…”

“…Of all these mechanisms, hormonal regulation/ imbalance was discussed but not extensively studied [10][11][12][13]. For example, thyroid hormone (TH) was shown to play a role in regulating HSV-1 key genes and participate in latency/ reactivation but detailed molecular pathways are still unclear [14][15][16].…”

“…Normalization was accomplished by using plasmid pICP4-SEAP containing reporter secreted alkaline phosphatase (SEAP) driven by ICP4 promoter [25]. Site-directed mutagenesis at the TK TRE was essentially described previously [15].…”