My general research interests lie in understanding how viruses manipulate cellular pathways to ensure their own propagation. The goal is to use this knowledge to improve antiviral agents by interfering with these viral functions. Initial studies in our laboratory are focused on understanding the mechanisms that control cell death in herpes simplex virus-infected cells.
Herpes Simplex Virus
Herpes simplex virus 1 (HSV-1) infects over two-thirds of the United States population. Depending on the types of tissues affected, infection can lead to disease as minor as a cold sore or as devastating as life-threatening encephalitis. HSV-1 primary infection usually results in asymptomatic viral shedding or mucosal lesions. Subsequently, an immune response clears HSV-1 from the epithelium and lesions resolve. Epithelial viral clearance, however, does not eradicate the virus from the body because HSV-1 persists in a latent state in neurons. HSV-1 can reactivate from this reservoir to produce progeny virions and recurrent disease. Although current antiviral agents are effective at limiting lytic replication, they do not eliminate HSV-1 from latently infected tissues.
A common cellular defense mechanism against viral invasion is the elimination of infected cells through apoptotic cell death. Recently, apoptosis has been associated with both latency (Bloom, 2004 Int Rev Immunol,23, 187-98) and the severity of herpes associated disease (Miles, Willcox et al., 2004 Curr Eye Res, 29, 79-90).
Apoptosis induction during HSV-1 infection was initially recognized in human carcinoma HEp-2 cells, which undergo apoptosis when treated with a protein synthesis inhibitor during infection (Koyama and Adachi, 1997 J Gen Virol, 78, 2909-12). Further studies using this and other cancer cell lines have provided insight into the delicate balance of apoptotic signals during infection (reviewed in (Nguyen and Blaho, Adv Virus Res, 69, 67-97)).
A model based on our current understanding of apoptosis during HSV-1 infection is presented in Figure 1. HSV-1 modulation of apoptosis is tied to its stepwise pattern of viral gene regulation. Initially, apoptosis is triggered through the transcription of immediate early (α) viral genes (Sanfilippo, Chirimuuta et al., 2004 J Virol, 78, 224-39). The a gene products facilitate the expression of the second set of viral genes (ß), many of which are involved in viral DNA synthesis. Once viral genome replication is underway, the viral γ genes are expressed. During these later stages of the viral life cycle anti-apoptotic proteins are synthesized that prevent apoptotic cell death from proceeding. A number of viral gene products possess anti-apoptotic activities.
Susceptibility to HSV-1-dependent apoptosis correlates with cellular transformation status.Even though early studies that utilized the HEp-2 strain of HeLa cell provided information crucial to understanding the viral factors involved in HSV-dependent apoptosis (HDAP), analysis of host factor involvement was limited. When studies were expanded to include other cell types, it became apparent that a range of sensitivities to HDAP exists. Cell lines derived from brain, colon, and breast cancer were highly sensitive to HDAP (Nguyen, Kraft et al., J Gen Virol, 88, 1866-75). These data indicate that the exquisite sensitivity of transformed cells to HDAP is nearly universal. The only cancer cell lines found to be resistant to HDAP were those that failed to undergo apoptosis with all environmental stimuli tested. This resistance is likely due to mutations acquired in their cellular apoptotic machinery (Kraft, Nguyen et al., 2006 Virus Res, 120, 163-75). Unexpectedly, unlike both transformed and immortalized cells, all of the cells derived from normal epithelial tissues tested so far have failed to undergo HDAP. These findings suggested that biochemical pathways altered during oncogenesis mediate sensitivity to HDAP.
The Human Papillomavirus E6 protein is a key mediator of HSV-1-dependent apoptosis in HeLa cells. To address whether genetic changes occurring during oncogenesis sensitize cells to HDAP, we exploited the fact that continuous oncogene expression is essential for HeLa cells to maintain their tumorigenic properties (Goodwin and DiMaio, 2000 Proc Natl Acad Sci U S A, 97, 12513-8). HeLa cells harbor integrated Human Papillomavirus (HPV) genomes and express two viral oncogenes, E6 and E7, which inactivate the p53 and pRB tumor suppressor proteins, respectively. In a typical HPV infection, E6 and E7 expression is limited by the HPV E2 transcriptional repressor. In HeLa cells, however, the viral genome is integrated into the host genome such that the E2 ORF is disrupted, thereby allowing for unrestrained oncogene expression and rampant cellular proliferation. Reconstituting HeLa cells with E2 inhibits E6 and E7 expression, reactivates p53 and pRb, and represses growth (Goodwin, Yang et al., 2000 Proc Natl Acad Sci U S A, 97, 10978-83).
To test whether altering transformation status alters sensitivity to HDAP, we transduced HeLa cells with an E2 expressing virus (in collaboration with Dr. DiMaio, Yale University) prior to HDAP induction. E2 expression reduced HDAP in HeLa cells (Nguyen, Kraft et al., J Virol, 81, 12985-95), demonstrating that HeLa cells require continuous oncogene expression to efficiently undergo HDAP. Sole expression of E6 mediated HDAP sensitization.
Next, we independently modulated the activities of two known cellular targets of E6. These studies demonstrated that E6 sensitizes HeLa cells to HDAP through hTERT and p53 (Nguyen, Kraft et al., J Virol,81, 12985-95). Given the universality of the apoptotic antiviral response, p53 and telomerase regulation will likely be important for counteracting host defenses in many other viral infections. The mechanism whereby hTERT and p53 alter sensitivity to viral apoptosis is currently under investigation in my laboratory. We are also interested in identifying other cellular factors involved in this process.
Depending on the tissues affected Herpes Simplex Virus (HSV) infections lead to disease as minor as a cold sore or as devastating as encephalitis. Our goal is to elucidate host factors regulating HSV disease. A form of cell death, apoptosis, is a common response to viral infections. During HSV infection, an intricate balance between pro- and anti-apoptotic factors leads to a cell state in which apoptotic enzymes are activated in the absence of cell death. Our current research focus is to (i) define the impact of apoptosis on HSV replication and (ii) identify cellular players in this process.
Pradhan, P., Nguyen, M.L. Early passage neonatal and adult keratinocytes are sensitive to apoptosis induced by infection with an ICP27-null mutant of herpes simplex virus 1. Apoptosis. 18: 160-70. 2013.
Cotter, CR, Kim, W., Nguyen, ML, Yount, JS, López, CB, Blaho, JA, and Moran, TM. The virion host shut-off (vhs) protein of HSV-1 blocks the replication-independent activation of NF-κB in dendritic cells. J.Virol. 85:12662-12672., 2011 – link
Nguyen, M.L. and Blaho, J.A., Telomerase Activity during Herpesvirus Infection. Future Virology. 6(8) 901-904., 2011 – link
Nguyen, M.L., Cotter, C.R., Yount, J.S., López, C.B., Blaho, J.A., and Moran, T.M., The virion host shut-off (vhs) protein blocks a TLR-independent pathway of Herpes Simplex Virus type 1 recognition in human and mouse dendritic cells. PLoS ONE. 5(2): e8684. doi:10.1371/journal.pone.0008684., 2010 – link
Nguyen, M.L. and Blaho, J.A., Cellular Players in the Herpes Simplex Virus Dependent Apoptosis Balancing Act. Viruses, 1: 965-978., 2009 – link
Nguyen, M.L., Kraft, R.M., Aubert, M., Goodwin, E., DiMaio, D., and Blaho, J.A., p53 and hTERT Determine Sensitivity to Viral Apoptosis. J.Virol. 81:12985-12995., 2007 – link
Nguyen, M.L., Kraft, R.M., and Blaho, J.A., Susceptibility of Cancer Cells to Herpes Simplex Virus Dependent Apoptosis. J. Gen. Virol.88: 1866-1875, 2007 – link
Shai, A., Nguyen, M.L., Wagstaff, J., Jiang, Y., and Lambert, P.F., HPV16 E6 Confers p53-dependent and p53-independent Phenotypes in vivo in the Absence of Functional E6AP. Oncogene 26:3321-3328, 2007 – link
Nguyen, M.L. and Blaho, J.A. Apoptosis During Herpes Simplex Virus Infection. Advances in Virus Research 69: 67-97, 2007 – link
Kraft, R.M., Nguyen, M.L., Yang, XH, Thor, A.D, and Blaho, J.A., Caspase 3 activation during herpes simplex virus 1 infection. Virus Research 120: 163-175, 2006 – link
Nguyen, M.L., Kraft, R.M., Blaho, J.A. African green monkey kidney Vero cells require de novo protein synthesis for efficient herpes simplex virus 1-dependent apoptosis. Virology 336: 274-290, 2005 – link
Schaeffer A.J., Nguyen, M.L., Liem, A., Lee, D., Montagna, C., Lambert, P.F., Ried, T., Difilippantonio, M.J. E6 and E7 Oncoproteins Induce a Distinct Pattern of Chromosomal Aneuploidy in Skin Tumors from Transgenic Mice. Cancer Research 64: 538-546, 2004 – link
Nguyen, M.N., Nguyen, M.L., Caruana, G., Bernstein, A., Lambert, P.F., Griep, A.E. Requirement of PDZ Containing Proteins for Cell Cycle Regulation and Differentiation in the Mouse Lens Epithelium. Mol. Cell. Biol. 23: 8970-8981, 2003 – link
Nguyen, M.L., Nguyen, M.N., Lee, D., Griep, A.E., and Lambert, P.F. The PDZ Ligand Domain of the Human Papillomavirus 16 E6 Protein is Required for E6’s Induction of Epithelial Hyperplasia In Vivo. J. Virol. 77: 6957-6964, 2003 – link
Nguyen, M,L. Song, S., Liem, A, Liu, Y, Androphy, E., Lambert, P.F. A Mutant of Human Papillomavirus Type 16 Deficient in Binding a-Helix Partners Mediate the Oncogenic Potential in vivo. J. Virol. 76:13039-13048, 2002 – link