An investigation of the effect of herpes simplex virus 1 infection on host cell metabolism

Abstract

Herpes simplex virus 1 (HSV-1) is a prevalent alphaherpesvirus capable of infecting a wide range of human cells. Although the HSV-1 genome encodes several proteins possessing metabolic activity, the virus relies primarily on the host cell for the production of energy and macromolecular precursors. This dissertation focuses on the interaction between HSV-1 and host cell metabolism, leading to a more complete view of the metabolic program induced by infection. First, we used stable isotope labeling and liquid chromatography-mass spectrometry (LC-MS) techniques to show that HSV-1 infection induces anapleurotic flux into the citric acid (TCA) cycle through the enzymatic activity of pyruvate carboxylase (PC). Infection also upregulated flux through aspartate to nucleotides, and this required the activity of the enzyme glutamic-oxaloacetic transaminase 2 (GOT2). When combined with steady state metabolite levels, these changes suggested that HSV-1 placed a high priority on nucleotide synthesis. We next completed an siRNA screen to determine if the virus depended on any additional metabolic enzymes for its replication. Multiple enzymes were identified, including the previously mentioned PC and GOT2. In addition, we found that knockdown of argininosuccinate synthetase 1 (AS1) increased virus yield. AS1 knockdown triggered a metabolic program reminiscent of the one induced by infection, including a significant increase in the levels of nucleotide synthesis metabolites. As wild-type infection resulted in the loss of AS1 mRNA and protein levels, we have concluded that decreased AS1 activity is responsible for at least part of the metabolic phenotype induced by HSV-1 infection.

Analysis of steady state metabolite levels after the addition of HSV-1 also showed a significant drop in NAD+, an important co-factor in multiple metabolic reactions. This decrease was due to activation of the enzyme poly(ADP-ribose) polymerase (PARP) during infection. PARP activation required viral DNA synthesis, and was likely a response to DNA damage pathways activated by replication of the viral genome. PARP over-activity can trigger cell death, and the virus appears to have found a way around this problem by degrading an isoform of poly(ADP-ribose) glycohydrolase (PARG), which regulates PARP.

Together, this work clearly shows that HSV-1 infection robustly alters host cell central carbon metabolism, both directly and indirectly, and that interactions with host metabolic enzymes play an integral role in this process.