The molecular mechanisms that control how herpesviruses modulate between productive and latent states are poorly understood. Research projects led by members of the Wilson Lab at NYU Langone elucidate these mechanisms in cultured neurons and in Kaposi sarcoma.

Herpes Simplex Virus Latency in Cultured Neurons: Who’s in Control, the Virus or the Host?

Herpes simplex virus type 1 (HSV-1) and its close relative HSV-2 establish latency in the peripheral nervous system and reemerge as painful lesions around the mouth, eyes, and genitals. The viral latent genome resides in the neuronal nucleus located in the nerve ganglia, where it is circularized and assembled into chromatin similar to that of the host cell. Silencing of the productive (lytic) cycle genes occurs through epigenetic mechanisms involving histone modifications, but this repressive state can be reversed during reactivation allowing lytic gene transcription. Details of the reactivation process are shrouded in mystery despite decades of careful research in many laboratories.

One major reason for this gap in knowledge is the reliance on live-animal infection models that are difficult to manipulate and study. Through a very productive collaboration with colleagues in the laboratories of Ian J. Mohr, PhD, in the Department of Microbiology, and Moses V. Chao, PhD, in the Departments of Cell Biology, Neuroscience and Physiology, and Psychiatry, we have developed a tractable cell culture model for HSV-1 latency and reactivation using primary sympathetic neurons. We are now using this in vitro system to understand the role of the virus-encoded transcription factor VP16 and its cellular cofactor HCF-1 in overcoming epigenetic barriers to reactivation.

Our data indicates that VP16 and HCF-1 are initially held in the neuronal cytoplasm but enter the nucleus in cells that support reactivation. We hypothesize that HCF-1 plays critical roles in ejecting repressors including the polycomb group (PcG) repressor complexes from viral chromatin and in establishing an active chromatin state around lytic promoters propelling the program forward. In all likelihood, relocalization of HCF-1 represents a pre-existing process in neurons that has been co-opted by the virus to couple its lifecycle to signaling pathways within the host. Understanding the viral switch will help us understand how neuronal gene expression is modified by intra- and extra-cellular signals.

Molecular Basis of Latency and Reactivation in Kaposi Sarcoma–Associated Herpesvirus

The challenges that face latent Kaposi sarcoma–associated herpesvirus (KSHV) are very different from HSV in that the virus persists in a rapidly dividing endothelial or lymphocytic cell and that it chooses to modify its environment through extensive reprogramming of cellular gene expression and through the effusive production of extracellular signaling molecules (cytokines and chemokines). Because KSHV promotes the survival and proliferation of infected cells, it can form tumors in immunocompromised people, such as organ transplant recipients or those with HIV infection and AIDS. These tumors are disfiguring and can be life-threatening.

Our interests have focused on two viral transcription factors that serve as major regulators of the latency and lytic programs: LANA and RTA. LANA is expressed in all infected cells and manipulates viral and cellular gene expression, enables the viral DNA genome to replicate in synchrony with the host, and also physically tethers the viral genome to host chromosomes ensuring passage through cell divisions. Recent projects in the lab have explored the chromosome tethering function of LANA and the control of LANA expression.

RTA is only expressed in cells undergoing reactivation and like the HSV initiator protein VP16, RTA is needed to propel the lytic transcription program forward. Our recent studies have focused on identifying viral promoters that respond to RTA with the goal of understanding how these different genes are expressed in an ordered fashion. This has led to the identification of factors that cooperate with RTA to confer promoter- and presumably context-specific activity, so-called co-regulators.

Our studies concentrate on two examples: the cellular co-regulator CSL (RBPJk, CBF-1), an endpoint of the Notch signaling pathway, and the viral co-regulator vIRF4, an interferon regulatory factor (IRF) homologue that was captured from the cell at some relatively recent point in the evolution of the rhadinovirus family that includes KSHV.

We are interested to discover the significance of these factors for the virus. Does KSHV manipulate Notch signaling during reactivation? Why has KSHV selected, incorporated, and then modified a master regulator of the antiviral response? These and other studies emphasize the very tight intertwining of virus and host control processes and perhaps help to explain why herpesviruses can persist for long intervals in a host.