Membrane traffic pathways in viral replication and pathogenesis

"We study the human immunodeficiency virus (HIV) that is linked to AIDS and is believed to have infected 60 million people worldwide. During infection these viruses must interact with receptor proteins expressed on the surfaces of target cells. Using the natural binding proteins of these receptors, it is possible to abrogate HIV infection. We are trying to understand the cellular and molecular mechanisms that underlie this regulation with the aim of developing novel strategies to down regulate the receptors. Such strategies may have efficacy against HIV. Importantly, the receptors exploited by HIV are representative of a large class of similar proteins, so-called G protein coupled receptors (GPCRs), that control a range of biological functions including the immune system. So what we learn about HIV receptors may have relevance to other important biological events.||In addition to understanding virus entry, we also investigate the molecular and cellular mechanisms involved in the formation of new virus particles within infected cells. This requires that the key structural proteins that make up virus particles are bought together at the same time and place within an infected cell so that virus particles can form. We have identified molecular signals in the envelope glycoprotein that are required for the correct trafficking within infected cells. When one of these signals is mutated in a simian model system, the virus is attenuated and is no longer pathogenic. Moreover, animals treated with this virus are protected against challenge with normal pathogenic viruses. Together these studies are providing novel insights to the mechanisms of pathogenesis and may give hints to the key components of immune control."

We focus on understanding; (i) the mechanisms controlling cell surface expression of cellular receptors for HIV and (ii) the mechanisms involved in the assembly of HIV virions.||Project 1: Human immunodeficiency virus (HIV), the etiological agent of AIDS, infects cells by membrane fusion following interaction with receptors expressed on surfaces of target cells. Two different cell surface glycoproteins form the functional virus receptor - CD4 and a member of the chemokine receptor family of G protein-coupled receptors (GPCRs). Two chemokine receptors have been linked to HIV infection, CC chemokine receptor 5 (CCR5) and CXC chemokine receptor 4 (CXCR4). The agonists for these receptors can protect cells from infection by inducing their endocytosis. We are studying the mechanism of chemokine receptor internalisation and the fate of the internalised molecules. Although focused on chemokine receptors, these studies have relevance for understanding the regulation of a variety of other GPCRs.||Project 2: To make new HIV particles, the essential proteins of the virus must be bought together at the same time and place within infected cells. Structurally, HIV is a relatively simple virus and requires just three virally encoded proteins to make new infectious particles, namely Envelope (Env), Gag and Gag-Pol. We are trying to understand how these proteins are sorted and traffic within cells in order to produce viral particles. We have found that in primary human macrophages, infected with HIV in tissue culture, the assembly events occur on the membranes of a novel intracellular multivesicular compartment. This internal assembly may convey distinct biochemical and functional properties on virus particles and enable release of infectious virus to be regulated and coordinated with the presence of appropriate target cells, such as CD4 positive T cells. Experiments are in progress to determine how the viral components target this compartment and the normal biological functions of this compartment. Significantly, we have found that the Env protein contains conserved signals that control its intracellular trafficking, in part these signals mediate endocytosis and keep the level of Env expressed at the cell surface low. Significantly, in a monkey model, mutation of these signals in the closely related simian immunodeficiency viruses (SIV) leads to attenuation of the pathological potential of the virus. Moreover, animals treated with such viruses show protection when challenged with pathogenic viruses. We hypothesis that correct signals in the structural components of the virus are required to allow efficient assembly of infectious virions and that in animals challenged with viruses lacking the correct signals in the viral structural proteins, the immune system is able to control infection and protect against challenge with non-attenuated viruses.