Carlos M. de Noronha, ScD

According to United Nations statistics, 36.7 million people are living with HIV/AIDS globally with an estimated 5,500 new infections occurring every day. Numerous antiviral drugs have been developed and are being used to extend the lives of many individuals living with HIV. Despite the efficacy of the antiviral drugs in the populations that have access to both the drugs and good health care, challenges still exist. Among these are eradication of HIV from infected individuals and/or the ability to continuously, durably suppress virus expression. Further, development of safe, effective vaccines, to prevent HIV-1 infection in uninfected individuals continues to be an important research focus.

Presently available anti-viral drugs were designed to counteract the best-understood HIV functions. These include protease-mediated cleavage of viral components, entry, reverse transcription and integration. These targets were experimentally most accessible because their activities parallel those of cellular homologs. The similarities made deciphering the proteins' functions easier but also made development of high specificity drugs more difficult.

Evolutionarily conserved proteins among the 15 encoded by HIV may serve as highly suitable alternative antiviral targets, specifically by virtue of their difference from cellular components. Determining the role that each viral protein plays in HIV infection will provide vital insight into how HIV evades and cripples the immune system.

A focus of research in our laboratory is on how one of these HIV-1 proteins, designated Vpr, contributes to virus replication and pathogenesis. Vpr has two known biological functions. One function is to halt the cell cycle at or near the G2 phase, specifically after most or all cellular DNA has been duplicated but before cell division. The other function is to promote infection of non-dividing cells. This role may be related to the Vpr-encoded nuclear import and export signals, but alternative possibilities are emerging. Interestingly, the capacities to cause cell cycle arrest and to promote virus replication in non-dividing cells are separated on two related proteins, Vpr and Vpx respectively, in HIV-2 and some simian immunodeficiency virus counterparts.

We and others have demonstrated that HIV1 and HIV2 Vpr as well as HIV2 Vpx engage a ubiquitin ligase complex which is characterized by inclusion of the proteins DDB1 and cullin4. Further we found that Vpr uses the protein DCAF1 as an adapter to the engage the ubiquitin ligase complex. Blocking the function of this complex in various ways prevented Vpr from triggering G2 cell cycle arrest but did not interfere detectably with normal cell cycle progression or with the ability of the cells to arrest in response to DNA damage. We are presently working to identify the cellular target or targets of this ubiquitin ligase complex and to determine whether there are functions of Vpr that have been overlooked in previous work. Our long-term goal is to determine how the interaction between Vpr and the ubiquitin ligase complex impacts HIV replication and pathogenesis.

While investigating the interplay between HIV and macrophages, we determined that mobilization of Nrf2, a transcription factor and anti-oxidant response regulator, blocks HIV infection. This block occurs after reverse transcription of the viral genome, but apparently before the viral genome is exposed to the nucleoplasm. Analysis of transcription changes resulting from Nrf2 mobilization showed no increase in mRNAs known to encode anti-viral products or decreases in mRNAs known to encode cellular factors on which the virus relies for infection. This block is already partially active in untreated macrophages.

Expression of the cystine/glutamate antiporter xCT increases dramatically after Nrf2 mobilization. Exploring the role for xCT in the blocking HIV infection, we found that increased xCT expression is sufficient, but not necessary for the infection block. Specifically, the import of cystine, a limiting glutathione precursor, is one of many ways that cells respond to oxidizing conditions, and that response is important for the observed antiviral activity.

Nrf2 mobilization does not change the quantity of known anti-HIV proteins, however it decreases the fraction of phosphorylated anti-HIV protein SAMHD1. The non-phosphorylated form of SAMHD1 counters HIV infection while the phosphorylated form does not. We are working to discover the role of SAMHD1 in the Nrf2-dependent infection block.