The Marta Gaglia Lab
Virus - Host Cell Interaction in KSHV and Influenza Virus
Viruses employ multiple factors to take over the infected cells, with the goal of repurposing resources and avoiding immune system detection. We want to unravel the mechanisms that viruses use to usurp cellular function and the co-ordinate contribution of viral and cellular proteins to viral control of host gene expression and innate immune responses.
The infectious agents at the centers of our studies are an AIDS-related pathogen, Kaposi’s sarcoma-associated herpesvirus (KSHV), which causes Kaposi’s sarcoma and aggressive lymphomas, and the ubiquitous influenza A virus. Both these viruses are deadly. Kaposi’s sarcoma remains the major cause of cancer-related death in parts of Africa. In the US 1 in 200 transplant recipients develop Kaposi’s sarcoma. There are no targeted therapies for this disease. Influenza A virus infections affect 5-15% of the world-wide population yearly, with up to half a million deaths per year. Vaccines and therapeutic options are still only partly effective.
Understanding the biology of KSHV and influenza A virus will help in the design new therapeutic strategies. Also, studying the complex interaction of viral and cellular factors allows us to probe the normal working of the cell and uncover fundamental new aspects of gene regulation.
Viral RNases in host gene regulation
Regulation of RNA stability is often used by cells to fine-tune gene expression, especially of highly regulated genes. Several viruses, and in particular KSHV and influenza A virus, hijack RNA degradation pathways to enact “host shutoff”, a wide-spread reduction in host gene expression, using virus-encoded ribonucleases (RNases). The RNases these divergent viruses encode share a similar mechanism to degrade host RNAs that involves host-encoded RNA degradation enzymes, essentially tricking the cell into degrading its own transcripts. We have found that, the action of these RNases is targeted in two different ways: 1) they specifically target transcripts generated by the RNA polymerase II complex; 2) in the case of the KSHV protein SOX, cleavage occurs at specific sites defined by a degenerate sequence/structural element; in the case of influenza A virus PA-X other determinants may determine the position of cleavage sites. This suggests that there is more to host gene regulation by viral RNases than currently understood. Questions we are working on include: 1) how is the specificity of these enzymes is achieved?; 2) how is this specificity used by the viruses to sculpt gene expression and affect host pathways? In particular, we want to unravel the mechanism of action of the influenza A virus RNase PA-X, which has a key role in the regulation of immune-related damage during influenza A virus infection.
Figure 1. Viral RNases and cell-virus interaction mechanisms.
Caspase-mediated regulation of interferon responses
Interferon responses are the key first line of defense against viruses. Surprisingly, reactivation of the oncogenic herpesvirus KSHV from latency does not elicit a strong type I IFN response. We have exciting new data that during infection, the host enzymes caspases are usurped by the virus to block interferon responses, another example of the virus turning core host pathways against itself. Caspases are druggable targets and could be exploited to potentiate innate immunity against this and other viruses to suppress viral replication. We are currently investigating how caspases control IFN and how KSHV controls caspases.
Identification of novel regulators of gene expression encoded by KSHV
During replicative (lytic) KSHV infection, the transcriptome of the infected cells is severely altered. This is proposed to contribute to immune evasion and tumorigenesis, but the viral factors that govern these changes are poorly understood. Using an overexpression screen, we have identified several previously uncharacterized KSHV proteins that may regulate host gene expression. These novel functions may be conserved among gamma-herpesviruses, as homologs of the candidate regulators in MHV68, a related mouse virus, also modulate gene expression. We are currently investigating how several of these putative regulators interact with cellular pathways, what genes they affect and what role they play in the viral life cycle using viral mutants.
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